Chapter 17: Imaging Considerations in Orthopedic Trauma

Andrew H. Schmidt, Kerry M. Kallas

Chapter Outline

General Considerations

Medical imaging in the setting of acute musculoskeletal trauma contributes greatly to the initial diagnosis and subsequent management of orthopedic injuries. In many instances, patients are able to provide details of the injury, and imaging studies often confirm or exclude diagnoses already suggested by the clinical history, mechanism of injury, and physical examination findings. Imaging plays a critical role in the management of multitrauma patients who arrive obtunded or unconscious or are intubated and therefore unable to localize symptoms or cooperate during the physical examination. Multitrauma patients may also have coexisting neurologic and visceral injury, and in this setting orthopedic imaging is often deferred for other imaging studies and surgical triage for life-threatening injuries. However, plain radiographs must be made of all potential musculoskeletal injuries as soon as possible so that appropriate early treatment decisions are made. 
A wide variety of imaging examinations are available in clinical practice today, and use of a particular modality may be influenced by multiple factors, such as availability, image resolution, invasiveness, cost-effectiveness, patient risk, and requirements for special handling of the trauma patient. Many imaging studies are routinely ordered for specific indications and need no justification; for example, conventional radiographs are used to evaluate acute bony trauma of the extremities. Particularly with regard to more advanced imaging techniques; however, clinicians must often consider these tradeoffs in deciding whether to pursue additional imaging. Clinicians also need to be aware of the limitations of imaging in selected patients. For example, in elderly female patients with trauma to the proximal femur and/or pelvis, plain radiographs are not nearly as accurate for the diagnosis of fractures as MR imaging.103 

Availability

Although there is widespread availability of conventional radiography in both clinical and hospital settings, there is more variable access to advanced imaging modalities, particularly in rural communities and after hours.76 In a random survey of 5% of US emergency departments (n = 262), CT scanners were present in 96% of institutions and were available 24 hours a day in 94%. Scanner resolution was variable; 39% had access to 16-slice or greater scanners. On-site MRI was available in two-thirds of the institutions, with another 20% having mobile MRI available. Smaller and rural hospitals had less access to CT and MRI, and when available, CT tended to be lower resolution.76 Although data are lacking, access to other imaging modalities, such as ultrasound (US) and nuclear medicine (NM) is also likely to be similarly variable, and may be available only on an “on-call” basis or not available at all after hours. 
Fortunately, all that is needed to evaluate the orthopedic trauma patient in the immediate setting are plain radiographs, which provide information sufficient to diagnose any fracture or dislocation. The primary exception to this is in the evaluation of the spine, especially in the comatose patient and in the setting of specific injury patterns, where both CT and MRI have well-defined roles.59,79,82 However, controversy continues over the relative merits of CT versus MRI in the evaluation of spine trauma, with one group considering that MRI is the new standard for the evaluation of blunt cervical spine trauma.141 Although MRI has the added benefit of more clearly demonstrating soft tissue injuries in general, and disc herniation in the spine in particular, the inconsistent after-hours availability of MRI, as well as the obvious logistic problems of transporting and monitoring a trauma patient within an MRI unit, means that CT will remain the most common method of imaging the spine in the early evaluation of the trauma patient.189 
The recent introduction of digital radiography (DR) and teleradiology provides a means to obtain after-hours interpretation of images by trained radiologists.57,135,167,194 Although this is most often done in the management of acute neurologic emergencies and in the assessment of cross-sectional imaging of the abdomen and chest, such technology will no doubt benefit musculoskeletal trauma patients as well. In a recent report describing the benefits of a nighttime teleradiology service for emergencies, 43 of 75 studies were musculoskeletal.57 

Image Resolution

The choice of a particular imaging examination may, in part, be influenced by spatial resolution and contrast resolution. The ability of an imaging modality to resolve small objects of high subject contrast (e.g., bone–muscle interface) as distinct entities is referred to as spatial resolution, which is typically measured in line pairs per millimeter (lp/mm); higher values of lp/mm indicate greater resolution. For comparison, the limiting spatial resolution of the human eye is approximately 30 lp/mm. Resolution may also be expressed in millimeters, whereby smaller values represent greater spatial resolution. Table 17-1 lists representative values of limiting spatial resolution for common imaging modalities. Conventional radiographs have considerably better spatial resolution than cross-sectional imaging techniques, although overlapping bony structures often complicate evaluation of osseous anatomy. CT has better spatial resolution than MRI and is more commonly performed for evaluating finer bony abnormalities, such as avulsion fractures and calcification within tumor matrix. 
Table 17-1
The Limiting Spatial Resolutions of Various Medical Imaging Modalities: The Resolution Levels Achieved in Typical Clinical Usage of the Modality
Resolution
Modality lp/mm mm Comments
Screen film radiography 6 0.08 Limited by focal spot and detector resolution
Digital radiography 3 0.17 Limited by size of detector elements
Fluoroscopy 4 0.125 Limited by detector and focal spot
CT 1 0.4 About V2-mm pixels
NM: Planar imaging <0.1 7 Spatial resolution degrades substantially with distance from detector
SPECT <0.1 7 Spatial resolution worst toward the center of cross-sectional image slice
PET 0.1 5 Better spatial resolution than other nuclear medicine imaging modalities
MRI 0.5 1.0 Resolution can be improved at higher magnetic fields
US 1.7 0.3 (5 MHz) Limited by wavelength of sound
 

CT, computed tomography; NM, nuclear medicine; SPECT, single-photon emission computed tomography; PET, positron emission tomography; MRI, magnetic resonance imaging; US, ultrasound.

 

Modified and reprinted with permission from: Brushberg JT, Seibert JA, Leidholt EM Jr, et al. The essential physics of medical imaging. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2002.

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Contrast resolution refers to the ability to resolve two tissues of similar subject contrast. Conventional radiographs typically have poor soft tissue contrast resolution, whereas CT and MRI, in particular, have much better contrast resolution, in part related to their tomographic nature. For example, on conventional radiographs, subcutaneous fat may be discerned from the underlying muscle groups, although the intermuscular fascial planes cannot be visualized. CT and MRI better demonstrate the subcutaneous fat and intermuscular fascial planes, although MRI shows superior soft tissue contrast resolution compared with CT. 

Invasiveness

Most medical imaging procedures are noninvasive, or may require minimally invasive procedures, such as placement of intravenous access for contrast administration. Some imaging techniques are more invasive; however, such as peripheral angiography for vascular assessment in the trauma patient, and not only carry more inherent risk to the patient but also require greater resources and coordination on an emergent basis. When used appropriately, the diagnostic and therapeutic advantages of these procedures can contribute substantially to the patient’s management. 

Cost-effectiveness

With increasing pressures on cost containment, studies have been performed to address the cost-effectiveness of algorithms incorporating conventional radiography in the diagnosis and follow-up of musculoskeletal trauma.6 Significant costs may be incurred at receiving hospitals as a result of repeating radiographic workups for patients who have been transferred from referring facilities along with their original radiographs.187 Several recent studies have shown the benefits of “rules” in deciding when to order radiographs for knee and ankle trauma, resulting in fewer radiographs ordered and reduced cost without increased incidence of missed fractures.6 Additional studies have also shown the ability to reduce postoperative and follow-up radiographs in treatment of ankle fractures.86 Similar studies have addressed the cost-effectiveness of routine pelvic radiography in the setting of blunt trauma, although with mixed results.52,97 Study of pediatric torus fractures has shown that postcasting radiographs are unnecessary and follow-up radiographs do not change fracture management, with the implication of significant cost savings as a result of decreased radiography.62 
Given the increases in health care costs each year in the United States, an area of particular concern is the perceived expense of advanced musculoskeletal imaging techniques such as MRI. According to one estimate, the use of musculoskeletal MRI has grown nearly 14 times faster than overall musculoskeletal imaging during the period 1996 to 2005 (353% increase vs. 26% increase).153 Parker et al.153 explored the possible cost savings that could be realized if ultrasound was used instead of MRI for the diagnosis of musculoskeletal disorders. According to their review of 3,621 musculoskeletal MRI reports, 45.4% of primary diagnoses and 30.6% of all diagnoses could have been made with US instead.153 By extrapolating these data into the future, Parker et al.153 predict that the substitution of musculoskeletal US for MRI in appropriate cases could save more than $6.9 billion in the period 2006 to 2020 and lead to large cost savings for Medicare.153 
Other studies have shown that advanced imaging can be very cost-effective to the degree that such imaging improves initial diagnostic accuracy and avoids delays in treatment that can contribute to increased morbidity to the patient or delay to return to work. For example, several studies have shown that early MRI in cases of wrist trauma can be cost-effective by providing accurate diagnosis of scaphoid fractures in cases where initial conventional radiography was normal.27,49,127 MRI also proved superior to follow-up radiography for diagnosis of occult fractures, resulting in a change in management in up to 89% of cases.165 Cost was found to be similar or reduced in all studies comparing early MRI with more traditional algorithms of casting and radiographic follow-up.27,49,173 Two studies showed cost benefits associated with earlier rather than later MRI scanning.27,165 Similar studies have shown the cost-effectiveness of early limited MRI in the diagnosis and management of occult hip fractures.118 

Patient Risk

As a rule, imaging procedures used in evaluating orthopedic trauma contribute very little increased risk to the patient. The exception is CT, for which there is increasing concern about the risks of radiation exposure, especially in children.26,145,162,177,178 In addition to the risk of ionizing radiation with CT, other potential risks include patient handling, contrast reactions, and potential risk with MRI in patients with implanted devices containing metal. 
Handling trauma patients requires special attention and care, especially when transferring patients from gurneys onto imaging equipment. Many trauma patients have potential spine injuries, necessitating the use of spinal precautions and special radiographic views during imaging procedures. Likewise, fractured limbs may be very painful when moved, and there may be changes in fracture reduction or redislocation of an injured joint during manipulation of an extremity for radiographs. Because of pain and disorientation, patients may be unable to lie still during imaging examinations and may require analgesia and sedation. Sometimes, mechanical ventilation and multiple lines as well as catheters must be managed. Life support equipment and external fixation devices may also be incompatible with or limit the usefulness of certain examinations, such as conventional radiography and MRI. 
The risk of cancer associated with medical imaging has been the subject of recent reports.66,145,162,177,178 Cancer risks associated with ionizing radiation vary with modality; CT generates considerably higher-radiation doses compared to conventional radiography, while US and MRI do not involve ionizing radiation. Radiation doses vary considerably among CT protocols and between manufacturers.168 One study showed a 61% to 71% decrease in radiation dose between standard-dose and low-dose multidetector CT (MDCT) in cervical spine trauma.142 The National Council of Radiation Protection and Measurements reported in 2009 that the average radiation dose in the United States has risen from 3.6 mSv in the early 1980s to a value of 6.2 mSv in 2006, with most of the increase attributed to CT and nuclear imaging.145 Another report documented an increase between 1996 and 2012 in the use of advanced imaging and the per capita radiation dose, as well as the proportion of patients receiving high and very high doses of radiation.178 It has been estimated that as many as 1.5% to 2% of all cancers in US patients may be attributable to radiation from CT studies.26 CT is often used to evaluate to evaluate the multiply-injured and unconscious patient. These patients typically undergo head and body CT for evaluation of intracranial and body trauma, and the use of CT to clear the cervical spine, in lieu of conventional radiography, may be increasing. Body CT generates the greatest radiation dose. In the cervical region, the greatest risk of ionizing radiation is induction of thyroid malignancy. One study suggests that use of CT to clear the cervical spine in unconscious major trauma patients is justified given the relatively minor concern for inducing thyroid malignancy. However, in those patients who are conscious or with a Glasgow Coma Scale score between 9 and 12, clinical evaluation is more likely to be helpful, and the risk of thyroid malignancy in a young cohort does not justify the use of CT to clear the entire cervical spine.168 A recent study reported radiation doses in common CT studies done at four hospitals in San Francisco, noting a mean 13-fold difference between the highest and lowest dose for each type of study.177 Prasarn et al.162 reported total radiation exposure in a cohort of 1,357 orthopedic trauma patients. The average effective radiation dose for all patients was 31.6 mSv. For patients with an Injury Severity Score of greater than 16, the average exposure was 48.6 mSv.162 To put these findings in perspective, the International Commission on Radiological Protection recommends a permissible annual radiation dose of 20 mSv.162 This suggests that more should be done to limit radiation exposure during routine medical imaging, and clinicians as well as radiologists should keep radiation exposure in mind when ordering and performing imaging studies, especially CT. Because of concerns regarding ionizing radiation, CT manufacturers are developing noise-reduction software tools that allow high-quality images to be provided at much lower-radiation doses. 
Intravenous administration of iodinated contrast medium carries a small risk of adverse events, which may be categorized as mild, moderate, severe, and end organ.4 With traditional high-osmolality ionic contrast media, most adverse reactions are mild to moderate and occur in 5% to 12% of all patients. This incidence is significantly decreased with use of the newer low-osmolality nonionic contrast agents. The occurrence of severe contrast reactions is approximately 1 to 2 per 1,000 patients receiving high-osmolality contrast agents, whereas this number decreases to approximately 1 to 2 per 10,000 patients receiving low-osmolality contrast media.3 Examples of end-organ adverse events include thrombophlebitis related to the injection site, nephrotoxicity, pulseless electrical activity, seizures, and pulmonary edema.4 Peripheral angiography carries a low risk of complications, including bleeding and further vascular injury, although these problems may be minimized with experience and careful technique. 
MRI has unique risks in patients with implanted devices.85 Ferromagnetic metals can experience strong forces, especially near the magnet when forces can be enough to cause motion of the implant. Secondly, some metals may experience heating, although the effects of this are negligible in orthopedic implants. Finally, metal implants always cause some degradation of the image, although this can often be mitigated using new signal processing techniques, as discussed later in this chapter. 

Specific Imaging Modalities

Radiography

Technical Considerations

Conventional Radiography.
Conventional radiography (screen film radiography, plain film radiography) involves the use of x-rays, which are high-energy electromagnetic radiation with wavelengths smaller than ultraviolet light but longer than gamma rays. X-rays are produced using an x-ray tube, whereby electrons are emitted from a heated tungsten filament and accelerated across a voltage potential to strike an opposing tungsten target. The flow of electrons from filament to the target results in a tube current, and its interaction with the tungsten target generates a spectrum of x-rays and heat. Before leaving the x-ray tube, the x-rays are filtered and collimated into a useable beam. Factors that are set by the technologist to vary the quality and/or quantity of the x-ray beam include the voltage potential (measured in peak kilovoltage [kVp]), tube current (milliamperes [mA]), and exposure time (seconds). The output of the x-ray tube is expressed in mAs, calculated by multiplying the tube current (mA) by the exposure time (s). These factors are routinely recorded on digital radiographs, whereas they may be handwritten on portable radiographs for use with future examinations. 
After leaving the x-ray tube, the x-ray beam is directed through the patient and onto a screen/film cassette. The x-ray beam is attenuated as it passes through the patient, primarily via two processes: The photoelectric effect and the Compton scatter. After passing through the patient and before reaching the screen/film cassette, the transmitted radiation may be further collimated using a lead grid to remove the scatted radiation. Scatter increases with increasing patient thickness and larger fields of view and is a significant source of image degradation. Scatter may be negligible with extremities, in part related to their smaller size and greater proximity to the cassette; hence, grids may not be required. 
Screen/film cassettes are used to capture the transmitted radiation and create the latent image. Intensifying screens absorb x-ray photons and subsequently emit a greater number of light photons, which are then absorbed by the film. The film consists of a base, which is covered on one or both sides by an emulsion containing silver grains. Absorbed light photons result in liberation of free electrons within the emulsion, which subsequently reduce the silver atoms. When the film is developed, the reduced silver atoms are amplified and appear black on the film. Most screen/film cassettes use a dual-screen and dual-emulsion film combination, which is enclosed in a light-tight cassette and ensures good contact between the screens and film. To improve bone detail, a single-screen, single emulsion system may be used. 
Portable Radiography.
Portable radiography is frequently used to evaluate acute trauma patients, and its use may be complicated by several factors not encountered in the radiology department’s controlled environment. Trauma patients frequently are immobile and require special handling precautions, which may make it difficult to obtain routine anteroposterior (AP) and lateral projections. Appropriate placement and alignment of the screen/film cassette may be especially challenging, and if placed behind a backboard or beneath the patient’s cart, it may introduce artifacts into the radiograph and obscure anatomy of interest. Objects outside of the patient’s body related to his or her resuscitation, including endotracheal tubes, nasogastric tubes, chest tubes, and intravenous access, frequently project onto the radiograph. Casts, splints, and other external fixation devices may also project onto extremity radiographs and limit visualization of underlying bony detail. 
Technical factors, such as levels of kilovoltage peak (kVp) and mA, also need modification with portable radiography. Portable examinations are often performed with higher kVp settings, which provide for a wider margin of error in selecting other technical factors. Higher kVp values will result in greater scattered radiation; however, and may necessitate the use of a grid with the screen/film cassette. Precise alignment of the grid and cassette to the central beam of the portable x-ray tube is also more difficult because each of the components are not fixed in space, and malalignment results in significant obscuration of the image and degradation in image quality. 
Digital Radiography.
Several digital technologies for acquiring radiographs are in use and continue to be refined. In all DR systems, the creation of x-rays and attenuation of the x-ray beam as it passes through the patient are similar to conventional radiography systems. What differentiates DR systems is the type of image receptor that interacts with the attenuated x-ray beam to create a medical image. 
Computed radiography (CR) was first introduced in the late 1970s and has gained wide popularity in radiology departments within the last decade. With CR, the screen/film cassette is replaced by a cassette containing a photostimulatable phosphor deposited onto a substrate. When this type of phosphor interacts with x-rays, electrons are elevated to and trapped at higher-energy levels within the phosphor. The amount of electron trapping is proportional to the incident x-rays and results in the creation of a latent image, which can later be read using a specialized CR cassette reader. The reader scans the phosphor plate using a laser, which releases the electrons from their higher-energy states, and results in emission of light as they drop down to lower-energy states. The emitted light is captured by a photomultiplier tube, which converts the light into an electrical signal, which is subsequently digitized and stored. This process is done on a point-by-point basis throughout the entire phosphor plate to create a digital image. 
Relatively recent advances in flat panel detectors have led to a new digital imaging technology that has been referred to as direct capture radiography, or alternatively, indirect and direct DR. Each of these systems uses flat panel detectors that incorporate a large array of individual detector elements; each one corresponds to a pixel in the final image. In indirect DR, the detector elements are sensitive to light; hence, an x-ray intensifying screen is used to convert the incident x-rays into light, which is then captured by the individual detector elements and stored as a net negative charge. In direct DR, the individual detector elements are coated with a photoconductive material (selenium is commonly used). On exposure to x-rays, electrons are liberated from the photoconductor and are captured by the underlying detector elements, resulting in a net negative charge within each detector element. With both systems, the negative charges within the array of detector elements are read out electronically, digitized, and stored to create the final image. 
Currently, the spatial resolution of conventional radiography is greater than for DR systems (Table 17-1). CR and DR; however, offer significant advantages over conventional radiography, including the ability to manipulate digital images and alter image contrast, decreased radiation dose to the patient and radiologic personnel, and greater ease of storage and transmission of radiographs both within and beyond the imaging department. New portable DR systems that incorporate wireless flat panel displays are much quicker and have improved workflow compared to conventional DR.114 Unfortunately, DR systems are expensive to implement, as they require replacement of the entire radiography suite. CR systems are much more economical to implement, as they only require replacement of the screen/film cassettes and purchase of a CR reader. Both digital systems, though, offer ongoing cost savings as a result of decreased numbers of retakes and reduction in film costs. 

