(Radiographics. 1999;19:1143-1160.)
© RSNA, 1999
Volume-rendered Three-dimensional Spiral CT: Musculoskeletal Applications1
E. Scott Pretorius, MD and
Elliot K. Fishman, MD
1 From the Russell H. Morgan Department of Radiology and Radiologic Science, Johns Hopkins University School of Medicine, Baltimore, Md. Presented as a scientific exhibit at the 1997 RSNA scientific assembly. Received October 28, 1998; revision requested November 23 and received December 30; accepted January 26, 1999. Address reprint requests to E.K.F., Department of Radiology, Johns Hopkins Hospital, 600 N Wolfe St, Baltimore, MD 21287.
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Abstract
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Spiral computed tomography (CT) is a powerful modality for evaluation of the musculoskeletal system, particularly when coupled with real-time, volume-rendering reconstruction techniques. Including volume-rendered spiral CT in routine musculoskeletal imaging protocols can change management in a significant number of cases. In cases of trauma, subtle fractures—particularly those oriented in the axial plane—are better seen on volume-rendered images. Complex injuries can be better demonstrated with volume-rendered images, and complicated spatial information about the relative positions of fracture fragments can be easily demonstrated to the orthopedic surgeons. The use of intravenously administered contrast material allows simultaneous evaluation of osseous and vascular structures within the affected area. Evaluation of suspected infectious or neoplastic disease is also aided by including volume-rendered imaging in the musculoskeletal spiral CT examination. The extent of disease can be thoroughly evaluated with volume-rendered images, and therapeutic planning—be it surgical or medical—is aided by the anatomic information available from volume-rendered images. Postoperative studies in patients with orthopedic hardware also benefit from volume-rendered imaging. Volume rendering eliminates most streak artifact and produces high-quality images on which the relationships among hardware, bones, and bone fragments are well demonstrated.
Index Terms: Bone neoplasms, 40.30 • Bones, infection, 40.20 • Bones, injuries, 40.40 • Computed tomography (CT), helical, 40.12115 • Computed tomography (CT), three-dimensional, 40.12117 • Computed tomography (CT), volume rendering, 40.12117
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INTRODUCTION
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The combination of subsecond spiral computed tomography (CT) and three-dimensional (3D) reconstruction with volume rendering allows rapid and detailed examination of the musculoskeletal system. In our experience, spiral CT combined with volume rendering has proved valuable in diagnosis of subtle abnormalities and in planning patient therapy. In a substantial number of cases, management is altered because of findings seen only on the 3D images or better demonstrated on these images: subtle fractures, complex injuries, and pathologic conditions masked by metallic streak artifact. In addition, volume-rendered images can display complex spatial information and are especially useful for conveying complicated anatomic information to clinical colleagues (1–3).
In this article, the technique of volume-rendered 3D spiral CT and its musculoskeletal applications are presented. These applications include trauma, infection, tumors, and postoperative imaging.
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TECHNIQUE
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Spiral CT
Musculoskeletal applications of spiral CT require an understanding of examination techniques and protocols to optimize study performance. Of course, the optimal scanning technique depends on the clinical question to be answered. Small areas of interest, such as the sternoclavicular joint or the wrist, benefit from spiral acquisition of a volume data set that combines narrow collimation (1–2 mm) and a pitch of 1–1.5 with small reconstruction increments (1 mm). Larger areas of interest, such as the pelvis or lower extrem
ity, may be examined with wider collimation (3 mm), a pitch of 1–2, and reconstruction every 2–3 mm. Specific values for milliampere seconds and kilovolt peak depend on the scanner used. Typical imaging parameters for several representative clinical protocols are listed in the Table (4,5).
If 3D imaging is contemplated, even minimal interscan or intrascan motion artifact may severely compromise the integrity of the data set. The use of subsecond spiral acquisition limits or eliminates this potential artifact, even in patients who have difficulty remaining motionless.
Studies performed to evaluate possible infection in muscle or soft tissue or a soft-tissue neoplasm require use of intravenously administered contrast material. Rapid-acquisition spiral CT allows the entire data set to be acquired at the peak of contrast enhancement. Three-dimensional images produced with maximum intensity projection or volume-rendering techniques require optimal administration of contrast material if vascular images are to be generated. Injection rates are typically 3 mL/sec; the scanning delay depends on the region of the body to be scanned. Typical scanning delays are usually 40 seconds for an abdominal examination and 70 seconds for an examination of the lower extremities.