Applications

Conventional radiography remains the primary diagnostic modality for assessing fractures and dislocations. Orthogonal views, occasionally supplemented by additional specific projections, are sufficient to identify and manage most fractures. Orthopedic surgeons’ immediate interpretation of conventional radiographs of simple fractures has been shown to be timely, accurate, and inexpensive and contributes to patient care, whereas formal interpretation of the same studies by a radiologist typically occurs after care is rendered, may be inaccurate, adds expense, and does not contribute to patient management.23 
For many injuries, including those in the spine, specific measurements have been reported that may characterize a given injury.19 In addition to delineating the fracture pattern, conventional radiographs are useful for assessing limb length and alignment and are the primary means by which fracture healing is monitored. Numerous examples of the use of conventional radiographs are found throughout this text. In many cases, more subtle indications of injury apparent on conventional radiographs can suggest the need for further diagnostic imaging or intervention. Examples of such cases would be the identification of a posterior fat pad sign in a pediatric elbow, indicating an occult elbow injury, a joint effusion, or the finding of a fat-fluid level in the knee joint capsule indicating osteochondral fracture. Surrounding soft tissues may also be evaluated for and show additional evidence of trauma, including swelling, foreign bodies, and gas. Although conventional radiographs are universally used for assessing fracture healing, one recent report noted that there is very poor interobserver agreement regarding the determination of fracture healing after internal fixation.43 
DR has largely replaced conventional radiography and has provided a platform on which to develop new methods of musculoskeletal imaging. Digital imaging facilitates computer processing of images, which may improve their diagnostic value. Botser et al.24 studied a series of nondisplaced proximal femoral fractures and found that digital enhancement with the use of specific filter techniques improved fracture diagnosis. One recent advance is a full body scanner that can take rapid digital images of the entire body in one or multiple planes (StatScan Critical Imaging System; Lodox Systems Ltd., South Africa). The use of StatScan in the evaluation of multiple trauma patients and pediatric patients has been reported.60,143,158 The primary advantages are the rapid detection of injuries and less time needed for resuscitation. In one study, 96% of fractures were identified on the initial StatScan.158 In another study focusing on 37 consecutive pelvic injuries, findings on StatScan images were compared to those seen with CR and CT.143 Of 73 abnormalities noted in these patients, 18 were not identifiable on the StatScan, although only one of the missed findings was considered significant for the initial management of the patient.143 Although many patients initially evaluated with StatScan still need formal CT, such studies can be more limited and result in less overall radiation exposure to the patient than conventional imaging algorithms.60 

Fluoroscopy

Technical Considerations

Conventional Fluoroscopy.
Fluoroscopy involves the use of low-dose x-rays to image patient anatomy at high temporal resolutions—that is, in real time. Typical components of a fluoroscopy system include an x-ray tube, filters, and a collimator, similar to that used in conventional radiography. The x-ray tube is energized continuously using a low exposure rate, and the x-ray beam is directed through the patient onto an image intensifier. The image intensifier is responsible for converting the attenuated x-ray beam into a visible light image, which is frequently coupled to a closed-circuit television camera to produce a “live” image on a video monitor. An optical coupling system, using high-resolution lenses and mirrors, may also be used to direct the light image to recording devices, such as video recorders and photospot cameras. 
The components of the image intensifier are housed in a glass vacuum tube and include a large input phosphor, a photo cathode, a series of electrostatic lenses, an anode, and a smaller output phosphor. Incident x-rays are directed onto the input phosphor and are converted into light photons, similar to a radiographic intensifying screen. The light photons are channeled by the phosphor to the adjacent photocathode as a result of the linear crystalline structure of the phosphor matrix. The photocathode is composed of a thin metal layer, containing cesium and antimony, applied to the posterior surface of the input phosphor, which interacts with the light photons and results in emission of electrons. The electrons are then accelerated from the photocathode to the anode by an applied voltage approximating 25,000 V. During the acceleration process, the electrons emitted across the entire cross-sectional area of the photocathode are kept in relative alignment by a series of electrostatic lenses, such that the spatial information they contain is preserved. The electrons are subsequently focused onto the output phosphor, which results in light emission and creation of an image. 
Fluoroscopy systems vary in configuration, from permanently installed biplane angiography suites to mobile C-arm designs. Mini C-arm units have become increasingly popular for outpatient clinics. Image intensifiers are produced in different sizes, and measurements refer to the size of the input phosphor. Typical diameters range from 10 to 40 cm (4 to 16 in), and various sizes may be better suited or standardized to specific applications. Many fluoroscopy systems offer additional magnification modes, which use a smaller area of the input phosphor to create the magnified image. The theoretical resolution of an image intensifier is approximately 4 to 5 lp/mm, with somewhat better resolution obtained in magnification modes (Table 17-1). This is achievable only when the images are output to film. The image intensifier output is usually coupled to a video monitor for real-time viewing, which results in degradation of the resolution achievable by the image intensifier. Resolution of such closed-circuit television systems is typically 1 to 2 lp/mm. 
Digital Fluoroscopy.
Advances in digital technology have led to the development of digital fluoroscopy systems, which are now common in clinical practice. The output of the image intensifier may be coupled to a high-resolution video camera with subsequently digitized output, or directed onto a charge-coupled device (CCD). A CCD is a small plate containing a large array of photosensitive elements, each of which corresponds to a single pixel in the final digital image. Each element stores charge in proportion to the amount of absorbed light, which is then read out electronically and digitized to produce a pixel value. The matrix of pixel values is then used to create the final digital image. The resolution of a CCD depends on the size of each of its array elements; CCDs with a 1024 × 1024 matrix may achieve a resolution of 10 lp/mm. The digital nature of the image lends itself to computer postprocessing, including digital subtraction techniques, which improves image contrast. More recent advances in flat panel detector technology using thin-film transistor (TFT) arrays may allow replacement of the image intensifier and video camera by TFT panels, resulting in even greater improvement in image contrast. 
Two- and Three-Dimensional Fluoroscopy.
New fluoroscopic devices obtain fluoroscopic images in an arc around the patient, and contain imaging processing software that provides an immediate two-dimensional (2D) or three-dimensional (3D) reconstruction of the target. Several 3D-fluoroscopic imaging systems are available. C-arms that are adapted for this purpose incorporate a motor that rotates the x-ray tube and image intensifier around the patient while taking hundreds of images. Immediate computer processing generates a reconstructed cross-sectional image that is similar to an axial CT image. Different manufacturers’ devices vary in the arc of rotation required to obtain an image, with newer devices capable of creating the reconstructed images with 136 degrees of arc compared to the 180 degrees required by first-generation scanners.183 Although the devices that obtain images through a full 180-degree arc produce quality images, the devices that image through the smaller arc can provide images in anatomic regions like the shoulder that cannot be imaged with the 180-degree devices.183 

Applications

Intraoperative Imaging.
Intraoperative radiography and fluoroscopy are almost universally used during the operative care of fractures. Imaging techniques are needed during surgery to verify the reduction of fractures, identify the starting portals for intramedullary nails, target cannulated or interlocking screws, and verify implant position (Fig. 17-1). Fluoroscopic assessment of tibial plateau fracture reduction leads to results as good as or better than those obtained with arthroscopy-assisted reduction.119 Norris et al.147 used intraoperative fluoroscopy during the repair of acetabular fractures and found it as effective as postoperative radiographs to assess fracture reduction and comparable to postoperative CT to evaluate for intra-articular extension of hardware. Recent advances in “minimally invasive” fracture fixation rely even more on the interpretation of fluoroscopic images.108 
Figure 17-1
 
Intraoperative fluoroscopic views of the proximal femur used to evaluate fracture reduction and position of the femoral head screw during cephalomedullary nailing of an unstable reverse obliquity fracture. Here, intraoperative fluoroscopy is used to target drilling of a guide pin in the center of the femoral head, using both (A) anteroposterior and (B) lateral images. Information available on these images includes coronal and sagittal plane fracture alignment, position of the intramedullary nail itself, and finally, position of the screw in the femoral head.
Intraoperative fluoroscopic views of the proximal femur used to evaluate fracture reduction and position of the femoral head screw during cephalomedullary nailing of an unstable reverse obliquity fracture. Here, intraoperative fluoroscopy is used to target drilling of a guide pin in the center of the femoral head, using both (A) anteroposterior and (B) lateral images. Information available on these images includes coronal and sagittal plane fracture alignment, position of the intramedullary nail itself, and finally, position of the screw in the femoral head.
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Figure 17-1
Intraoperative fluoroscopic views of the proximal femur used to evaluate fracture reduction and position of the femoral head screw during cephalomedullary nailing of an unstable reverse obliquity fracture. Here, intraoperative fluoroscopy is used to target drilling of a guide pin in the center of the femoral head, using both (A) anteroposterior and (B) lateral images. Information available on these images includes coronal and sagittal plane fracture alignment, position of the intramedullary nail itself, and finally, position of the screw in the femoral head.
Intraoperative fluoroscopic views of the proximal femur used to evaluate fracture reduction and position of the femoral head screw during cephalomedullary nailing of an unstable reverse obliquity fracture. Here, intraoperative fluoroscopy is used to target drilling of a guide pin in the center of the femoral head, using both (A) anteroposterior and (B) lateral images. Information available on these images includes coronal and sagittal plane fracture alignment, position of the intramedullary nail itself, and finally, position of the screw in the femoral head.
View Original | Slide (.ppt)
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Despite the benefits of intraoperative fluoroscopy, most surgeons insist on obtaining conventional radiographs at the completion of surgery. Although this practice requires further radiation exposure and adds time and expense, it is important for both clinical and medicolegal documentation. Fluoroscopic images have limited field-of-view and may not demonstrate the full extent of hardware fixation (as in the case of an intramedullary nail) or overall limb alignment as well as conventional radiographs. Finally, it may be difficult to compare intraoperative fluoroscopic images to later conventional radiographs, so the immediate postoperative radiograph represents an important baseline reference for future comparisons. 
Several studies have examined the amount of ionizing radiation that operating room personnel are exposed to during the care of fractures when fluoroscopy is used.16,95,186 Fortunately, with modern fluoroscopic systems, measurable radiation exposure is limited to the surgeon’s hands,16,95 although he or she needs to limit excessive use of the fluoroscope during surgical procedures. Recently, Matthews et al.132 showed that during surgery, repetitive fluoroscopic scout imaging is performed to reproduce a specific desired image. In a simulated test-rig, an average of seven scout images were required to reproduce a given C-arm position.132 In contrast, these investigators showed that the use of navigation-assisted repositioning using a standard, commercially available image-guided surgical navigation system did not require a single additional scout image, with comparable positioning times.132 
A recent advance in intraoperative fluoroscopy is the ability to generate cross-sectional, multiplanar 2D and 3D computer-reconstructed images in real-time.9,11,31,98,99,192 The ability to obtain immediate cross-sectional images during surgery can help the surgeon assess reduction during the repair of certain intra-articular fractures when direct visualization of the articular surface is not possible.31 Cross-sectional intraoperative imaging may also be of benefit in situations when hardware placement requires precision, such the insertion of pedicle screws or iliosacral screws.11 In cadaver models of calcaneal fracture98 and acetabular fracture,99 3D fluoroscopy was superior to standard 2D fluoroscopy and comparable to CT for the detection of intra-articular hardware and intermediate between the other modalities in demonstrating articular impaction of acetabular fracture99 or the articular reduction or medial screw protrusion in calcaneal fractures.98 In a clinical series of articular fractures, information obtained via intraoperative 3D fluoroscopy led to a decision to revise the fracture and/or fixation in 11% of cases.9 In another series of patients undergoing surgery for foot and ankle trauma, 39% of cases with adequate conventional C-arm images were revised intraoperatively after 3D fluoroscopy was performed.169 However, it is important to note that no one has documented that the use of 3D fluoroscopy improves outcomes, so for now this technology remains mostly investigational. 
Surgical Navigation.
Although computer-assisted surgical navigation techniques may be performed with cross-sectional imaging data obtained from preoperative CT, fluoroscopy is commonly used for surgical navigation because of its flexibility, convenience, low radiation exposure, and low cost. Although the field of surgical navigation is in its infancy, computer-assisted surgical navigation has already been applied to cervical and thoracic spine fracture fixation,7 placement of percutaneous iliosacral and anterior column screws in the pelvis,40,115,140 femoral neck fracture fixation,115 and intramedullary nailing.96,101,115,185 
Fluoroscopic surgical navigation requires a specialized computer-based system, which tracks the position of a hand-held tool in space. It is necessary to “register” the patient’s bone within the computer based on preoperative CT data or the use of a generic dataset. Fluoroscopic views need be taken only once; thereafter, all movements of the tool are recorded against the registered bone image and may be displayed in different planes simultaneously, superimposed on the static images by the computer system. This dramatically reduces the need for repeated intraoperative imaging, decreasing the time of surgery and the radiation exposure of the patient and surgical team. However, intraoperative changes in the patient’s position or in the dimensions of the registered bone (such as might occur during fracture reduction) decrease the accuracy of image registration. Surgical navigation has been used for hip fractures35,117 and placement of iliosacral screws.140 During intramedullary nailing of the femur, surgical navigation facilitates accurate entry-point location, fracture reduction, and insertion of interlocking and blocking screws and assists with determination of nail and screw length.74,101,196 Weil et al.196 used a cadaveric femur model to demonstrate that computerized navigation may increase the precision of fracture reduction, while at the same time lessening requirements for intraoperative fluoroscopy. In another cadaveric model, navigated distal interlocking was found to lead to less rotational deformity (2 degrees) compared with freehand distal interlocking (7 degrees).74 
Although this technology has been proved to be feasible, the clinical importance and cost-effectiveness of surgical navigation remain undetermined. Collinge et al.38 compared the safety and efficiency of standard multiplanar fluoroscopy with those of virtual fluoroscopy for use in the percutaneous insertion of iliosacral screws in 29 cadaver specimens. Interestingly, both methods were equally accurate; one screw was incorrectly inserted in each group, and both groups contained examples of screws with minor deviations in trajectory. Although the actual time for screw insertion was less with virtual fluoroscopy (3.5 minutes vs. 7.1 minutes), this was offset by the increased time needed to set up and calibrate the image-guided system.38 Liebergall et al.117 showed improved screw parallelism and screw spread when navigation was used during repair of femoral neck fractures, and this correlated with fewer reoperations and overall complications in the navigated group. 

Computed Tomography

Technical Considerations

CT has had the greatest clinical impact of any of the radiographic imaging modalities; its inventors (Godfrey Hounsfield and Allan Cormack) received the Nobel Prize for Medicine in 1979. Since its inception in the early 1970s, advances in technology and computer science have guided the development of several new generations of CT scanners, each capable of greater throughput and improved resolution. Although a more detailed review of the history of CT scanners is beyond the scope of this section, a brief description of current concepts in CT scanner technology is presented. 
Helical (spiral) CT scanners were developed in the late 1980s and are so named because of the helical path the x-ray beam takes through the patient. The development of “slip ring” technology allowed the gantry (x-ray tube and detectors) to rotate continuously around the patient, whereas with previous-generation scanners, gantry rotation was constrained by electrical cables, which needed to be unwound in between slice acquisitions. With nonhelical scanners, table position was incrementally advanced in between slice acquisitions; with slip ring technology, the table position is advanced continuously while the gantry rotates, resulting in a helical x-ray beam path. 
The first dual-slice helical scanner was demonstrated in 1992, with 4- and 16-slice models appearing in 1998 and 2001. On the whole, multislice (multidetector) scanners are similar to single-slice helical scanners in many respects. Instead of a single row of detectors; however, multiple rows of detectors are present within the gantry and are designed to allow acquisition of multiple slices at the same time. 
With these new technologies, scanning algorithms needed to be modified, which resulted in new terminology and imaging parameters to adjust. For single-slice helical scanners (and older-generation scanners as well), slice thickness is determined by x-ray beam collimation, whereas, for multislice scanners, it is determined by detector width. For single-slice scanners, pitch is defined as the ratio of table movement (mm) per 360-degree rotation to slice thickness (mm). A pitch of 1 is comparable to older-generation scanners where the table movement increment was the same as the slice thickness. A pitch of less than 1 results in overlapping of the x-ray beam and higher patient radiation dose; a pitch greater than 1 results in increased coverage through the patient and decreased radiation dose. In practice, pitch is generally limited to 1.5 to 2, although protocols vary. For multislice scanners, the definition of pitch changes to incorporate the detector array width rather than the single slice width and is referred to as detector pitch. 
The data sets from single-slice and multislice scanners are both helical in nature, and individual slices must be interpolated from the data set. Minimum slice thickness is set by the original x-ray beam collimation (single-slice scanners) or detector width (multislice scanners). Any number of slices may be reconstructed at any position along the long axis of the patient, and in any thickness equal to or greater than the minimal slice thickness. This allows reconstruction of overcontiguous slices (with typically 50% overlap), which increases the sensitivity for detecting small lesions that may otherwise be averaged between adjacent slices. This also results in twice as many images, although with no increase in scan time or additional radiation dose to the patient. 
Multiplanar reconstructions (MPRs) and 3D reconstructions are also routinely performed with both single-slice and multislice helical scanners. This, in part, is related to the fact that today’s CT examinations routinely produce hundreds of images, and MPR and 3D reformatting assist in interpreting these data. Advances in detector technology have allowed slice thickness to decrease such that slice thicknesses of 0.5 mm are routinely achieved clinically and allow acquisition of isotropic voxels. A voxel is the 3D equivalent of a pixel and represents the volume of tissue represented by a single pixel; isotropic voxels have uniform thickness in all directions (e.g., 0.5 × 0.5 × 0.5 mm). Acquisition of images with isotropic voxels results in multiplanar (nonaxial) reconstructions that have in-plane resolutions equal to those of the original axial image. In addition, the use of overcontiguous images is useful in 3D reconstructions to eliminate stair-step artifact. 
Developments in rapid-prototyping technology now readily allow Digital Imaging and Communications in Medicine (DICOM)-based CT data to be used to develop physical models of the imaging target. Such models have been extensively used in maxillofacial reconstruction, but are beginning to be utilized for complex fractures of the scapula and pelvis.58 
Orthopedic hardware results in metallic streak artifact on standard CT images, which frequently obscures surrounding bone and soft tissue detail.65 Standard CT protocols can be modified to reduce metal artifact by using a lower pitch setting, a higher tube current (250 to 350 mAs), and higher peak kilovoltage (140 kVp) during acquisition.65 In addition, soft-tissue filters rather than edge-enhancing algorithms can be used to further reduce metal artifacts, and use of wide window settings at image display (width, 3,000 to 4,000 HU; level, 800 HU) is advocated.65 Metal streak artifact is propagated on multiplanar reformatted images as well. Fortunately, volume rendering of an MDCT axial database can dramatically reduce streak artifact associated with hardware.64 Fayad et al.65 recently reviewed the use of 3D-CT images obtained using 64-MDCT in assessing postoperative complications in patients with orthopedic hardware, including nonunion, infection, new fracture, or hardware malposition. Postoperative MDCT after surgical repair of tibial plateau fractures has been shown to accurately image the articular surface in the early postoperative period and to be useful for assessing fracture healing later, despite the adjacent metal implant.144 
Overall, the advantages of multislice helical scanners include faster scan times and patient throughput, reduced motion artifacts, reduced intravenous contrast requirements, improved lesion detection, and improved MPRs and 3D reconstructions. Disadvantages include the potential for decreased resolution along the long axis of the patient (related to increased pitch) and a large number of images, resulting in increased reconstruction time and storage requirements. Another disadvantage of CT in general is the high radiation dose associated with this modality. However, radiation doses can be reduced by using low-dose, rather than standard-dose, scanning algorithms without differences in subjective image quality evaluation.142 Furthermore, use of MDCT with volume visualization and postprocessing (3D CT) limits exposure to radiation by using single-plane acquisitions with isotropic data sets. 
Advances are being made in not only the processing of images, but in image analysis. Although currently not in clinical use, automatic detection of fractures using a computer algorithm has been shown to be very fast and effective.203 

Applications

Complex Fractures.
CT remains the imaging modality of choice for evaluating complex fractures as well as ruling out injury to the spine. Axial (cross-sectional) CT cuts provide important information regarding 3D relationships that may not be evident on plain films (Fig. 17-2). In addition to high-resolution axial images, MPRs are commonly performed (Fig. 17-3). Such information provides critical data about the displacement of fracture fragments, including assessment of intra-articular displacement, articular surface depression, and bone loss.8 Three-dimensional reconstructions using surface rendering techniques are often less helpful in fracture management compared with MPRs. With 3D imaging techniques, fracture planes are frequently obscured by overlying fracture fragments and underestimate the true degree of comminution; however, they may be helpful in evaluating angulation and displacement of fracture fragments, in addition to depression of articular surfaces. With previous-generation CT scanners, evaluation of fracture planes parallel to the scan plane was suboptimal because of volume averaging of the fracture plane with adjacent intact bone. With multislice scanners, image data are obtained as a volume rather than as individual slices, and MPRs typically have resolution equal to the axial images (due to isotropic voxels). For this reason, detection of transversely oriented fracture planes is significantly enhanced. Typical indications for CT include fractures of the spine, scapula, proximal humerus, distal radius, pelvis and acetabulum, tibial plateau, tibial plafond, calcaneus, and midfoot. 
Figure 17-2
 
A: Anteroposterior and lateral plain images of an ankle with recurrent syndesmotic widening following previous screw fixation. Once cannot tell from the plain films alone whether the fibula is malpositioned anteriorly or posteriorly as well. B: Axial CT of both ankles demonstrates that the primary deformity is widening. C: Plain views after revision fixation. D: Postoperative CT showing that the syndesmosis has been reduced.
A: Anteroposterior and lateral plain images of an ankle with recurrent syndesmotic widening following previous screw fixation. Once cannot tell from the plain films alone whether the fibula is malpositioned anteriorly or posteriorly as well. B: Axial CT of both ankles demonstrates that the primary deformity is widening. C: Plain views after revision fixation. D: Postoperative CT showing that the syndesmosis has been reduced.
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Figure 17-2
A: Anteroposterior and lateral plain images of an ankle with recurrent syndesmotic widening following previous screw fixation. Once cannot tell from the plain films alone whether the fibula is malpositioned anteriorly or posteriorly as well. B: Axial CT of both ankles demonstrates that the primary deformity is widening. C: Plain views after revision fixation. D: Postoperative CT showing that the syndesmosis has been reduced.
A: Anteroposterior and lateral plain images of an ankle with recurrent syndesmotic widening following previous screw fixation. Once cannot tell from the plain films alone whether the fibula is malpositioned anteriorly or posteriorly as well. B: Axial CT of both ankles demonstrates that the primary deformity is widening. C: Plain views after revision fixation. D: Postoperative CT showing that the syndesmosis has been reduced.
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Figure 17-3
 
A: Anteroposterior view of a right acetabular fracture-dislocation. Axial computed tomography (CT) (B) better reveals the extent of comminution of the posterior wall, as well as demonstrating the persistent posterior dislocation of the hip. C and D: With high-resolution 3D reconstructions, a more “anatomic” appreciation of the fracture pattern is possible, similar to what the surgeon would view at surgery. Note the “ghosting” technique used to remove the other bones, most notably the femoral head, that would otherwise obscure the view. For complex fractures such as this, advanced CT scanning is unparalleled.
A: Anteroposterior view of a right acetabular fracture-dislocation. Axial computed tomography (CT) (B) better reveals the extent of comminution of the posterior wall, as well as demonstrating the persistent posterior dislocation of the hip. C and D: With high-resolution 3D reconstructions, a more “anatomic” appreciation of the fracture pattern is possible, similar to what the surgeon would view at surgery. Note the “ghosting” technique used to remove the other bones, most notably the femoral head, that would otherwise obscure the view. For complex fractures such as this, advanced CT scanning is unparalleled.
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Figure 17-3
A: Anteroposterior view of a right acetabular fracture-dislocation. Axial computed tomography (CT) (B) better reveals the extent of comminution of the posterior wall, as well as demonstrating the persistent posterior dislocation of the hip. C and D: With high-resolution 3D reconstructions, a more “anatomic” appreciation of the fracture pattern is possible, similar to what the surgeon would view at surgery. Note the “ghosting” technique used to remove the other bones, most notably the femoral head, that would otherwise obscure the view. For complex fractures such as this, advanced CT scanning is unparalleled.
A: Anteroposterior view of a right acetabular fracture-dislocation. Axial computed tomography (CT) (B) better reveals the extent of comminution of the posterior wall, as well as demonstrating the persistent posterior dislocation of the hip. C and D: With high-resolution 3D reconstructions, a more “anatomic” appreciation of the fracture pattern is possible, similar to what the surgeon would view at surgery. Note the “ghosting” technique used to remove the other bones, most notably the femoral head, that would otherwise obscure the view. For complex fractures such as this, advanced CT scanning is unparalleled.
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In the spine, helical CT has become the imaging modality of choice. The latest American College of Radiologists’ appropriateness criteria recommend axial MDCT with sagittal and coronal reconstructions as the primary imaging modality of choice for suspected spine trauma.41 In addition to its high diagnostic sensitivity, MDCT is more time effective, reducing imaging time by as much as 50% compared to radiography; time that may be critical to a trauma patient.42 A variety of measurements that incorporate CT data have been described that are useful in the assessment of the cervical spine following injury, including cervical translation and vertebral body height loss, canal compromise, spinal cord compression, and facet fracture and/or subluxation.19 Despite its greater initial expense, CT has been shown to have sensitivity and specificity of 96%, both greater than for conventional plain radiography.82 Grogan et al.82 present a decision analysis emphasizing cost minimization and conclude that helical CT is the preferred initial screening test for detecting cervical spine injury in moderate- to high-risk trauma patients. However, clinicians depending on MDCT of the spine for the imaging of trauma should be aware of potential diagnostic pitfalls, including accessory ossification centers, developmental disk abnormalities, vascular channels, and image artifacts.100 Finally, one recent study suggested that CT of the cervical spine in trauma is overutilized, and that strict adherence to NEXUS guidelines for imaging could reduce the need for CT of the neck by 20%.80 
In the upper extremity, CT is commonly performed to evaluate fractures of the scapula, proximal humerus, and distal radius.8,83,134,206 MPRs of these fractures assist in surgical planning. For proximal humeral fractures, simple axial images provide important information about the glenohumeral relationship, demonstrate glenoid rim fractures, and reveal whether the tuberosities of the humerus are fractured. Occult fractures of the coracoid process and lesser tuberosity are readily seen.83 Despite the valuable information that CT provides (with or without MPRs), several studies have shown that the interobserver assessment of proximal humeral and scapular neck fractures was not improved with the addition of CT.134 For distal radial fractures that mandate surgical reconstruction, CT is more accurate than conventional radiography in demonstrating involvement of the distal radioulnar joint, the extent of articular surface depression, and the amount of comminution.8,37,163,206 Three-dimensional CT was found to further improve the accuracy of fracture classification and to influence treatment decisions compared to standard 2D CT in a series of 30 intra-articular distal radius fractures.87 In another study, MDCT was compared to conventional radiography in a series of 120 distal radius fractures.8 In this study, MDCT was dramatically better at demonstrating central articular impaction than plain, and 26 radiographically occult injuries to the carpus were identified. Furthermore, the recommended treatment plan changed in 23% of the cases based on information provided by the CT evaluation.8 
CT is routinely used in evaluating pelvic fractures. CT with sagittal reconstruction is the best way to diagnose the so-called “U” fracture of the sacrum.160 A CT-based classification of acetabular fractures has been proposed.88 For the assessment of acetabular fractures, classification according to the system of Letournel is more accurate when CT is used compared to plain radiographs alone.148 CT is also better than conventional plain radiography at identifying intra-articular step-offs and gaps and is considered an essential part of the preoperative evaluation.20 Reformatted images can be obtained in oblique planes to simulate standard Judet radiographs (Fig. 17-4).75,148 Use of CT-reformatting avoids the pain and risk of fracture displacement or hip redislocation that might occur while repositioning the patient 45 degrees on each side for Judet views. A potential disadvantage is the slight loss of information resulting from volume averaging and computer reconstruction that could affect interpretation of the images. In one study, 5 orthopedic trauma surgeons with varying trauma experience compared 77 images from 11 different patients with acetabular fractures.22 The reviewers were asked to identify primary fracture lines and to classify each fracture according to the Judet-Letournel system; each patient had two sets of three images (one with traditional Judet radiographs and one with reformatted CT scans). When compared to the surgical findings, both sets of images performed equally well, and reviewers reported equivalent confidence in their ability to recognize fracture characteristics with each type of imaging.22 Postoperative CT after acetabular fracture repair identifies residual articular defects or incongruities better than plain radiographs.21 CT demonstrates intra-articular debris in a significant number of patients after hip dislocation,92 and CT should be performed in any patient whose conventional plain radiographs show an incongruent reduction. Because small intra-articular bodies may not be visible on radiographs, one should consider obtaining CT images in all patients who sustain a hip dislocation, even when conventional plain radiographs appear to be normal. 
The corresponding anteroposterior view is shown above. (Courtesy of Dr Rena Stewart.)
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Figure 17-4
Computed tomography of the pelvis reformatted in 45-degree right and left oblique planes (bottom) to simulate the traditional plain film Judet views of the pelvis.
The corresponding anteroposterior view is shown above. (Courtesy of Dr Rena Stewart.)
The corresponding anteroposterior view is shown above. (Courtesy of Dr Rena Stewart.)
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The impact of CT on tibial plateau fracture management is well described.33 In one study, when using just conventional radiographs for formulating a treatment plan, the mean interobserver kappa coefficient was 0.58, which increased to 0.71 after adding CT. The mean intraobserver kappa coefficient for fracture classification using radiographs was 0.70, which increased to 0.80 with addition of CT. The mean intraobserver kappa coefficient for treatment plan based on radiographs alone was 0.62, which increased to 0.82 after adding CT. With the addition of CT, the fracture classification was changed in 12% of cases, whereas the treatment plan was altered 26% of the time.33 In another study, Wicky et al.199 compared helical CT with 3D reconstructions to conventional radiography in patients with tibial plateau fractures and found that, for the purpose of classification, fractures were underestimated in 43% of cases by radiographs. Among a smaller subset of patients in whom operative plans were formulated with and without CT, the same investigators found that the addition of helical CT 3D reconstructions led to modifications in the surgical plan in more than half the cases.199 
Tornetta and Gorup190 evaluated the use of preoperative CT in the management of tibial pilon fractures. Twenty-two patients were studied with both conventional radiographs and CT. The fracture pattern, number of fragments, degree of comminution, presence of articular impaction, and location of the major fracture line were recorded. CT revealed more fragments in 12 patients, increased impaction in 6 patients, and more severe comminution in 11 patients. The operative plan was changed in 14 (64%) patients, and additional information was gained in 18 (82%) patients.190 
CT is valuable for assessing fractures of the hindfoot. CT reveals bone debris in the subtalar joint of patients with lateral process fractures of the talus.56 In children with Tillaux fractures of the anterolateral distal tibia, CT is better than conventional radiography in detecting displacement of more than 2 mm, which is considered an indication for surgery (Fig. 17-5).91 Helical CT is valuable for the preoperative planning of calcaneal fractures.68 Axial images of the calcaneus best show hindfoot deformity, whereas MPRs (including 3D imaging with dislocation of the joint) best reveal intra-articular involvement.68 
Figure 17-5
Computed tomography of a triplane fracture as viewed on a digital workstation.
 