Volume Rendering
Creation of a multiplanar two-dimensional reformatted image or a 3D CT image begins with the acquisition and reconstruction of axial image data (6). These data are usually composed of anisotropic voxels, which are longer in the z axis than in the transverse plane. The data must then be interpolated to create cuboid isotropic voxels, which are the same dimension in all three axes. Isotropic data can then be mapped into the appropriate plane or, with application of the appropriate rendering algorithm, into a 3D volume (7).
All 3D rendering algorithms attempt to display 3D spatial relationships in a two-dimensional image. Both shaded surface rendering and volume rendering have been advocated as reconstruction algorithms for 3D musculoskeletal imaging, but certain advantages of volume rendering make it our preferred algorithm for all 3D musculoskeletal imaging applications.
Shaded surface rendering algorithms take the first voxel encountered along a projection ray that exceeds a user-defined threshold value and define the position and attenuation value of that voxel as the surface of the bone (Fig 1a). No other CT information along that projection ray contributes to the viewed image. Therefore, shaded surface displays are capable of demonstrating gross 3D relationships but fail to display lesions hidden beneath the bone surface (8–10). Shaded surface displays also tend to demonstrate stair-step artifacts.
By making use of the entire data set, volume-rendering techniques convey more information than shaded surface rendering. In volume rendering, the contributions of each voxel along a line of sight from the viewer's eye through the data set are summed (Fig 1b). This process is repeated many times to determine each pixel value in the displayed image. Because the entire data set is incorporated into the resulting image, volume rendering requires considerably more computer power than other rendering algorithms. Ongoing advances in workstation hardware and software have made sufficient computer power clinically available at a reasonable cost. We currently use workstations (Silicon Graphics, Mountain View, Calif) that are capable of producing volume-rendered images at real-time rates. Real-time interactive display eliminates the need for the editing step prior to 3D rendering and thus minimizes the radiologist's time commitment and expedites communication of results to the referring clinician. Volume-rendered images may be viewed in any plane or projection and in a range of opacity from transparent to opaque.
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MUSCULOSKELETAL APPLICATIONS
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Trauma
Spiral CT has two major roles in musculoskeletal trauma: (a) to define or exclude a fracture that was equivocal at plain radiography and (b) to determine the extent of a previously diagnosed fracture and thus provide guidance for therapy. In institutions where a CT scanner is located in the emergency department, select patients with clinically apparent fractures may undergo spiral CT instead of plain radiography. Spiral CT will provide additional information about soft-tissue abnormalities and demonstrate osseous anatomy, especially in anatomically complex areas—such as the pelvis, scapula, and spine—where plain radiography is often limited in its ability to demonstrate fractures. In the setting of trauma, conventional radiographic series are often difficult to obtain because patients may be unable to cooperate fully with positioning requirements. In comparison with trauma radiography, the results of which are often of poor quality, volume-rendered spiral CT represents a significant advance in trauma imaging and a significant savings in terms of patient time spent in the radiology department.
We have found routine use of multiplanar imaging and volume rendering to be a critical part of the CT study of musculoskeletal trauma. In patients with pelvic fractures seen on axial CT images, treatment decisions have been shown to be altered in up to 30% of cases because of findings on multiplanar or volume-rendered images (Fig 2) (11). In general, these changes in treatment result when multiplanar or volume-rendered images reveal a more severe injury than was clinically suspected or than was seen on conventional axial images. In our experience, patients whose treatment changes tend to fall into two groups: (a) those who might otherwise have received conservative treatment but become surgical candidates and (b) those in whom urgent surgery is delayed in favor of later, definitive arthrodesis or arthroplasty.
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Figure 2. Anterior volume-rendered spiral CT image of the sacrum reveals a subtle fracture of the right aspect of S1 (arrows). The fracture was not seen on conventional axial images because the fracture line lay relatively in the axial plane.
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Shoulder.—Fractures of the scapula are often extremely subtle on plain radiographs, but volume-rendered spiral CT is extremely sensitive in the detection and characterization of such fractures (Figs 3, 4). One study of patients with scapular fractures demonstrated a high frequency of associated traumatic injury to the shoulder, chest wall, and lung: pulmonary contusion in 54% of cases; rib fracture in 54%; clavicular fracture in 27%; and subclavian, brachial, or axillary artery injury in 11% (12). Volume-rendered spiral CT can demonstrate these injuries and display the relevant anatomy in this complex region (Fig 3).
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Figures 3, 4. (3) Scapular fracture in a pedestrian who was struck by an automobile. Posterior volume-rendered spiral CT image shows a complex fracture of the right shoulder. The scapular body is shattered (black arrows), and the scapular spine has become separated from the remainder of the bone. An associated ipsilateral rib fracture is also seen (white arrow). (4) Right posterior oblique volume-rendered spiral CT image of the shoulder shows a comminuted fracture of the scapular body (arrow). The scapular spine, coracoid process, and acromioclavicular joint are intact.
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Figures 3, 4. (3) Scapular fracture in a pedestrian who was struck by an automobile. Posterior volume-rendered spiral CT image shows a complex fracture of the right shoulder. The scapular body is shattered (black arrows), and the scapular spine has become separated from the remainder of the bone. An associated ipsilateral rib fracture is also seen (white arrow). (4) Right posterior oblique volume-rendered spiral CT image of the shoulder shows a comminuted fracture of the scapular body (arrow). The scapular spine, coracoid process, and acromioclavicular joint are intact.
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In evaluation of three- and four-part fractures of the proximal humerus, the superiority of spiral CT with multiplanar and 3D reconstruction over plain radiography has been demonstrated (13). Spiral CT is extremely sensitive in the detection of such fractures, and volume-rendered images can display the spatial relationships of
fracture fragments in this complex anatomic region. In particular, the number of fracture fragments and their degree of rotation—critical factors in determining whether a proximal humeral fracture should be managed surgically (14,15)—are well demonstrated.
Sternoclavicular Joint.—Most injuries to the sternoclavicular joint result from blunt, closed-chest trauma such as motor vehicle accidents. Although the sternoclavicular joint may be injured in isolation (Fig 5), accompanying fractures of superior ribs or of the shoulder joint are common. Posterior dislocation of the sternoclavicular joint is associated with injury to the aorta and great vessels (Fig 6). Intravenous contrast material should always be given to exclude vascular injury, and, if possible, CT angiography should be performed.
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Figures 5, 6. (5) Inferior volume-rendered spiral CT image shows a comminuted fracture of the medial right clavicle (arrow). The medial aspect of the clavicle is displaced posteriorly with respect to the more lateral portions of the bone. The right sternoclavicular joint is disrupted, as manifested by widening of the joint space. The superior ribs and the great vessels were not injured. (6) Inferior volume-rendered spiral CT image shows posterior dislocation of the right sternoclavicular joint (white arrow). A mediastinal hematoma is present (arrowhead); axial contrast material-enhanced images (not shown) demonstrated the site of venous hemorrhage. There is an associated fracture of the right fourth rib (black arrows).
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Figures 5, 6. (5) Inferior volume-rendered spiral CT image shows a comminuted fracture of the medial right clavicle (arrow). The medial aspect of the clavicle is displaced posteriorly with respect to the more lateral portions of the bone. The right sternoclavicular joint is disrupted, as manifested by widening of the joint space. The superior ribs and the great vessels were not injured. (6) Inferior volume-rendered spiral CT image shows posterior dislocation of the right sternoclavicular joint (white arrow). A mediastinal hematoma is present (arrowhead); axial contrast material-enhanced images (not shown) demonstrated the site of venous hemorrhage. There is an associated fracture of the right fourth rib (black arrows).
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Multiplanar and 3D imaging are a routine part of our imaging examination of the sternoclavicular joint. The sternum is best evaluated on coronal and coronal oblique images (16). Volume-rendered images, particularly those in the z-axis projection, are optimal for evaluating the orientation of sternoclavicular joint dislocations. These images are most useful after nearby bone structures, such as the rib cage and spine, have been edited.
Elbow.—Spiral CT of the elbow is indicated for fracture detection when plain radiographs are equivocal; it is indicated for fracture evaluation when plain radiography shows a complex injury. In such cases, volume-rendered images are ideal for anatomic evaluation and for communicating the interrelationships of fracture fragments to the orthopedic surgeon (Fig 7). We have found that 2-mm collimation with a 1-mm reconstruction interval is needed to obtain satisfactory detail in patients with such injuries.
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Figure 7a. Lateral (a) and dorsal (b) volume-rendered spiral CT images obtained for surgical planning show a comminuted, intraarticular fracture of the olecranon (arrow), which was identified on plain radiographs. The ulnar diaphysis is displaced distally and slightly volarly relative to the major proximal fracture fragment.