Users can visualize axial and reconstructed coronal and sagittal images simultaneously.
Users can visualize axial and reconstructed coronal and sagittal images simultaneously.
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Figure 17-5
Computed tomography of a triplane fracture as viewed on a digital workstation.
Users can visualize axial and reconstructed coronal and sagittal images simultaneously.
Users can visualize axial and reconstructed coronal and sagittal images simultaneously.
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Postoperative Evaluation of Fracture Reduction.
CT is also useful for postoperative assessment of complex fractures. Moed et al.136 compared the functional outcome of 67 patients with posterior wall acetabular fractures with the findings on postoperative CT. In this study, postoperative CT more accurately revealed the degree of residual fracture displacement compared with conventional radiographs, and the accuracy of surgical reduction seen on postoperative CT was highly predictive of the clinical outcome.136 In a series of operatively treated tibial plateau fractures, clinically relevant information regarding articular depression or fracture healing that were not apparent on plain radiographs was found in 81% of cases imaged with MDCT.144 Vasarhelyi et al.191 found side-to-side torsional differences of greater than 10 degrees in one-quarter of 61 patients undergoing fixation of distal fibula fractures.191 Kurozumi et al.111 correlated postoperative radiographs and CT with functional outcomes in 67 patients with intra-articular calcaneal fractures and found that better reduction of the calcaneocuboid joint and posterior facet of the subtalar joint correlated with improved outcome. 
Healing of Fractures.
Conventional radiographs are often limited in demonstrating persistent fracture lines, while such nonunions are more readily demonstrated on CT (Fig. 17-6).14 CT has replaced conventional tomography in most centers for the identification of fracture nonunions. Multiplanar CT reconstructions may be needed if the fracture pattern is complex. Assessing partially united fractures can also be difficult, even with CT. The accuracy of CT in detecting tibial nonunion was evaluated. Bhattacharyya et al.14 studied 35 patients with suspected tibial nonunion and equivocal plain radiograph findings. In this series, the sensitivity of CT for detecting nonunion was 100%, but its accuracy was limited by a low specificity of 62%, because three patients who were diagnosed as having tibial nonunion by CT were found to have a healed fracture at surgery.14 
Figure 17-6
 
A: Anteroposterior radiograph of a patient who had persistent knee pain after surgical repair of a medial femoral condyle fracture. B: Computed tomography of the distal femur clearly with 2D reconstructions in the coronal and sagittal planes provides unambiguous evidence of fracture nonunion.
A: Anteroposterior radiograph of a patient who had persistent knee pain after surgical repair of a medial femoral condyle fracture. B: Computed tomography of the distal femur clearly with 2D reconstructions in the coronal and sagittal planes provides unambiguous evidence of fracture nonunion.
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Figure 17-6
A: Anteroposterior radiograph of a patient who had persistent knee pain after surgical repair of a medial femoral condyle fracture. B: Computed tomography of the distal femur clearly with 2D reconstructions in the coronal and sagittal planes provides unambiguous evidence of fracture nonunion.
A: Anteroposterior radiograph of a patient who had persistent knee pain after surgical repair of a medial femoral condyle fracture. B: Computed tomography of the distal femur clearly with 2D reconstructions in the coronal and sagittal planes provides unambiguous evidence of fracture nonunion.
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A more interesting role for CT is evaluation of early fracture healing. CT reveals external callus formation earlier than conventional radiography and allows for more complete and detailed visualization of fracture healing, which may be obscured by overlying casts and/or fixation hardware on radiographs.81 Lynch et al.126 developed a means of measuring changes in CT density at fracture sites by quantifying the formation of mineralized tissue within fracture gaps, while ignoring loss of bone mineral caused by disuse osteoporosis. In a preliminary study of seven patients with distal radial fractures, this technique demonstrated increased CT density 2 weeks postfracture that correlated with the visual appearance of sclerosis and blurring of the fracture line on conventional radiographs.126 It is not yet known whether such information will be predictive of fracture healing complications. The use of new multidetector 3D-CT techniques can reduce metal artifact to further improve the visualization of nonunion adjacent to metal hardware.65 
Evaluation of Combat Injuries.
Combat injuries are now frequently caused by multiple ballistic fragments causing multiple penetrating injuries. State-of-the-art MDCT is available at all US deployed combat hospitals.67 Use of 3D CT at the point of initial patient triage allows radiologists to identify retained fragments and depict wound paths, providing trauma surgeons with vitally important information regarding potential injury to vital organs and neurovascular structures. Such information assists in the initial stabilization of injured soldiers in the combat hospital without delaying emergent life-preserving intervention.67 In a simulated mass casualty incident, use of 64-MDCT increases throughput and facilitates imaging of patients more rapidly.107 

Magnetic Resonance Imaging

Technical Considerations

MRI does not use ionizing radiation. Rather, MRI uses radiofrequency (RF) waves, in the presence of a strong magnetic field, to interact with the patient’s hydrogen atoms (protons) to create images of superb soft tissue contrast. Although the physics of MRI is complex and too detailed to review in this section, the more practical aspects of MRI relevant to the evaluation of orthopedic imaging will be discussed. 
Present-day MRI scanners may be classified according to field strength. The basic unit of measurement of magnetic field strength is the Gauss (G); the earth’s magnetic field measures approximately 0.5 G. Field strengths for MRI are much greater and are measured in Tesla (T), which is defined as 10,000 G. Low-field-strength scanners are typically 0.2 to 0.3 T and are commonly used in outpatient settings as “extremity” or “open” scanners. High-field-strength scanners are generally those greater than 1 T, with 1.5-T scanners dominating the market and representing more than 90% of installed scanners worldwide. The 3-T scanners have also become clinically available, although their acceptance has been limited because of the higher cost of these systems and relatively limited selection of receiver coils. Advantages to higher-field-strength scanners include increased capability, increased resolution and image quality, and decreased scan time. 
The RF coils are an important element of any MRI system. RF coils are used to transmit RF waves into the patient, as well as receive RF signals (“echoes”) from the patient during the course of the examination. A standard “body” coil is incorporated into scanners as a default coil from which to both send and receive RF signals. The body coil is located within the housing of the magnet and, as a result, is located some distance from the patient. This distance factor decreases the strength of the RF signal received from the patient, although this is not a problem for imaging larger body parts such as the abdomen and pelvis. For smaller body parts, such as extremities in orthopedic imaging, specialized RF coils are available and are widely used to increase the quality of MRI studies. These coils are usually “receive only” coils, meaning the body coil transmits the RF pulse; some specialty coils; however, incorporate both transmit and receive functions. These smaller coils are placed around or over the body part to be scanned, which decreases the distance from the patient’s anatomy to the coil and results in greater signal return from the underlying tissue. This increases the signal-to-noise ratio (SNR) of the resulting images and produces images of greater contrast resolution and higher image quality, which may be used to improve image quality, increase spatial resolution, or decrease scan time. 
Advances in RF coil technology have led to a wide variety of RF coil designs available today. Volume coils encircle the anatomy of interest and provide increased signal homogeneity. Surface coils are placed over the anatomy of interest and significantly improve near-field signal strength returning from the underlying anatomy. Quadrature and phased-array coil designs incorporate multiple coil elements with electronic coupling to increase signal strength and SNR. Specialized coils are available for orthopedic imaging and include dedicated phased-array coils, as well as various sizes of flexible surface coils. 
MR images are generated using a series of pulse sequences. The term pulse sequence refers to sequence of RF pulses that are applied in concert with a series of magnetic gradients. These pulses are applied in a particular order and with a particular timing scheme, with the RF coils listening for the resulting “echoes” at specific time intervals. Pulse sequences determine the type of image contrast produced. During each pulse sequence, magnetic gradients are applied to the main magnetic field to achieve spatial localization. A magnetic gradient along the long axis of the bore of the magnet (and patient) is used for slice selection, whereas gradients along the transverse plane are responsible for frequency and phase encoding, which result in localization within the transverse plane. Most MRI examinations are particularly loud as a result of rapidly switching the gradients on and off, which necessitates use of earplugs or headphones during the test study. Inherent in all pulse sequences are specifications for parameters such as geometry (imaging plane, field of view, number of slices), resolution (number of frequency and phase encoding steps, slice thickness), and image contrast (repetition time [TR], echo delay time [TE]). A collection of multiple pulse sequences used for a particular examination is often referred to as a protocol. 
Common sequences used in orthopedic imaging include spin-echo (SE) and gradient-echo (gradient recalled echo [GRE]) imaging. SE sequences are most frequently used in conjunction with a fast imaging technique, termed fast spin-echo (FSE) or turbo spin-echo (TSE) imaging, depending on the manufacturer. SE sequences provide T1-weighted (T1W), proton density (PD), and T2-weighted (T2W) image contrast based on selection of the parameters TR and TE. T1W images tend to depict anatomy well and are sensitive, but not specific, for pathology. T2W images are fluid-sensitive images and tend to depict pathology well. PD images are neither T1W nor T2W, and contrast is derived from differences in PD within the tissues. PD images are commonly used in orthopedic imaging, as they result in high SNR images and depict anatomy and pathology well. PD images are often acquired in conjunction with T2W images during the same pulse sequence; in this case, the PD image is referred to as the first echo, and the T2W image is called the second echo. This combination may also be referred to as a double echo (DE, 2E) sequence. 
One consequence of FSE/TSE techniques is that fat, like fluid, is relatively bright on PD and T2W sequences. Fat suppression (FS) techniques are necessary to evaluate for edema or fluid with fat-containing tissues, such as bone marrow. Two techniques are commonly used: Short T1 inversion recovery (STIR) and chemical saturation (“fat-sat,” spectral saturation, frequency-selective presaturation). STIR is a distinctive spin echo pulse sequence that results in suppression of a particular tissue based on the choice of an additional parameter, TI. A relatively short TI value of 150 ms results in suppression of fat-containing tissues. This sequence tends to be relatively low in SNR and, as a consequence, is often performed at lower resolution. The sequence is less affected by variations in magnetic field homogeneity; however, and results in fairly uniform FS throughout the image. Chemical saturation is a frequency-selective RF pulse, which is applied before the normal RF pulse, and effectively eliminates the signal from fat-containing tissues. This may be applied to any of the SE sequences (T1W, PD, T2W); T1W FS sequences are typically used after contrast (gadolinium) enhancement, whereas PD FS and T2W FS sequences are used in evaluating a variety of tissues, including bone marrow and articular cartilage. Chemical saturation is often used in conjunction with lower-resolution FSE sequences, as the technique decreases SNR as a result of eliminating fat signal, resulting in “grainier” images at higher resolutions. Chemical saturation is also sensitive to inhomogeneities in the external magnetic field, which may result in nonuniform FS across the field of view. This is particularly a problem with extremities positioned off-center with the bore of the magnet, such as the elbow, where the magnetic field is not as uniform compared with isocenter. When uniformity of FS is a problem, STIR images may be substituted. STIR images are not sensitive to gadolinium and cannot be used to evaluate gadolinium-contrast enhancement, and hence are less useful for MR arthrography or intravenous contrast studies. 
Developing orthopedic imaging protocols is a challenging task that involves balancing tradeoffs in signal (SNR), spatial resolution, contrast resolution, and image acquisition time. Low SNR images tend to be “noisy” or “grainy” and unpleasant to view. Higher-resolution techniques result in both lower SNR and longer acquisition times and may not be practical for all patients; for this reason, lower-resolution techniques may be required. Many patients are unable to tolerate long scan times because of pain and limitations on movement during the examination, and, motion artifact may become a problem. MR artifacts (wrap around, motion artifact, pulsation artifact, metallic artifact) represent additional sources of image degradation and can be difficult at times to eliminate. Metal artifact is a particular problem in orthopedic imaging applications,85 but fortunately the amount of artifact can be reduced with certain MR imaging techniques on high-field (1.5 to 3 T) MRI. These include the use of TSE sequences with smaller interecho spacing, with the frequency-encoding direction oriented away from the site of interest and the readout bandwidth increased.63,85 When difficulties arise during an MRI examination, pulse sequences often need to be modified to obtain the information needed from the examination. 

Applications

MRI is frequently performed in evaluating both osseous and soft tissue injuries after trauma. It is capable of defining fractures that are radiographically occult, pediatric articular fractures, and associated soft tissue injuries that may not be suspected or evaluable after physical examination and plain radiography.34 Although MR angiography is a well-established technique for noninvasive evaluation of the arterial system, it may be impractical for evaluating the multitrauma patient. Evaluation of vascular trauma is accomplished much more rapidly with CT angiography (CTA) or conventional angiography, which also allows for interventional procedures (e.g., embolization of arterial bleeding). A more controversial application is MR venography (MRV) to detect deep venous thrombosis (DVT) of the proximal thigh and pelvic veins. In a recent review of the imaging of DVT, Orbell et al.149 note that MRV has many advantages, including lack of exposure to ionizing radiation and avoidance of any need for vein cannulation and injection of contrast (for nonenhanced studies). MRV is as sensitive and specific for proximal leg DVT as ultrasonography (US) or venography30 and is reported to be more accurate in the detection of isolated pelvic thrombi.138 Unfortunately, the cost and logistical problems of MRI have limited its usefulness in the imaging of DVT. 
MRI has been advocated to be the gold standard for imaging of the cervical spine following trauma,141 and faster imaging protocols certainly make the use of MRI much more feasible in the acutely injured patient.59 However, the practicality of using MRI in trauma patients may be limited by difficulties associated with transporting patients to the MRI suite, as well as MRI incompatibilities with various life-support equipment and patient implants. MRI scan times are also much longer than with CT and other imaging modalities and may not be tolerated by potentially unstable patients or those in considerable pain. Thus, for practical reasons, MRI continues to have only a limited role in the immediate management of the trauma patient. 
Osseous Injury.
Recent advances in MRI have made it possible to quantitatively assess bone structure and function, so that MRI may someday supplant bone densitometry as a tool to assess fracture risk caused by osteoporosis as well as the response to treatment.195 It is now well known that bone marrow edema (bone bruise, bone marrow contusion) is frequently identified on MRI after extremity trauma. Histologically, these imaging findings correlate with cancellous bone microfractures as well as edema and hemorrhage within the fatty marrow.166 The long-term sequelae of these radiographically occult lesions have not been well defined. Roemer and Bohndorf172 evaluated 176 consecutive patients with acute knee injuries and found that nearly three-fourths had bone marrow abnormalities. The majority of lesions (69%) involved the lateral compartment of the knee; 29% were medial, and 2% were patellofemoral. Many of the lesions resembled edema of the subchondral bone, without other osseous or cartilage injury, while nearly one-fourth represented subchondral impaction fractures and one-third comprised osteochondral or chondral lesions. Forty-nine of these patients had repeat MR studies conducted at least 2 years after their injury. In these patients, only 7 of 49 (14%) had persistent signal changes within the marrow space. The extent of signal abnormality was less than originally seen, and none of the patients developed degenerative changes, regardless of the injury type that was initially present. No cases of posttraumatic osteonecrosis were found. Therefore, one must be careful to avoid interpreting marrow signal abnormalities alone on MRI as evidence of a true fracture, as this may lead to overtreatment. This distinction is especially problematic in the assessment of hip pain after a fall, where trochanteric bone marrow edema might be interpreted as a fracture, leading to a decision to perform internal fixation. 
MRI is very useful in the evaluation of radiographically occult fractures. Fracture lines are distinctly visualized on PD or T2W images as linear, lower-signal intensity abnormalities silhouetted by higher-signal intensity marrow fat. Fracture lines can also be seen on STIR and PD/T2W FS images, which also show the degree of surrounding reactive marrow edema. Care is needed in interpreting T1W images; however, images as fracture lines may be obscured by surrounding marrow edema, both of which are hypointense in signal intensity on T1W images.77 
MRI has become the imaging modality of choice for identifying occult fractures for which correct early diagnosis is essential, such as femoral neck fractures (Fig. 17-7),118,124 scaphoid fractures,49,109,165,173 and pediatric elbow injuries.164 In elderly patients with hip pain after a fall, early MRI when radiographs are normal can avoid delays in diagnosis and treatment of hip fractures. In one study, 25 patients with hip pain were evaluated for occult fracture with conventional radiographs, scintigraphy, CT, or a combination of studies.159 A final diagnosis was ultimately determined from repeat radiographs in 10 patients and by scintigraphy in 15 patients. The time to final diagnosis averaged 9.6 days when the diagnosis was made by serial radiographs and averaged 5.3 days when the diagnosis was made by scintigraphy. Given the delay in diagnosis associated with using more conventional methods of imaging, the authors point out that use of immediate MRI instead can dramatically decrease the number of imaging examinations performed and the time to diagnosis, resulting in decreased costs of care and possibly reduced complications.159 In a more recent study, six elderly patients with hip pain after a fall had both MRI and CT, while seven others had MRI alone.124 In the first group, four of the six CT studies were inaccurate, while all MRI studies were correctly defined the pathology.124 In cases of occult hip fracture, the fracture pattern can be delineated using MRI, which may be of therapeutic importance. Occult fractures of the femoral neck that frequently are treated with screw fixation may be distinguished on MRI from occult intertrochanteric fractures, greater trochanter fractures, or pubic rami fractures that do not require surgical stabilization. Finally, if the MRI does not demonstrate fracture, it often does indicate another finding that explains a given patient’s symptoms.69 Clinicians may be more apt to rely on MRI alone than on NM studies; in one report, clinicians always requested additional imaging for cases in which the bone scan was positive.46 MRI may also identify additional comorbid conditions such as pre-existing osteonecrosis or metastatic disease.84 
Figure 17-7
A: Conventional plain anteroposterior radiograph of a patient’s hip demonstrates a femoral neck fracture.
 