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Figure 7b. Lateral (a) and dorsal (b) volume-rendered spiral CT images obtained for surgical planning show a comminuted, intraarticular fracture of the olecranon (arrow), which was identified on plain radiographs. The ulnar diaphysis is displaced distally and slightly volarly relative to the major proximal fracture fragment.
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Wrist.—As with other small regions of the body, the wrist is best evaluated with a narrow section thickness (1–2 mm) and reconstruction at 1 mm. The indications for volume-rendered spiral CT of the wrist are similar to those for volume-rendered spiral CT of the elbow: clinically suspected fractures not seen on plain radiographs and complex fractures that require further evaluation (Figs 8, 9). Although direct coronal scanning can eliminate the need for multiplanar imaging in this important plane (17), volume rendering allows evaluation of the wrist from any perspective. Unlike plain radiography, volume-rendered CT can be performed through cast material without significant image degradation (Fig 9).
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Figure 8. Lateral volume-rendered spiral CT image shows an impacted, comminuted, intraarticular fracture of the distal radius with dorsal angulation of the distal fracture fragments (arrow). In this case, spiral CT required 2-mm-thick sections reconstructed at 1-mm intervals to provide detail of small bone structures. No evidence of associated carpal bone fracture was seen at spiral CT.
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Figure 9. Dorsal volume-rendered spiral CT image shows an impacted fracture of the distal radius (thin arrows) with an associated fracture of the distal ulna (thick arrow). A small fracture fragment of the radial styloid process is also seen (arrowhead). The image was obtained through a cast, which would limit the usefulness of plain radiography.
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Spine.—Spinal trauma can be routinely evaluated with a combination of axial spiral CT and volume-rendered images. Specific applications include identification of fractures, subluxation, and locked facets and localization of foreign bodies (Fig 10) and fracture fragments (Fig 11). Spiral CT is especially valuable in the detection of subtle sacral fractures, which are often missed on plain radiographs. The relationship of the fracture to the sacral foramina is well seen on 3D views (Figs 12, 13).
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Figure 10. Spinal fracture secondary to a gunshot wound. Inferior volume-rendered spiral CT image shows an impacted bullet at the pedicle of T11 with extradural extension of both bullet fragments and bone fragments (arrow). Streak artifact is minimal despite the large caliber of the bullet.
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Figure 11a. (a) Lateral volume-rendered spiral CT image shows a flexion teardrop fracture at C6 with posterior displacement of the posterior portion of the vertebral body (arrow). (b) Cutaway anterior volume-rendered spiral CT image best shows the full extent of involvement.
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Figure 11b. (a) Lateral volume-rendered spiral CT image shows a flexion teardrop fracture at C6 with posterior displacement of the posterior portion of the vertebral body (arrow). (b) Cutaway anterior volume-rendered spiral CT image best shows the full extent of involvement.
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Figures 12, 13. (12) Sacral fracture in a 34-year-old woman who was unable to walk or control her bladder after a high-speed motor vehicle accident. Anterior volume-rendered spiral CT image with the pubis and ischium removed by editing shows a fracture of the right hemisacrum that extends through the neural foramina of S1-S4 (arrows). The patient was ultimately able to walk but did not regain normal bladder control. (13) Anterior volume-rendered spiral CT image shows a sacral stress fracture. The fracture lines extend through both the left and right foramina of S1 and S2 (arrows).
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Figures 12, 13. (12) Sacral fracture in a 34-year-old woman who was unable to walk or control her bladder after a high-speed motor vehicle accident. Anterior volume-rendered spiral CT image with the pubis and ischium removed by editing shows a fracture of the right hemisacrum that extends through the neural foramina of S1-S4 (arrows). The patient was ultimately able to walk but did not regain normal bladder control. (13) Anterior volume-rendered spiral CT image shows a sacral stress fracture. The fracture lines extend through both the left and right foramina of S1 and S2 (arrows).
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Pelvis.—Spiral CT data sets coupled with real-time volume rendering allow visualization of the entire pelvis through any plane or perspective (18). Any inlet or tangential view desired may be created, thus eliminating the need for time-consuming radiographic series (Figs 14–16). The data set may be edited to isolate the fracture, and in select cases the femur may be disarticulated from the acetabulum. Concurrent sacral and sacroiliac injuries may also be identified and mapped (19) (Figs 12, 13). Although CT and 3D imaging are not necessary in every case of pelvic trauma, we have found that the interpretation of complex injuries benefits most from 3D imaging (20).
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Figures 14-16. (14) Pelvic fracture in a 15-year-old boy who was struck by an automobile. Volume-rendered spiral CT image (superior view of left anterior oblique projection) shows the extent of an acetabular fracture (arrow), which was surgically repaired. (15a) Volume-rendered spiral CT image (angled inlet view) shows the extent of a right acetabular fracture (arrows). (15b) Edited right posterior oblique volume-rendered spiral CT image best shows the orientation of the fracture lines and the acetabulum. (16) Pelvic fracture with an intraar-ticular fragment in a 15-year-old boy who was struck by a bus. Inferior volume-rendered spiral CT image shows a posterior acetabular fracture (curved arrow). An intraarticular bone fragment is seen (straight arrow); this fragment was a result of the initial injury, which was a posterior fracture with dislocation.
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Figures 14-16. (14) Pelvic fracture in a 15-year-old boy who was struck by an automobile. Volume-rendered spiral CT image (superior view of left anterior oblique projection) shows the extent of an acetabular fracture (arrow), which was surgically repaired. (15a) Volume-rendered spiral CT image (angled inlet view) shows the extent of a right acetabular fracture (arrows). (15b) Edited right posterior oblique volume-rendered spiral CT image best shows the orientation of the fracture lines and the acetabulum. (16) Pelvic fracture with an intraar-ticular fragment in a 15-year-old boy who was struck by a bus. Inferior volume-rendered spiral CT image shows a posterior acetabular fracture (curved arrow). An intraarticular bone fragment is seen (straight arrow); this fragment was a result of the initial injury, which was a posterior fracture with dislocation.
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Figures 14-16. (14) Pelvic fracture in a 15-year-old boy who was struck by an automobile. Volume-rendered spiral CT image (superior view of left anterior oblique projection) shows the extent of an acetabular fracture (arrow), which was surgically repaired. (15a) Volume-rendered spiral CT image (angled inlet view) shows the extent of a right acetabular fracture (arrows). (15b) Edited right posterior oblique volume-rendered spiral CT image best shows the orientation of the fracture lines and the acetabulum. (16) Pelvic fracture with an intraar-ticular fragment in a 15-year-old boy who was struck by a bus. Inferior volume-rendered spiral CT image shows a posterior acetabular fracture (curved arrow). An intraarticular bone fragment is seen (straight arrow); this fragment was a result of the initial injury, which was a posterior fracture with dislocation.
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Figures 14-16. (14) Pelvic fracture in a 15-year-old boy who was struck by an automobile. Volume-rendered spiral CT image (superior view of left anterior oblique projection) shows the extent of an acetabular fracture (arrow), which was surgically repaired. (15a) Volume-rendered spiral CT image (angled inlet view) shows the extent of a right acetabular fracture (arrows). (15b) Edited right posterior oblique volume-rendered spiral CT image best shows the orientation of the fracture lines and the acetabulum. (16) Pelvic fracture with an intraar-ticular fragment in a 15-year-old boy who was struck by a bus. Inferior volume-rendered spiral CT image shows a posterior acetabular fracture (curved arrow). An intraarticular bone fragment is seen (straight arrow); this fragment was a result of the initial injury, which was a posterior fracture with dislocation.
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The use of intravenous contrast material in these studies allows a vascular map of the iliac and femoral vessels to be created from the same CT data set (21) (Fig 17). Associated major vessels can be identified and evaluated, although hemodynamically unstable patients with suspected injury to smaller vessels should still be evaluated with conventional angiography (22).
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Figure 17. Anterior volume-rendered spiral CT image of the pelvis shows that the osseous and major vascular structures are normal. In this trauma case, evaluation of the aorta and major pelvic vasculature was performed at the same time as evaluation of the osseous pelvis.
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Knee.—Although magnetic resonance (MR) imaging is the preferred modality for evaluating the ligaments and menisci of the knee, spiral CT with multiplanar reconstruction and volume rendering is ideal for evaluating the osseous structures of the acutely traumatized knee, even if the joint is wrapped or in a partial cast (Figs 18–20). For example, the reconstructed images can be used to identify and quantitate depression of the tibial plateau, even if the knee cannot be easily positioned for conventional radiography (23). Although the menisci and ligaments of the knee are better imaged with MR imaging, 3D CT images have been used to guide surgical replacement of the cruciate ligament (24).