Although the fracture can be seen on routine radiographs, the patient was at risk for osteonecrosis because of corticosteroid use related to a kidney transplant. Some apparent changes are seen in the bone density of the femoral head. Magnetic resonance imaging of the pelvis confirms the presence of an acute left hip fracture and demonstrates that there was no osteonecrosis. Incidentally noted is a small developing fracture with surrounding stress reaction in the right femoral neck medially: (B) STIR, (C) T1-weighted, and (D) T2-weighted images. Higher-resolution images of the left hip fracture demonstrating mild impaction at the fracture site without significant angulation. Axial proton density (E), axial fat-suppressed proton density (F), and coronal T2-weighted (G) images. Note inferior pole of kidney transplant in the lower left pelvis with surrounding complex fluid collection.
Although the fracture can be seen on routine radiographs, the patient was at risk for osteonecrosis because of corticosteroid use related to a kidney transplant. Some apparent changes are seen in the bone density of the femoral head. Magnetic resonance imaging of the pelvis confirms the presence of an acute left hip fracture and demonstrates that there was no osteonecrosis. Incidentally noted is a small developing fracture with surrounding stress reaction in the right femoral neck medially: (B) STIR, (C) T1-weighted, and (D) T2-weighted images. Higher-resolution images of the left hip fracture demonstrating mild impaction at the fracture site without significant angulation. Axial proton density (E), axial fat-suppressed proton density (F), and coronal T2-weighted (G) images. Note inferior pole of kidney transplant in the lower left pelvis with surrounding complex fluid collection.
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Although the fracture can be seen on routine radiographs, the patient was at risk for osteonecrosis because of corticosteroid use related to a kidney transplant. Some apparent changes are seen in the bone density of the femoral head. Magnetic resonance imaging of the pelvis confirms the presence of an acute left hip fracture and demonstrates that there was no osteonecrosis. Incidentally noted is a small developing fracture with surrounding stress reaction in the right femoral neck medially: (B) STIR, (C) T1-weighted, and (D) T2-weighted images. Higher-resolution images of the left hip fracture demonstrating mild impaction at the fracture site without significant angulation. Axial proton density (E), axial fat-suppressed proton density (F), and coronal T2-weighted (G) images. Note inferior pole of kidney transplant in the lower left pelvis with surrounding complex fluid collection.
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Figure 17-7
A: Conventional plain anteroposterior radiograph of a patient’s hip demonstrates a femoral neck fracture.
Although the fracture can be seen on routine radiographs, the patient was at risk for osteonecrosis because of corticosteroid use related to a kidney transplant. Some apparent changes are seen in the bone density of the femoral head. Magnetic resonance imaging of the pelvis confirms the presence of an acute left hip fracture and demonstrates that there was no osteonecrosis. Incidentally noted is a small developing fracture with surrounding stress reaction in the right femoral neck medially: (B) STIR, (C) T1-weighted, and (D) T2-weighted images. Higher-resolution images of the left hip fracture demonstrating mild impaction at the fracture site without significant angulation. Axial proton density (E), axial fat-suppressed proton density (F), and coronal T2-weighted (G) images. Note inferior pole of kidney transplant in the lower left pelvis with surrounding complex fluid collection.
Although the fracture can be seen on routine radiographs, the patient was at risk for osteonecrosis because of corticosteroid use related to a kidney transplant. Some apparent changes are seen in the bone density of the femoral head. Magnetic resonance imaging of the pelvis confirms the presence of an acute left hip fracture and demonstrates that there was no osteonecrosis. Incidentally noted is a small developing fracture with surrounding stress reaction in the right femoral neck medially: (B) STIR, (C) T1-weighted, and (D) T2-weighted images. Higher-resolution images of the left hip fracture demonstrating mild impaction at the fracture site without significant angulation. Axial proton density (E), axial fat-suppressed proton density (F), and coronal T2-weighted (G) images. Note inferior pole of kidney transplant in the lower left pelvis with surrounding complex fluid collection.
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Although the fracture can be seen on routine radiographs, the patient was at risk for osteonecrosis because of corticosteroid use related to a kidney transplant. Some apparent changes are seen in the bone density of the femoral head. Magnetic resonance imaging of the pelvis confirms the presence of an acute left hip fracture and demonstrates that there was no osteonecrosis. Incidentally noted is a small developing fracture with surrounding stress reaction in the right femoral neck medially: (B) STIR, (C) T1-weighted, and (D) T2-weighted images. Higher-resolution images of the left hip fracture demonstrating mild impaction at the fracture site without significant angulation. Axial proton density (E), axial fat-suppressed proton density (F), and coronal T2-weighted (G) images. Note inferior pole of kidney transplant in the lower left pelvis with surrounding complex fluid collection.
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MRI is similarly advantageous in the assessment of pediatric elbow injuries. In one series, seven of nine pediatric patients with an elbow effusion after injury were found to have a radiographically occult fracture.164 In the same series, MRI provided further useful diagnostic information in 16 other patients despite the presence of a visible fracture and/or dislocation of the elbow on plain radiographs.164 
Although CT with multiplanar reformatting remains the modality of choice for imaging complex fractures, recent studies indicate that MRI may be valuable in assessing such injuries as well. In one such study, the impact of MRI on the treatment of tibial plateau fractures was assessed.204 Patients presenting with tibial plateau fracture were assessed with conventional radiography, CT, and MRI. Three sets of images were prepared for each injury: Radiographs alone, radiographs with CT, and radiographs with MRI. Three surgeons were asked to determine the fracture classification and suggest a treatment plan based on each set of images. The investigators found that the best interobserver variability for both fracture classification and fracture management was seen with the combination of conventional radiographs and MRI. The Schatzker classification of tibial plateau fractures based on conventional radiographs changed an average of 6% with the addition of CT and 21% with the addition of MRI. MRI changed the treatment plan in 23% of cases. Holt et al.90 studied 21 consecutive patients with tibial plateau fractures who were evaluated with both conventional radiography and MRI before treatment. MRI was more accurate in determining fracture classification, in revealing occult fracture lines, and in measuring the displacement and depression of fragments. The MRI findings resulted in a change in the classification of 10 fractures (48%) and a change in the management of four patients (19%). MRI also allowed diagnosis of associated intra-articular and periarticular soft tissue injuries preoperatively. 
The role of CT is well recognized in the assessment of spinal trauma, but MRI is increasingly being used to evaluate for associated injuries such as herniated discs with cervical spine injuries and possible spinal cord injury associated with thoracolumbar spine fracture/dislocations. Green and Saifuddin79 have shown that 77% of patients with spine injury have a secondary injury level identified by whole spine MRI. Most commonly, these secondary injuries were bone marrow contusions, but 34% of patients had noncontiguous compression or burst fractures diagnosed by MRI. 
Soft Tissue Injury.
Because of its superb soft tissue contrast resolution and good spatial resolution, MRI provides an accurate means to assess soft tissue injury. MRI of the shoulder and knee is commonly ordered for evaluation of tendons, ligaments, and cartilage after trauma, frequently related to athletic injuries. Common indications for shoulder MRI following trauma include evaluation of the rotator cuff tendons for tearing, the superior glenoid labrum for superior labral anterior-posterior (SLAP) tears, and the anteroinferior labral-ligamentous complex after glenohumeral joint dislocation.12,39,188 Standard indications for knee MRI following trauma include evaluation of the cruciate and posterolateral corner ligaments for sprain or disruption, the menisci for tears, and the articular cartilage for osteochondral injury.53,70,198,200 Lonner et al.120 compared MRI findings to examination under anesthesia in 10 patients with acute knee dislocations who had later surgical intervention, at which time the pathology was defined. Although the investigators considered MRI to be useful for defining the presence of ligamentous injuries in knee dislocations, the clinical examination under anesthesia was more accurate in this series when correlated with findings at surgery.120 MRI has recently been shown to be useful at defining the nature of associated ligamentous injuries to the deltoid and tibiofibular syndesmosis in cases of distal fibular fracture, which may affect surgical decision making.34 
MR arthrography is a potentially valuable technique for assessing intra-articular derangement in many joints. Common indications include distinguishing partial- from full-thickness rotator cuff tears and evaluating labral-ligamentous pathology in the shoulder, evaluating the collateral ligaments in the elbow and intercarpal ligaments in the wrist, demonstrating labral tears in the hip, evaluating postoperative menisci in the knee, assessing stability of osteochondral lesions, and delineating intra-articular bodies.182 Direct MR arthrography is performed by intra-articular injection of a dilute gadolinium solution, resulting in distention of the joint capsule and improved delineation of intra-articular structures. Indirect MR arthrography is performed using intravenous injection of gadolinium, with a delay before scanning during which mild exercise may be performed. The indirect technique is based on recognition that the intravenous gadolinium diffuses from the highly vascular synovium into the joint space. The indirect technique does not produce controlled joint distention; however, and is best applied in smaller joints such as the elbow, wrist, ankle, and shoulder.13 
Orthopedic Hardware.
Orthopedic hardware presents a challenge in MRI because metal distorts the magnetic field and results in large areas of signal void, which frequently obscures adjacent anatomy.63 Modifications of traditional MR pulse sequences have been developed on high-field MR scanners to reduce artifact associated with orthopedic implants. FSE (turbo) sequences are used, which inherently decrease metallic artifact compared with routine SE and GRE sequences. Modifications to FSE sequences include increasing receiver readout bandwidths, decreasing interecho spacing and reducing effective echo times to maintain SNRs.63,179 These protocols are now commonly found in the sequence libraries of many newer MR scanners. Protocols based on modification of receiver bandwidth have been shown to reduce metallic artifact by an average of 60%, whereas additional experimental protocols (not commercially available) using a combination of several susceptibility artifact reduction techniques further reduce metallic artifact by an average of 79%.106 The degree of artifact is also dependent on the metallic composition of the orthopedic hardware, with titanium generally exhibiting the least amount of artifact. Applications for these sequences include evaluation of painful joint replacements, particularly knee and hip prostheses,161,179,180 and osteonecrosis of the femoral head after pinning femoral neck fractures (Fig. 17-8). 
Figure 17-8
Metal artifact reduction sequences.
 
A: A femoral neck fracture after pinning with four screws demonstrates nonunion. Magnetic resonance imaging using metal artifact reduction sequences shows no evidence of avascular necrosis of the femoral head. B: An additional case of nonunion of an intertrochanteric fracture that demonstrates avascular necrosis of the femoral head without subchondral fracture or collapse. The intramedullary rod and screw are titanium, which results in fewer artifacts than stainless steel or other alloys.
A: A femoral neck fracture after pinning with four screws demonstrates nonunion. Magnetic resonance imaging using metal artifact reduction sequences shows no evidence of avascular necrosis of the femoral head. B: An additional case of nonunion of an intertrochanteric fracture that demonstrates avascular necrosis of the femoral head without subchondral fracture or collapse. The intramedullary rod and screw are titanium, which results in fewer artifacts than stainless steel or other alloys.
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Figure 17-8
Metal artifact reduction sequences.
A: A femoral neck fracture after pinning with four screws demonstrates nonunion. Magnetic resonance imaging using metal artifact reduction sequences shows no evidence of avascular necrosis of the femoral head. B: An additional case of nonunion of an intertrochanteric fracture that demonstrates avascular necrosis of the femoral head without subchondral fracture or collapse. The intramedullary rod and screw are titanium, which results in fewer artifacts than stainless steel or other alloys.
A: A femoral neck fracture after pinning with four screws demonstrates nonunion. Magnetic resonance imaging using metal artifact reduction sequences shows no evidence of avascular necrosis of the femoral head. B: An additional case of nonunion of an intertrochanteric fracture that demonstrates avascular necrosis of the femoral head without subchondral fracture or collapse. The intramedullary rod and screw are titanium, which results in fewer artifacts than stainless steel or other alloys.
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Ferromagnetic material placed within a magnetic field may experience linear force, torque, and heating. In general, most contemporary orthopedic implants are not ferromagnetic and are MRI compatible in terms of heating and migration. Most fracture implants are made of 316L stainless steel, titanium, or titanium alloy; none of these materials contain delta ferrite, so they are not magnetic.47 MRI can be safely performed about plates, screws, and total joint implants, although artifacts may degrade the image as described earlier. In contrast, some external fixator components, especially clamps, contain strongly ferromagnetic materials and can be potentially unsafe in an MRI scanner.44,110 Davison et al.44 studied 10 sets of commercially available tibial external fixators that they applied to sawbone tibia. The external fixators were tested for magnetic attraction using a hand-held magnet while positioned 30 cm outside the entry portal of a 1.5-T scanner, at the level of the entry portal, and 30 cm inside the MRI tube. The EBI Dynafix with Ankle Clamp, EBI Dynafix, and EBI Dynafix Hybrid, along with the Hoffman II, Hoffman II Hybrid, Ilizarov with stainless steel rings, and Synthes Hybrid, all had more than 1 kg of magnetic attraction at all three locations, which is a significant enough force to cause potential movement of implant and pain. These devices were not scanned. Three devices—the Ilizarov fixator with carbon fiber rings, Richards Hex-Fix, and Large Synthes External Fixator—had less than 1 kg of magnetic attraction at all three locations and were scanned for 30 minutes while temperature measurements were obtained with a digital thermometer and thermocouple. No component of these three fixators experienced more than –16.67°C (2°F) of temperature elevation during a 30-minute MRI scan. Davison et al.44 conclude that many commercially available external fixators have components that have significant magnetic attraction to the MRI scanner. Fixators that have less than 1 kg of attraction do not experience significant heating during MRI. 
The American Society for Testing and Materials (ASTM) has established standards for MRI compatibility of implants.202 Many orthopedic manufacturers have redesigned their implants to make them MRI compatible. Luechinger et al.125 recently studied new MRI-compatible large external fixator clamps made by Synthes and found dramatic reductions in forces experienced in a 3-T field compared with older devices. All orthopedic surgeons should check with the manufacturer and be aware of the MRI compatibility of their particular external fixator inventory. 

Arthrography

Technical Considerations

Conventional Arthrography.
Arthrography involves distention of a joint capsule using positive or negative contrast agents. Water-soluble, iodinated contrast media is typically used to provide positive contrast, whereas air has been historically used to produce negative contrast. Double-contrast examinations may also be performed using both agents simultaneously, although these techniques are largely of historical interest, as advances in cross-sectional imaging have supplanted double-contrast arthrography techniques. 
Injection technique involves placement of a needle into the joint capsule, usually under fluoroscopic or CT guidance. Typically, a 22-gauge needle is used for larger joints, including the shoulder, hip, and knee, and a 25-gauge needle is used for smaller joints, such as the elbow, wrist, ankle, and smaller joints of the hands and feet. The anatomic approach varies according to each joint; for example, a lateral approach into the radiocapitellar joint space is frequently used for the elbow, and anterior approaches are typically used for the shoulder, hip, and tibiotalar joint. Table 17-2 lists technical considerations for arthrography of selected joints. After needle placement, small amounts of contrast are injected until the intra-articular location of the needle tip is confirmed. Contrast is then injected with subsequent distention of the joint capsule; the amount also varies by joint. 
 
Table 17-2
Arthrographic Techniques of Selected Joints
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Table 17-2
Arthrographic Techniques of Selected Joints
Joint Injection Approach Needle Size Volume of Contrast (mL)182
Shoulder Anterior glenohumeral joint space 22-gauge 3 V2-inch spinal needle 15
Elbow Lateral radiocapitellar joint space 25-gauge 1 V2-inch needle 10
Wrist Dorsal radioscaphoid joint space 25-gauge 1 V2-inch needle 4
Hip Anterior femoral head/neck junction 22-gauge 3 V2-inch needle 15
Knee Medial or lateral patellofemoral joint space 22-gauge 1 V2-inch needle 40
Ankle Anterior tibiotalar joint space 22-gauge 3 V2-inch spinal needle 10–12
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Frequently, the injection is performed under fluoroscopy, and sequential spot films are obtained before and during the injection to evaluate the flow of contrast. Pathology is inferred by abnormal communication of contrast with extracapsular structures. Passive and active range of motion are often required to demonstrate pathology, as abnormalities may only be shown after contrast is allowed to work its way through defects in the capsule and into the surrounding soft tissues. Contrast extravasation through capsular abnormalities can be fairly rapid and may occur during passive or active range of motion. Extravasation may also occur during periods when the fluoroscope is not energized. In addition, the fluoroscope only provides 2D views of bony anatomy, and it is extremely limited in its evaluation of surrounding soft tissues. Consequently, localizing the site of extravasation during conventional arthrography can be quite challenging. Care is also needed to avoid overdistention of the joint capsule, as extravasation through the capsule can occur, leading to subsequent decompression of intra-articular contrast and possible false-positive interpretations. 
Complications of arthrography are uncommon but may include bleeding and infection at the injection site, in addition to allergic reactions related to iodinated contrast media. A small number of patients experience postprocedural pain, possibly related to a mild synovial inflammatory response to the contrast media. Although patients are generally apprehensive about the procedure, they generally tolerate the procedure with less discomfort than expected.170 
Digital Subtraction Arthrography.
With the advent of digital imaging, digital subtraction techniques have been developed for fluoroscopy. Typically, a preliminary scout film serves as a “mask,” which is subsequently subtracted from images following contrast injection. This significantly improves contrast resolution of the fluoroscopic spot films and enables visualization of contrast that would otherwise not be apparent when adjacent to similar high-density objects, such as joint prostheses. Digital subtraction arthrography (DSA) also allows sequential injection and evaluation of adjacent joint compartments, as a new mask is obtained after injection of the first compartment, which is subsequently subtracted from images acquired during injection of the second compartment. DSA techniques are sensitive to patient motion; however, which produces misregistration artifact as a result of misalignment of the mask and subsequent images. DSA also requires specialized equipment, which may not be available outside of radiology departments. 
CT and MR Arthrography.
Cross-sectional techniques, such as CT and MRI, have largely replaced conventional arthrography for evaluating internal derangement, but these imaging modalities may be combined with arthrography using appropriate contrast agents for each modality.61,182 For CTA, an arthrogram is first obtained using a contrast solution containing saline and water-soluble, iodinated contrast media, typically in a 1:1 dilution. Thin-section CT is then performed through the joint, and images in orthogonal planes are reconstructed. For MR arthrography, a very dilute gadolinium solution (typically 1:200 dilution) is injected into the joint, and MRI is subsequently performed. In addition to routine sequences, fat-suppressed T1W images are used to visualize the injected contrast. With both imaging modalities, evaluation is aided not only by silhouetting intra-articular structures by relatively bright contrast but also by distention of the joint capsule. This results in separation of intra-articular ligaments and capsular structures and allows more precise evaluation of complex anatomy (Fig. 17-9). Bony and soft tissue abnormalities are directly visualized with these cross-sectional techniques, compared to conventional arthrography, whereby pathology is inferred based on the appearance of the contrast collection in relation to the bony landmarks. 
Figure 17-9
 
A: Lateral radiograph of the proximal femur after fixation of a femoral neck fracture, showing malunion with retroversion of the femoral neck. B: The patient had persistent hip pain, and magnetic resonance arthrography revealed a tear of the anterior acetabular labrum. Note angular deformity at the site of fracture malunion and residual micrometallic artifact related to insertion of prior screws. (Reprinted with permission from: Eijer H, Myers SR, Ganz R. Anterior femoroacetabular impingement after femoral neck fractures. J Orthop Trauma. 2001;15:475–481.)
A: Lateral radiograph of the proximal femur after fixation of a femoral neck fracture, showing malunion with retroversion of the femoral neck. B: The patient had persistent hip pain, and magnetic resonance arthrography revealed a tear of the anterior acetabular labrum. Note angular deformity at the site of fracture malunion and residual micrometallic artifact related to insertion of prior screws. (Reprinted with permission from: Eijer H, Myers SR, Ganz R. Anterior femoroacetabular impingement after femoral neck fractures. J Orthop Trauma. 2001;15:475–481.)
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Figure 17-9
A: Lateral radiograph of the proximal femur after fixation of a femoral neck fracture, showing malunion with retroversion of the femoral neck. B: The patient had persistent hip pain, and magnetic resonance arthrography revealed a tear of the anterior acetabular labrum. Note angular deformity at the site of fracture malunion and residual micrometallic artifact related to insertion of prior screws. (Reprinted with permission from: Eijer H, Myers SR, Ganz R. Anterior femoroacetabular impingement after femoral neck fractures. J Orthop Trauma. 2001;15:475–481.)
A: Lateral radiograph of the proximal femur after fixation of a femoral neck fracture, showing malunion with retroversion of the femoral neck. B: The patient had persistent hip pain, and magnetic resonance arthrography revealed a tear of the anterior acetabular labrum. Note angular deformity at the site of fracture malunion and residual micrometallic artifact related to insertion of prior screws. (Reprinted with permission from: Eijer H, Myers SR, Ganz R. Anterior femoroacetabular impingement after femoral neck fractures. J Orthop Trauma. 2001;15:475–481.)
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Applications

Before advanced cross-sectional imaging techniques, arthrography was traditionally used for assessing periarticular soft tissue injuries associated with trauma. Today, there are more limited indications for arthrography, although it is frequently performed in combination with CT and MRI to increase the sensitivity and specificity for internal derangement. 
Arthrography may be substituted in patients with contraindications to MRI, such as pacemakers or intracranial aneurysm clips. CTA is preferred; however, as advances in CT scanner technology have led to marked improvements in resolution and scan time, resulting in high spatial resolution images and MPRs of intra-articular structures. 
Upper Extremity.
In the upper extremity, shoulder arthrography may be performed to evaluate for full-thickness rotator cuff tears. Extravasation of contrast into the subacromial/subdeltoid bursa is diagnostic of a full-thickness tear. Even with careful fluoroscopic observation during the injection process, it is frequently impossible to delineate the site or extent of the tear, as contrast medium may accumulate in the bursa without visualization of an obvious tract through the torn tendon. Occasionally, no extravasation is seen after completing the injection; however, after passively and/or actively exercising the shoulder, subsequent fluoroscopy reveals contrast flooding the bursa as a result of the medium working itself through a full-thickness tear. Special care is needed in interpreting arthrography of the postoperative rotator cuff, because intact cuff repairs may continue to leak contrast into the bursa. 
The value of three-compartment arthrography has been documented in the setting of acute wrist trauma,78 as has the value of digital subtraction techniques in wrist arthrography.45,205 Arthrography has historically been applied to the evaluation of ulnar collateral ligament injuries of the thumb (“gamekeeper’s thumb”). Recent literature has shown MR arthrography to be more accurate in detecting ulnar collateral ligament injuries and in evaluating displacement of the torn ligament1 as well for evaluation of triangular fibrocartilage complex injury.176 
Lower Extremity.
In the lower extremity, arthrography alone is rarely performed for trauma but may be combined with CT or MRI for evaluating osteochondral abnormalities (Fig. 17-9).121 A recent study comparing CTA with MR arthrography suggests that CTA may be more accurate in evaluating cartilage lesions of the ankle joint.174 
Arthrography may also be useful in the evaluation of pain after treating calcaneal fractures with intra-articular extension. Matsui et al.131 performed posterior subtalar joint arthrography at a mean of 6 months postinjury in 22 patients; 15 had undergone surgical repair and 7 had been treated nonoperatively. The patients were separated into four groups based on arthrographic findings: Normal, narrow, irregular, and ankylosis. Clinical follow-up performed at a mean of 23 months postinjury correlated very well with the earlier arthrographic findings, suggesting that subtalar arthrosis is responsible for much of the symptoms that develop after calcaneal fracture. 
Pediatric Injuries.
Arthrography is valuable in assessing pediatric physeal injuries (especially the elbow)2,15,51,113,129 that are not visible on conventional radiographs. It is also used intraoperatively to assist with the reduction of pediatric radial head fractures.94 The use of arthrography to assess pediatric injuries has been largely supplanted by MRI (when available), although in the pediatric population both procedures may require sedation. 
Dynamic Imaging.
Arthrography remains the investigation of choice when dynamic imaging is necessary. Using arthroscopy as the diagnostic standard, Kim et al.102 compared dynamic arthrography with MRI arthrography for the diagnosis of wrist pain in 38 patients, finding both modalities had similar sensitivity and specificity for the diagnosis of scapholunate ligament, lunotriquetral ligament, and triangular fibrocartilage complex tears. 

Ultrasonography

Technical Considerations

Conventional Ultrasonography.
US refers to the spectrum of sound waves with frequencies greater than 20 kHz (20,000 Hz), which are beyond the audible range of the human ear. Typical frequencies used in medical diagnostic US range from 2 to 12 MHz, although frequencies of 20 MHz and higher are in clinical use for more specialized applications involving very small regions of anatomy. Lower frequencies within this range (2 to 5 MHz) allow deeper penetration of the US beam for evaluation of thicker body parts, although at lower spatial resolutions. Higher-frequency US beams (10 to 12 MHz) provide greater spatial resolution and are frequently used in evaluating superficial anatomy, such as tendons. 
US beams are generated by transducers, which make use of piezoelectric materials to convert electrical energy into mechanical energy (sound waves). Today’s transducer designs are complex and may incorporate hundreds of individual piezoelectric elements, each of which is energized in turn or in combination, such that the individual sound waves combine into a US beam. The US beam propagates into the underlying tissues and is partially reflected back at tissue boundaries because of differences in acoustic impedance between tissues. Acoustic impedance is defined as the product of tissue density and the speed of sound, both of which vary among tissues. Small differences in acoustic impedance will produce smaller reflections of sound waves, whereas large differences in acoustic impedances will result in larger reflections. The reflected echoes travel back to the transducer, where the transducer elements convert the sound waves into electrical signals, which are then used to create the US image. 
A US image is composed of an array of pixels, each corresponding to a tissue element at a particular depth and location. Echoes returning from underlying soft tissue elements are generated from reflections of the US beam at tissue interfaces of different acoustic impedance. In addition, smaller cellular elements within tissues can also act as individual “scatterers.” Each of these scatterers reflects a small portion of the US beam in all directions. A portion of these scattered echoes are reflected back toward the transducer and are displayed in the image as background “echogenicity” that is characteristic for that tissue. Echoes returning from superficial soft tissues have shorter round trip distances to travel back to the transducer and are detected earlier than those for deeper soft tissues. For this reason, the depth of a tissue element can be calculated using the return time of its corresponding echo. The amplitude of the returning echo determines the brightness, or echogenicity, of a tissue element. As the US beam travels deeper within the soft tissues, it progressively loses energy and subsequent echoes from deeper tissue elements are smaller in amplitude. A correction factor, termed time gain compensation, is applied to deeper soft tissues to account for this drop off in echo amplitude. Thus, differences in echogenicity in the resulting US image will be less dependent on tissue depth and more related to differences in acoustic impedance and scattering. 
During a single cycle, the transducer sends a short burst of US waves (the US pulse) into the underlying tissues, and then listens for the returning echoes. The time spent sending out the US pulse is a tiny fraction of the listening time, typically about 0.5% of the total cycle time. The pulse repetition frequency determines how many pulses are sent into the underlying tissue over time, and typically ranges between 2000 and 4000 cycles/s (2 to 4 kHz). During routine scanning, the transducer is constantly steering and refocusing the US beam within the underlying tissue to generate echoes that will correspond to each pixel in the US image. The 2D US image that is generated is typically referred to as B-mode (“brightness” mode) imaging. The designs of current transducers are quite complex and rely on advanced electronics and scanning algorithms, but have resulted in greater spatial resolution and more advanced feature sets, including 3D, four-dimensional, and Doppler imaging. 
Echogenicity is a term used to describe the relative brightness of echoes returning from tissues or tissue interfaces. Tissues may be described as hypoechoic or hyperechoic with regard to a reference tissue, in addition to isoechoic if two distinct soft tissues share the same level of echogenicity. The descriptor anechoic refers to a tissue or medium that produces no reflected echoes and is black on the corresponding US image. Water is the best example of an anechoic medium, because all of the sound waves are transmitted through the medium without any reflections. In such situations, the energy within the US beam will be greater as it reaches the tissues on the far side of the medium and the distal tissues will appear brighter; this is referred to as increased through transmission. Conversely, any tissue or medium that blocks transmission of all sound waves will appear highly echogenic at its proximal interface with the US beam and will exhibit “distal acoustic shadowing,” whereby the more distal tissues appear black, resembling a shadow. Cortical bone and air are examples where the large differences in acoustic impedance result in marked attenuation of the US beam, producing distal acoustic shadowing. 
US examinations are highly operator dependent, and the quality of the examination can be influenced by the sonographer’s training, experience performing certain examinations, and understanding of normal anatomy and disease states. US is a real-time examination, and although images that represent the underlying anatomy are saved, these 2D images cannot provide the depth of understanding that real-time visualization provides. For this reason, it may be necessary for the interpreting physician to be present or to image the patient to interpret complex examinations. 
Doppler US.
Doppler US is used to evaluate moving tissues, such as blood flow within vessels. Velocity measurements and directions of flow may be ascertained on the basis of frequency shifts of the returning echoes. When the US beam is reflected from a tissue moving toward the transducer, the returning echoes undergo a slight increase in frequency. Similarly, when interacting with a tissue moving away from the transducer, the US beam will be reflected such that the returning echoes will incur a slight decrease in their frequency. These frequency shifts are used to calculate the speed of the moving tissue, whereas the direction of frequency shift (positive vs. negative) is used to determine the direction of motion relative to the transducer. 
Various modes of Doppler operation are available on today’s scanners and are frequently used for vascular evaluation. Duplex Doppler imaging combines 2D B-mode imaging with pulsed Doppler imaging; the 2D B-mode image provides an anatomical map to identify vessels for subsequent Doppler interrogation. Color Doppler combines B-mode grayscale imaging with color flow superimposed over vessels, as determined by Doppler imaging. Shades of red and blue are assigned to the vessels based on their velocities and directions and represent flow toward and away from the transducer, respectively. Power Doppler imaging is a signal processing algorithm that uses the total amplitude of the Doppler signal to generate maps of flow, which are then superimposed on B-mode grayscale images. The corresponding images demonstrate greater sensitivity to slow flow, although no directional information is available. 

Applications

US is a simple, noninvasive, relatively inexpensive imaging modality that is now widely available in most hospitals and in many clinics. Diagnostic US has an established role in the immediate diagnosis of trauma patients according to the ATLS protocol, where it is used in the “Focused Abdominal Sonography for Trauma” (FAST) examination for intra-abdominal injury. US also has applications in evaluation of fractures, fracture healing, soft tissue trauma including ligamentous injury, and venous thromboembolism. 
Fractures.
US has potential in the assessment of fractures, and may be underused in this regard.153 US compares favorably to conventional radiography in the assessment of occult scaphoid fracture in patients with wrist pain.89 Durston and Swartzentruber55 used US to assess the reduction of pediatric forearm fractures in the emergency department, thereby avoiding multiple trips to the radiology suite while gaining much more rapid assessment of the quality of fracture reduction. Assessing pediatric elbow injuries is notoriously difficult because of the complex joint anatomy and the multiplicity of its ossification centers, many of which are relatively unossified in childhood. US has proved to be valuable in evaluating lateral condylar fractures, in that it is able to assess the extent of the fracture line through the unossified capitellum and trochlea, to distinguish unstable intra-articular fractures from their stable extra-articular counter parts.193 
US may also be clinically useful in evaluating fractures in settings where conventional radiography may not be readily available, such as in military or aerospace settings.104 Dulchavsky et al.54 prospectively evaluated 158 injured extremities by US. Nonphysician cast technicians, who had received limited training and were blinded to the patient’s radiographic diagnoses, performed the US evaluations. Examinations only required an average of 4 minutes and accurately diagnosed injury in 94% of patients with no false-positive results. Injuries that were diagnosed by US included fractures in the upper arm, forearm, femur, tibia/fibula, hand, and foot. 
Fracture Healing.
US is a useful method to monitor fracture healing. Moed et al.137 performed sonographic evaluation of patients 6 and 12 weeks after unreamed tibial nailing and found that persistent nail visualization indicated poor callus formation and predicted later healing complications. Color Doppler sonography has been shown to demonstrate progressive vascularization of fracture callus and predict delayed callus formation in another study of patients with tibial fractures.32 
Soft Tissue Trauma.
US is also well suited for diagnosing musculoskeletal soft tissue injuries and is of proven value in the assessment of many tendon injuries, such as those of the tendo Achillis, rotator cuff, and ankle.17,29,36,171 US has been used to assess muscle injury, depicted as a tear or hematoma and subsequent complications such as fibrosis, cystic lesions, or heterotopic ossification.154 US is valuable in localizing foreign bodies within soft tissues; an advantage over conventional radiography is that foreign objects do not need to be radiopaque to visualize them.116 
Venous Thromboembolism.
US has come to play a very important role in managing venous thromboembolism in trauma patients.207 All trauma patients are at risk for developing DVT, and venous US has become the most widely used imaging modality for DVT diagnosis. Venous scanning performed by skilled operators is the most practical and cost-effective method for assessing DVT of the proximal and distal lower-extremity veins. Several US modalities are used to evaluate DVT, including B-mode for real-time visualization of compression of larger veins. 
Duplex Doppler for evaluating waveforms and velocities and color Doppler for depicting patency of veins are particularly useful in the calf and iliac veins.207 The diagnostic accuracy of US is well documented, and the sensitivity and specificity of venous US (including all types) for the diagnosis of symptomatic proximal DVT is 97% and 94%, respectively.207 The high specificity of venous US is sufficient to initiate treatment of DVT without further confirmation, and the high sensitivity for proximal DVT makes it possible to withhold treatment if the examination is negative.207 When US examinations cannot be performed (e.g., uncooperative patient, presence of bandages, casts), an alternative diagnostic procedure, such as contrast venography, may be needed. More advanced imaging modalities, such as CT or MRV are also available. US is less accurate in the diagnosis of proximal DVT involving the pelvis; MRV has been suggested as a more accurate modality for detecting intrapelvic DVT.138 

Nuclear Medicine Imaging

Technical Considerations

Nuclear scintigraphy involves intravenous injection of a radio-pharmaceutical with subsequent imaging using a gamma scintillation camera. The radiopharmaceutical is typically composed of two moieties: A radionuclide and a pharmaceutical compound. The pharmaceutical is responsible for localization of the molecule in the body, and the radionuclide allows imaging of the pharmaceutical distribution. 
Radionuclides are radioactive isotopes that undergo spontaneous decay, which results in the emission of photons. Photons that are generated in the nucleus of the atom are gamma rays, whereas photons generated by electron transitions within their orbital shells are x-rays. Either may be used for imaging, although the particular choice of a radionuclide predetermines the types and energies of photons that are emitted. In many NM imaging applications, technetium (99mTc) is commonly used as the radionuclide because of its favorable imaging properties (140-keV gamma energy), clinically suitable half-life (6 hours), availability (99Mo/99mTc generator) and ease in labeling of pharmaceuticals. Other radionuclides used in orthopedic imaging include gallium (67Ga) and indium (111In) and are discussed later in this section. 
Pharmaceuticals are metabolically active molecules that are designed to localize to target tissues once injected intravenously. There are many different mechanisms of localization, but for orthopedic imaging, regional blood flow is important for all administered radiopharmaceuticals. Specific radiopharmaceuticals for orthopedic imaging and their method of localization are discussed later in this section. 
Gamma scintillation cameras are specialized detectors that capture photons within a large flat crystal, commonly made of sodium iodide activated with thallium. Photons interact with the scintillation crystal and are converted to visible light, which is then captured by photomultiplier tubes (PMTs) coupled to the crystal. The PMT converts the light photon into an electrical signal, which is subsequently amplified and electronically processed. This process results in a single “count” in the final NM image corresponding to a single radioactive decay in the patient. 
NM images are formed by placing the gamma scintillation camera over the anatomy of interest and accumulating counts for a specific amount of time or for a minimum number of counts, typically on the order of hundreds of thousands of counts. Imaging is often performed after a delay to allow localization and/or uptake of the radiopharmaceutical within the target tissues. Delayed imaging demonstrates characteristic patterns of distribution throughout the body for a particular radiopharmaceutical, in addition to abnormal accumulation or absence of activity corresponding to disease states. Consequently, nuclear imaging studies are based on visualization of metabolic function, rather than anatomy. Anatomic features are frequently visualized on NM images, although spatial resolution is typically quite poor compared with other imaging modalities (Table 17-1). 
During routine acquisition of NM images, the gamma scintillation camera is left stationary in a single projection, resulting in a planar image. Single-photon emission computed tomography (SPECT) is an extension of planar imaging, whereby the gamma camera rotates around the patient, stopping at predefined intervals, to acquire multiple static planar images. Using techniques similar to those in CT, these planar data sets are then processed by computers. Images are typically created in orthogonal tomographic planes (axial, coronal, sagittal), in addition to 3D volumes. Although the main advantage of SPECT over planar images is the improved image contrast resolution as a result of eliminating radioactivity from overlapping anatomy, spatial resolution is similar or slightly decreased compared to planar imaging (Table 17-1). 
Indwelling orthopedic hardware may affect image quality by introducing artifacts into the diagnostic image. Hardware can shield the gamma camera from photons arising behind the hardware, resulting in a photopenic defect. Knowledge of indwelling hardware and their characteristic photopenic appearances alleviates misinterpretation of these defects. Multiple projections are also frequently performed during a single examination, which allows evaluation of the activity on multiple sides of the hardware. 
NM techniques relevant to trauma and orthopedics are described in the sections to follow. 
Skeletal Scintigraphy.
Skeletal scintigraphy, commonly referred to as a bone scan, is the most commonly performed NM study with respect to the skeletal system. The radiopharmaceutical used is typically a 99mTc-labeled diphosphonate, which localizes to bone based on chemiadsorption of the phosphorus compound to the mineral phase of bone, particularly at sites of increased osteoblastic activity. Regional blood flow is also important for tracer distribution, as areas of increased regional blood flow deliver greater tracer to the adjacent skeleton, and result in greater uptake. The term bone scan typically refers to images obtained after a 2- to 4-hour delay, to allow localization of the diphosphonate compound. Three-phase bone scans incorporate additional dynamic and immediate imaging phases. A radionuclide angiogram (first phase) is obtained during transit of radiopharmaceutical through the arterial system. Immediate static images are then obtained for an additional 5 minutes (second phase) and represent “blood pool” or “tissue phase” images. Both of these earlier imaging phases are used to evaluate for regional hyperemia, as evidenced by both increased blood flow and increased surrounding soft tissue uptake. 
Normal bone scan images show a characteristic appearance of the skeleton, with slightly greater uptake in the axial skeleton (spine, pelvis) than the extremities. In skeletally immature individuals, there is normal avid uptake in the growth plates, resulting in symmetrically increased bands of activity occurring adjacent to joints and apophyses. Many diseases are characterized by both increased osteoblastic and osteoclastic activity within the bone, in addition to regional hyperemia, and result in greater tracer uptake (“hot” lesions) than normal bone. These abnormalities may be solitary or multiple, and focal or diffuse in nature. Some pathologic processes, particularly permeative processes (small round cell tumors) or those that elicit little surrounding bone reaction, result in regions of decreased tracer uptake, or “cold” lesions. These lesions maybe difficult to detect on routine bone scans. Bone scans are highly sensitive for disease processes, although specificity is poor. A normal bone scan may rule out underlying skeletal abnormality, but a positive bone scan necessitates further workup of the underlying abnormality. 
Marrow Imaging.
Marrow imaging is performed using 99mTc-labeled sulfur colloid. The sulfur colloid is composed of particles measuring between 0.1 and 2 micron, which are taken up by the reticuloendothelial cells within the liver (85%), spleen (10%), and bone marrow (5%). Uptake is rapid (half-life is 2 to 3 minutes), and imaging is performed after a 20-minute delay. Current indications for marrow imaging are limited but include evaluation of osteomyelitis in conjunction with 111In-labeled white blood cell (WBC) imaging. 
Gallium Imaging.
Gallium-67 citrate is a radiopharmaceutical that was originally developed as a bone-imaging agent but was later found to be useful in imaging infection and inflammation. After intravenous injection, gallium binds to transferrin and circulates in the bloodstream. At sites of inflammation or infection, increased regional blood flow and increased vascular permeability result in greater accumulation of gallium. In addition, neutrophils release large amounts of lactoferrin as a part of their inflammatory response; gallium has a higher binding affinity for lactoferrin than transferrin and localizes at the site of inflammation. Gallium is a relatively poor imaging agent, as its photons are not optimum for imaging with present-day gamma cameras, and total body clearance is slow with considerable background activity. Imaging is typically performed at 48 hours, which contributes to delays in diagnosis. 
Gallium scans are often interpreted with bone scans for evaluation of osteomyelitis. Gallium activity that is greater than, or in different distribution than, corresponding activity on the bone scan is diagnostic for osteomyelitis. 
White Blood Cell Imaging.
There are several approaches for using labeled WBCs for diagnosing infection and/or inflammatory processes. Of these, 111In oxine– and 99mTc-labeled hexamethylpropyleneamine oxime (HMPAO)–labeled WBCs are discussed briefly. 
Indium-111 is complexed with oxine, which results in a lipid-soluble complex that readily crosses the cell membranes. Approximately 50 mL of blood must be withdrawn and the leukocytes need to be separated from the plasma and red cells. Labeling is accomplished by incubating the leukocytes with the 111In oxine complex for 30 minutes. The leukocytes are then resuspended in plasma and reinjected into the patient within a total of 2 to 4 hours. Imaging is typically performed at 24 hours to allow for leukocyte localization and clearance from the blood pool. 
99mTc HMPAO is a cerebral perfusion agent that also crosses cell membranes and may be used to label WBCs, preferentially granulocytes. Approximately 50 to 75 mL of blood is withdrawn and incubated with the radiopharmaceutical; however, the labeling process is performed in plasma, and cell separation is not needed. The labeled cells are then reinjected, and imaging is performed at 4 hours for the peripheral skeleton. 
Labeled WBC studies should be interpreted in combination with sulfur colloid marrow studies for evaluation of osteomyelitis and infected joint replacements. When used alone, labeled white cell studies may result in false-positive results, because labeled WBCs normally distribute to the bone marrow, in addition to the liver and spleen, after reinjection. The sulfur colloid marrow study is used to map out areas of normal residual marrow activity. Congruent activity is seen within the bone marrow on both examinations. Osteomyelitis results in replacement of marrow activity on the sulfur colloid study, resulting in a photopenic defect, whereas there is significantly increased activity on the corresponding labeled WBC study. 

Applications

NM imaging is frequently used for further evaluation when conventional radiographs are normal or to evaluate the significance of abnormalities seen on radiographs. Although typically highly sensitive for disease processes, its poor specificity makes it necessary to correlate the findings with additional clinical history, laboratory evaluation, or imaging examinations. Applications of NM to orthopedic trauma include evaluation of fractures, osteomyelitis, and osteonecrosis. 
Fractures.
Bone scans are highly sensitive for acute fractures. Matin130 demonstrated positive scans in 80% of fractures at 24 hours, and in 95% by 72 hours. Advanced age and debilitation contributed to nonvisualization of fractures beyond this time frame. The minimum time to return to normal was 5 months, and 90% of fractures returned to normal by 2 years. Because of its poor specificity, scintigraphy can lead to false-positive diagnoses of fracture. Garcia-Morales et al.73 reported five cases of false-positive scans for hip fracture because of collar osteophytes; subsequent MRI in these patients was negative. 
Radiographically negative stress fractures and insufficiency fractures are also well delineated on bone scintigraphy as focal areas of increased radiotracer uptake. Characteristic sites of stress fractures depend on the activity that produced them, although there is considerable overlap. Some fracture patterns show characteristic appearances on scintigraphy. For example, in elderly patients with chronic low back or hip pain, sacral insufficiency fractures reveal a classic “H” pattern of uptake, known as the “Honda” sign.71,155 Not uncommonly, several focal areas of increased tracer uptake are seen in the skeleton, which presumably represent a combination of acute and more chronic findings. In these cases, three-phase scintigraphy can provide additional information regarding hyperemia and may help to differentiate acute from chronic injuries. Typically, hyperemia resolves within 4 to 8 weeks after initial injury, with the blood flow, then the blood pool, images normalizing. 
Scintigraphy may be useful in the early identification of fracture healing complications. Barros et al.10 performed scintigraphy at 6, 12, and 24 weeks with 25 mCi of MDP-99mTc in 40 patients with tibial shaft fractures that were treated nonsurgically. Using the normal leg as a control, an activity index (the ratio of the uptake counts of the injured leg to the normal leg) was calculated. All fractures in this series healed within 20 weeks and the activity ratio index progressively decreased at the three evaluations.10 The investigators speculate that a persistently increased activity index would indicate future development of healing complications, such as delayed union or nonunion, although they did not have any such healing complications in their series.10 
Bone scintigraphy may also be used in evaluating a child with nonaccidental trauma. In a study from Australia, studies of 30 children who were the victims of suspected child abuse and who had both skeletal surveys and bone scintigraphy were retrospectively reviewed.128 Excluding rib fractures, there were 64 bony injuries, of which 33% were seen on both imaging modalities, 44% were seen on skeletal survey only, and 25% of the injuries were seen on bone scans alone. Metaphyseal lesions typical of child abuse were found in 20 cases (31%) on skeletal survey; only 35% of these were identified on bone scan. The investigators believed that both skeletal survey and bone scintigraphy should be performed in cases of suspected child abuse. 
Infection.
Osteomyelitis may result from hematogenous spread of microorganisms to bone, from direct extension from areas of adjacent soft tissue infection, or as a result of open fractures and/or surgery. Persistent pain or delayed healing after surgery can be difficult to evaluate with regard to infection, as conventional radiographs may show only more advanced destructive changes and MRI may be very difficult to interpret in light of recent surgery. 
Radionuclide imaging has evolved over time with respect to imaging orthopedic infections. In addition to three-phase bone scans, dual gallium/bone scintigraphy and labeled WBC studies, including combination leukocyte/bone and leukocyte/marrow studies, are valuable in diagnosing both acute and chronic osteomyelitis as well as infected joint replacements. However, no one study is equally applicable to all clinical situations.151 
Although three-phase bone scans have excellent accuracy for detecting osteomyelitis in normal underlying bone, the specificity of this test is markedly reduced in the presence of underlying bone disease. 
Dual gallium (67Ga)/bone scintigraphy has been used to evaluate osteomyelitis. Gallium scintigraphy demonstrates greater accuracy (86%) in diagnosing spinal osteomyelitis compared with 111In-labeled WBCs (66%).152 A recent evaluation of imaging techniques in spinal osteomyelitis and surrounding soft tissue infections has recommended SPECT 67Ga as the radionuclide study of choice when MRI is unavailable or as an adjunct in patients with possible spinal infection in whom the diagnosis remains uncertain.122 Gallium is also better suited for imaging of chronic osteomyelitis compared with 99mTc HMPAO–labeled WBCs, which are better for imaging acute infections.157 
99mTc HMPAO–labeled WBC scintigraphy exhibits high sensitivity (97.7%) and specificity (96.8%) for acute osteomyelitis, although its sensitivity for chronic osteomyelitis is slightly decreased.201 99mTc HMPAO–labeled WBC scintigraphy is preferred for evaluating children because the radiation dose to the spleen is smaller and less blood is needed for labeling.175 99mTc HMPAO–labeled WBC scintigraphy is superior to 99mTc bone scintigraphy for children younger than 6 months because of the poor sensitivity of bone scintigraphy at this age.157 111In-labeled WBC scintigraphy is preferred in evaluating chronic osteomyelitis, as dual 111In WBC/99mTc sulfur colloid studies result in improved accuracy for diagnosis of osteomyelitis in regions containing active bone marrow.157,175 In more complex regions with overlapping bone and soft tissues, such as the skull and hips, simultaneous 111In WBC/99mTc bone SPECT imaging has been recommended.175 
Dual 111In WBC/99mTc bone scans have been used to evaluate for osteomyelitis at sites of delayed union or nonunion.146 The sensitivity, specificity, positive and negative predictive values, and accuracy of this approach were 86%, 84%, 69%, 94%, and 82%, respectively. 
Recently, a meta-analysis of 99mTc-radiolabeled antigranulocyte monoclonal antibodies has shown a sensitivity of 81% and specificity of 77% in the diagnosis of osteomyelitis. The authors conclude that antigranulocyte scintigraphy can be used as a major diagnostic method in patients with suspected osteomyelitis but cannot replace traditional methods such as histologic examination and cell culture.150 Similarly, Stucken et al.184 reported the results of a prospective protocol designed to identify the presence of occult infection in patients with a nonunion of an open or previously operated fracture. The protocol included labeled leukocyte/sulfur colloid imaging, as well as measurement of inflammatory markers (C-reactive protein [CRP] and erythrocyte sedimentation rate [ESR]), serum WBC count, and histopathology. In this study of ununited fractures, the labeled leukocyte/sulfur colloid scan had a sensitivity of just 19%, and did not add anything to the positive predictive value of the combination of serum WBC, ESR, CRP and histopathologic examination alone.184 The authors concluded that the addition of labeled WBC/sulfur colloid imaging had no clinical benefit and was not cost effective when trying to assess occult infection when fracture nonunion is present.184 
Osteonecrosis.
Because scintigraphy is able to demonstrate the vascularity of bone, it is often used to try to assess the risk of osteonecrosis after an injury. Although largely supplanted by MRI, bone scanning can be used to identify osteonecrosis of the femoral head before it is apparent on conventional radiographs (Fig. 17-10).18 Studies by Drane and Rudd50 and Mortensson et al.139 have shown that bone scintigraphy cannot predict the risk of osteonecrosis after femoral neck fracture. Subsequent work has suggested that SPECT imaging may be more accurate in assessing vascularity of the femoral head in fractures of the femoral neck.28 
Figure 17-10
Pinhole bone scintigraphy (anteroposterior views) showing a photon-deficient area centrally in the right femoral head and increased uptake in the femoral neck and subcapital area compared with normal left hip findings.
 
(Reprinted with permission from: Yoon TR, Rowe SM, Song EK, et al. Unusual osteonecrosis of the femoral head misdiagnosed as a stress fracture. J Orthop Trauma. 2004;18:43–47.)
(Reprinted with permission from: Yoon TR, Rowe SM, Song EK, et al. Unusual osteonecrosis of the femoral head misdiagnosed as a stress fracture. J Orthop Trauma. 2004;18:43–47.)
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Figure 17-10
Pinhole bone scintigraphy (anteroposterior views) showing a photon-deficient area centrally in the right femoral head and increased uptake in the femoral neck and subcapital area compared with normal left hip findings.
(Reprinted with permission from: Yoon TR, Rowe SM, Song EK, et al. Unusual osteonecrosis of the femoral head misdiagnosed as a stress fracture. J Orthop Trauma. 2004;18:43–47.)
(Reprinted with permission from: Yoon TR, Rowe SM, Song EK, et al. Unusual osteonecrosis of the femoral head misdiagnosed as a stress fracture. J Orthop Trauma. 2004;18:43–47.)
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Angiography

Technical Considerations

Conventional Angiography.
Techniques in conventional angiography are well established and involve cannulation of a vessel, commonly a major artery, for subsequent diagnostic and therapeutic interventions. Typically, the right common femoral artery is accessed, although less common access sites include the left common femoral artery, the axillary and brachial arteries, and translumbar aortic approaches, the selection of which depend on the clinical situation and goal of angiography. The Seldinger technique, the standard procedure for cannulating the common femoral artery, involves placing an 18-gauge needle into the artery at the level of the midfemoral head under fluoroscopic guidance. A double wall puncture is preferred, whereby the needle is advanced through both the anterior and posterior arterial walls until contact is made with the femoral head. The needle tip is pulled back slowly until it is within the arterial lumen and pulsatile flow is observed from the needle hub. A guidewire is then passed through the needle and into the vessel lumen, and the needle is then exchanged over the guidewire for a catheter or sheath. Selective catheterization of individual vessels involves advancing the guidewire into the arterial tree, with subsequent advancement of the catheter over the guidewire. 
Diagnostic angiography is performed by positioning the catheter tip proximal within the artery of interest and rapidly injecting nonionic iodinated contrast medium, the rate and volume of which are proportional to the size of and flow within the vessel lumen. Rapid fluoroscopic spot filming is timed to coincide with contrast opacification of the arterial tree and documents progressive filling and washout of the vessels. Venous return may also be demonstrated with appropriate delays in filming. Abnormal findings associated with vascular trauma include transection, laceration, dissection, arteriovenous fistula, pseudoaneurysm, mural hematoma, intimal tears, and vasospasm. 
Digital subtraction angiography (DSA) is a commonly used technique, whereby a preliminary fluoroscopic spot film (the “mask”) is taken before contrast injection and is subsequently subtracted from dynamic images obtained during contrast injection. The background tissues (bones, soft tissues) are removed from the dynamic arterial images, resulting in greater image contrast resolution. The concentration of iodinated contrast may be reduced using this technique, resulting in a lower total volume of injected contrast medium. Disadvantages of this technique include lower-spatial resolution and misregistration artifact, which occurs as a result of patient motion after the mask image has been performed and results in misalignment of the mask during subtraction. 
Therapeutic interventions may be performed during angiography and, for trauma patients, most commonly include embolization of bleeding arterial vessels in association with both visceral and bony fractures. Superselective catheterization of the bleeding vessel is first performed, with subsequent occlusion of the vessel using agents administered through the catheter. Temporary and permanent embolic agents are available, and their use is directed by the clinical situation and therapeutic goal. Temporary agents include autologous blood clots and Gelfoam pledgets, whereas permanent agents include microcoils and macrocoils, detachable balloons, polyvinyl alcohol, as well as various tissue adhesives and glues. Pre-embolization and postembolization angiograms are performed not only to document occlusion of the bleeding vessel but also to evaluate for collateral flow around the occluded vessel. 
Complications of angiography include puncture site complications (e.g., groin hematoma, arteriovenous fistula, pseudoaneurysm), contrast complications (e.g., anaphylactoid reactions, renal failure), catheter-related complications (e.g., vessel wall dissection, thromboembolism), and therapy-related complications (e.g., tissue necrosis distal to embolization). Complications may be reduced with experience and careful technique by the angiographer. 
Computed Tomography Angiography.
CTA has become an established application of multislice helical CT technology. Intravenous nonionic iodinated contrast medium is injected, usually through an antecubital vein, using a volume of 120 to 150 mL at a rate of approximately 3 to 4 mL/s. Scanning is performed after an appropriate delay to ensure passage of contrast through the lungs and heart and into the arterial tree, so that imaging occurs during peak intravascular enhancement throughout the arterial segment of interest. Images are typically reconstructed from the helical dataset at 1-mm slice thicknesses with a 50% overlap. Because a typical CTA study generates hundreds to thousands of images, evaluation of the data is performed using 3D workstations, whereby the images may be viewed using cine modes, MPRs, and interactive real-time volume-rendering techniques. In addition to arterial injury, concomitant complex fractures are well evaluated on the same study. Factors that can limit accurate interpretation of CTA images include vasospasm, anatomic variants, atherosclerosis, displaced fracture fragments, metal hardware artifacts, foreign bodies, and patient motion or positioning problems. 

Applications

Vascular Trauma.
CTA using MDCT has become the imaging method of choice for the initial evaluation of vascular injury. CTA can be an important diagnostic and therapeutic modality in trauma patients with hemodynamic instability because of severe abdominal and pelvic trauma or extremity injuries with vascular damage (Fig. 17-11). Although management of a hemodynamically unstable patient with a pelvic fracture remains controversial, many experts suggest emergent angiography in these situations.48 The yield in terms of identifiable arterial injury is low; however, when vascular injury is present, embolization using interventional techniques can be life saving. If necessary, pelvic angiography can be performed concomitantly with external fixation of the pelvis in patients with severe “open-book” injuries of the pelvic ring (Fig. 17-11). 
Figure 17-11
Pelvic angiography in a hemodynamically unstable trauma patient with a pelvic ring injury.
 
A: The anteroposterior pelvic radiograph shows wide diastasis of the pubic symphysis. After emergent application of an anterior pelvic external fixator, the patient underwent selective embolization of both right and left internal iliac arteries. B: Spot film of the left internal iliac artery demonstrates dissection and nonfilling of multiple medial branches. Contrast fills the left internal iliac artery and its branches before embolization. C: Postembolization spot film demonstrates no flow of contrast distal to the embolization coils.
A: The anteroposterior pelvic radiograph shows wide diastasis of the pubic symphysis. After emergent application of an anterior pelvic external fixator, the patient underwent selective embolization of both right and left internal iliac arteries. B: Spot film of the left internal iliac artery demonstrates dissection and nonfilling of multiple medial branches. Contrast fills the left internal iliac artery and its branches before embolization. C: Postembolization spot film demonstrates no flow of contrast distal to the embolization coils.
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Figure 17-11
Pelvic angiography in a hemodynamically unstable trauma patient with a pelvic ring injury.
A: The anteroposterior pelvic radiograph shows wide diastasis of the pubic symphysis. After emergent application of an anterior pelvic external fixator, the patient underwent selective embolization of both right and left internal iliac arteries. B: Spot film of the left internal iliac artery demonstrates dissection and nonfilling of multiple medial branches. Contrast fills the left internal iliac artery and its branches before embolization. C: Postembolization spot film demonstrates no flow of contrast distal to the embolization coils.
A: The anteroposterior pelvic radiograph shows wide diastasis of the pubic symphysis. After emergent application of an anterior pelvic external fixator, the patient underwent selective embolization of both right and left internal iliac arteries. B: Spot film of the left internal iliac artery demonstrates dissection and nonfilling of multiple medial branches. Contrast fills the left internal iliac artery and its branches before embolization. C: Postembolization spot film demonstrates no flow of contrast distal to the embolization coils.
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More recently, CTA has emerged as a simple and effective means of assessing possible vascular injury of the pelvis and extremities. It is as accurate, less invasive, more time-efficient, and less expensive than standard angiography. CTA of the pelvis can be easily and successfully incorporated into standard CT evaluation protocols in patients with blunt trauma and is capable of differentiating active arterial and venous bleeding that can be useful information in guiding further care.5 In a study of 48 trauma patients, contrast-enhanced CT was compared to formal angiography in detecting pelvic bleeding; CT had 94.1% sensitivity and 97.6% negative predictive value for the detection of active hemorrhage, and 92.6% sensitivity and 91.2% negative predictive value for predicting need for surgical or endovascular intervention.133 
One traditional indication for angiography has been in the assessment of popliteal artery injury in the patient with definite or suspected knee dislocation. Recently, several studies have clarified the role of angiography in such patients, showing that urgent angiography is not needed unless there are deficits in distal pulses, ideally quantified by determination of the ankle-brachial index.105,181 
CTA has significant advantages for the assessment of potential vascular injury in the lower extremity because of its noninvasiveness and immediate availability. CTA has supplanted arteriography as the imaging method of choice for the initial radiographic evaluation of peripheral vascular injuries.72,156 Inaba et al.93 used multislice CTA in 59 patients who underwent a total of 63 studies. In their series, multislice CTA was both 100% sensitive and 100% specific for detecting clinically significant arterial injury.93 A recent study by LeBus and Collinge112 suggests that routine use of CTA in the evaluation of patients with high-energy tibial plafond injuries may be beneficial. Twenty-five consecutive patients were treated with a standard protocol that included preoperative CT (and CTA). In 13 of the patients (52%), notable arterial injury was identified, most involving the anterior tibial artery. The authors thought that information about associated vascular injury allowed them to make better decisions about surgical tactics to be used for a given procedure, including whether to use traditional open or minimally invasive approaches, as well as in choices about placement of incisions.112 

Management of Imaging Data

Advances in digital imaging modalities have necessarily been paralleled by advances in distributing, viewing, and storing imaging data. In many instances, the traditional light box has been replaced by digital workstations, the file room has been upgraded with digital archives, and the transport of films has been replaced by digital transmission of images across networks to remote workstations. Many of these changes have evolved in response to the increasing size of digital imaging studies, in addition to the need to use and distribute this information more efficiently within the health care environment. All of these changes have relied on continued improvements in computer networks, workstations, storage devices, and display media, in addition to implementation of standards, to support the evolving digital imaging infrastructure. Although a thorough discussion of digital image management is beyond the scope of this section, a brief review of some of the more common concepts and standards will be presented. 

Distribution of Imaging Information

Distribution of medical images is influenced by several factors, including size and volume of imaging studies, computer network infrastructure, and clinical needs by interpreting and referring physicians. Current trends in digital imaging technology have resulted in greater image resolution and greater numbers of images, both of which substantially contribute to increasing sizes of imaging studies. For example, a typical 256 × 256 matrix image, using 2 bytes of storage for each pixel, requires approximately 125 KB of storage per image, whereas a 512 × 512 matrix image requires approximately 500 KB, or four times as much as its lower-resolution counterpart. CT and MRI studies routinely contain 100 to 200 images, resulting in storage requirements of 12 to 100 MB per study. Newer 64-slice and 256-slice CT scanners may result in data files of up to 2.5 and 10 GB, respectively, per study. 
Media for distribution include printed films, CD-ROMs, and networks for remote viewing or processing on workstations (Fig. 17-5). When trauma patients are transported from one institution to another, import of images taken at the referring institution to a picture archive and communications system (PACS) at the receiving hospital, thereby reducing the need for repeat imaging at the receiving hospital.123 
Many imaging devices are connected to networks for transmitting image data to remote locations for image viewing and storage, to which the term teleradiology applies. There is a wide variety of network configurations, with descriptors such as local or wide area networks (LAN, WAN), intranets, and the internet. Speed of transmission across networks depends on the various types of communication links within the network (modem, ISDN, DSL, cable modem, T1, T3, fiberoptic cable), as well as the level of network traffic. Data compression is used to decrease the size of imaging studies before electronic transmission, and compression schemes are categorized as “lossless” (no loss of original data, typically 3:1 compression) or “lossy” (some loss of data in original image, typically 15:1 or greater compression). Use of the internet to transmit imaging studies is growing, although patient confidentiality and security issues have received considerable attention. 
Imaging studies sent to interpreting physicians are commonly viewed on workstations, which are able to display images at full resolution using specific formats (“hanging protocols”) and provide advanced capabilities for image processing (Fig. 17-5). Such workstations allow 3D images to be manipulated and reviewed in real-time; some can save movie files of the 3D image onto a disk. Of course, such capabilities are of limited value if they are not available to the orthopedic trauma surgeon in a timely manner. Current, high-end workstations are expensive and are usually not available outside of the radiology department; normally, less-sophisticated viewing stations provide basic access to images outside of the imaging department. In certain environments, use of hardcopy images will remain necessary. Examples of this situation include the operating room, where multiple images of different imaging modalities need to be viewed together by a surgeon in sterile operating garb, and in the clinic, where the viewing of multiple studies in chronologic order is necessary to observe fracture healing or changes in fracture alignment. 

Picture Archive and Communications Systems

A PACS represents a network of mechanisms used to acquire, view, and store digital images and at its most basic level includes devices used to acquire digital images (e.g., CT and MRI scanners), workstations whereby images may be viewed and manipulated for diagnostic interpretations, and archives where digital images are stored for later retrieval. PACS may also include viewing stations for departments outside of the radiology department (e.g., emergency department, intensive care unit), and may be contained within their own LAN or exist as a part of a larger WAN. PACS may also communicate with Radiology Information Systems and Hospital Information Systems to share and/or modify patient information. 
There are many advantages of PACS, including prompt access to clinical images, postprocessing of image data (window levels, MPRs and 3D reconstructions, measurement and annotations tools), the ability of more than one user to simultaneously view the same images, and reduced filming costs and lost films. On the other hand, significant disadvantages include initial and recurring expenses related to installing and maintaining PACS, massive storage requirements for image archival, and the necessity of support personnel to maintain the network and its components. One study showed that LCD personal computer monitors and PACS workstations did not differ significantly in the diagnostic quality of cervical spine fracture radiographs, suggesting that LCD personal computer monitors are sufficient for fast, accurate diagnosis in the emergency department for evaluation of cervical spine injuries at considerably reduced cost.25 

Digital Imaging and Communications in Medicine Standards

In 1983, the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) formed a joint committee to develop a standard by which users could retrieve images and associated information from digital imaging equipment in a form that would be compatible across all manufacturers. Two years later, the first version of the ACR-NEMA standard was published, and in 1988, an updated second version was published, which corrected errors and inconsistencies and added new data elements. The first two versions relied on point-to-point connections between equipment, and by 1988, the growing implementation of networks and PACS necessitated a complete rewriting of the standard, which is currently known as DICOM version 3.0. 
The DICOM standard sets forth a uniform set of rules for communication of medical images and associated information, which are complex but practical and adaptable. The standard is flexible enough to accommodate a variety of images and image information across a broad range of medical imaging platforms. Conformance with the standard is voluntary, and manufacturers of medical imaging equipment or software who support the standard must provide conformance statements describing their particular implementation of the standard. This does not guarantee that two DICOM-compliant devices will communicate properly with one another; rather, the conformance statement serves as a guide to rule out obvious incompatibilities between equipment. 

Digital Imaging and Teleradiology in Orthopedics

Digital Imaging

Digital imaging is the future of radiology and has definite advantages and disadvantages in the management of musculoskeletal injuries. In a recent review, Wade et al.194 noted the many potential advantages of digital imaging: Reduction of foot traffic between clinics, wards, and the radiology department; increased availability of investigations; increase in the speed of availability; the virtual elimination of missing studies; less radiation exposure; fewer wasted films; and reduction in retrieval times. However, there are logistical problems associated with the adoption and use of filmless systems in an emergency department setting that must be overcome.197 In addition, DR remains inferior to conventional radiography in terms of image spatial resolution (Table 17-1). Work is progressing in digital detector technology that may eventually provide spatial resolution equal to or exceeding that of conventional radiography. Miller et al.135 describe the medical application of total-body DR for screening trauma patients, using a C-arm–based system initially developed in South Africa to detect theft by diamond miners. Full implementation of DR and PACS can be expensive and subject to the nuisances of technologic failure and requires technical support skills that may not be universally available. Traditional printed images will continue to have a role in the operating room, in the clinic, and in other venues where access to the PACS system is not available or appropriate. 

Teleradiology

Teleradiology can affect the practice of fracture management in many ways. Teleradiology allows emergency physicians and/or house staff to send digital images of radiographs or clinical photographs to off-site attending orthopedic staff. There is potential application for community-based orthopedists to obtain second opinions about fracture management from specialists at tertiary care centers. Traditionally, such consultation required the referring orthopedic surgeon to obtain, duplicate, and mail hardcopies of radiographs to the consulting surgeon, who has then had to communicate his or her opinion to the referring surgeon by telephone. Using teleradiology, the transmission of patient information, imaging studies, and the consultant’s evaluation can all be accomplished with greater convenience and less cost. 
Ricci and Borrelli167 demonstrated that teleradiology improved clinical decision making in the management of acute fractures. A series of 123 consecutive fractures was studied; in all cases, a junior orthopedic resident performed the initial orthopedic evaluation. All radiographs were digitized and electronically sent to the attending orthopedist. Treatment plans were formulated and documented at three different times: After verbal communication of the patient’s history and injuries, after the digitized radiographs were viewed, and after the original hardcopy radiographs were viewed. The investigators recognized two different types of changes that were made to the initial plan of management: Acute treatment changes and changes in the definitive management of the fracture. Overall, the viewing of digitized radiographs resulted in a change of management in 21% of the fractures. No further changes in management were decided on after review of the original radiographs. The investigators concluded that the routine use of digitized radiographs improves fracture management.167 

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