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Figures 18-20. (18) Knee fracture in a patient with a history of a gunshot wound. Anterior (a) and lateral (b) volume-rendered spiral CT images show a comminuted fracture of the distal femur (arrow). (19) Anterior (a) and lateral (b) volume-rendered spiral CT images clearly show a comminuted fracture of the tibial plateau (arrow). Because of the presence of a partial cast, the examination would have been extremely limited if performed with plain radiography. (20) Knee fracture in a 12-year-old boy who hit a stationary automobile while riding a moped. Anterior (a) and posterior (b) volume-rendered spiral CT images show a fracture of the lateral aspect of the distal femur (arrow in a). Avulsion of the tibial spines is best seen on the posterior view (arrow in b).
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Figures 18-20. (18) Knee fracture in a patient with a history of a gunshot wound. Anterior (a) and lateral (b) volume-rendered spiral CT images show a comminuted fracture of the distal femur (arrow). (19) Anterior (a) and lateral (b) volume-rendered spiral CT images clearly show a comminuted fracture of the tibial plateau (arrow). Because of the presence of a partial cast, the examination would have been extremely limited if performed with plain radiography. (20) Knee fracture in a 12-year-old boy who hit a stationary automobile while riding a moped. Anterior (a) and posterior (b) volume-rendered spiral CT images show a fracture of the lateral aspect of the distal femur (arrow in a). Avulsion of the tibial spines is best seen on the posterior view (arrow in b).
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Figures 18-20. (18) Knee fracture in a patient with a history of a gunshot wound. Anterior (a) and lateral (b) volume-rendered spiral CT images show a comminuted fracture of the distal femur (arrow). (19) Anterior (a) and lateral (b) volume-rendered spiral CT images clearly show a comminuted fracture of the tibial plateau (arrow). Because of the presence of a partial cast, the examination would have been extremely limited if performed with plain radiography. (20) Knee fracture in a 12-year-old boy who hit a stationary automobile while riding a moped. Anterior (a) and posterior (b) volume-rendered spiral CT images show a fracture of the lateral aspect of the distal femur (arrow in a). Avulsion of the tibial spines is best seen on the posterior view (arrow in b).
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Figures 18-20. (18) Knee fracture in a patient with a history of a gunshot wound. Anterior (a) and lateral (b) volume-rendered spiral CT images show a comminuted fracture of the distal femur (arrow). (19) Anterior (a) and lateral (b) volume-rendered spiral CT images clearly show a comminuted fracture of the tibial plateau (arrow). Because of the presence of a partial cast, the examination would have been extremely limited if performed with plain radiography. (20) Knee fracture in a 12-year-old boy who hit a stationary automobile while riding a moped. Anterior (a) and posterior (b) volume-rendered spiral CT images show a fracture of the lateral aspect of the distal femur (arrow in a). Avulsion of the tibial spines is best seen on the posterior view (arrow in b).
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Figures 18-20. (18) Knee fracture in a patient with a history of a gunshot wound. Anterior (a) and lateral (b) volume-rendered spiral CT images show a comminuted fracture of the distal femur (arrow). (19) Anterior (a) and lateral (b) volume-rendered spiral CT images clearly show a comminuted fracture of the tibial plateau (arrow). Because of the presence of a partial cast, the examination would have been extremely limited if performed with plain radiography. (20) Knee fracture in a 12-year-old boy who hit a stationary automobile while riding a moped. Anterior (a) and posterior (b) volume-rendered spiral CT images show a fracture of the lateral aspect of the distal femur (arrow in a). Avulsion of the tibial spines is best seen on the posterior view (arrow in b).
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Figures 18-20. (18) Knee fracture in a patient with a history of a gunshot wound. Anterior (a) and lateral (b) volume-rendered spiral CT images show a comminuted fracture of the distal femur (arrow). (19) Anterior (a) and lateral (b) volume-rendered spiral CT images clearly show a comminuted fracture of the tibial plateau (arrow). Because of the presence of a partial cast, the examination would have been extremely limited if performed with plain radiography. (20) Knee fracture in a 12-year-old boy who hit a stationary automobile while riding a moped. Anterior (a) and posterior (b) volume-rendered spiral CT images show a fracture of the lateral aspect of the distal femur (arrow in a). Avulsion of the tibial spines is best seen on the posterior view (arrow in b).
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Ankle.—Routine fractures of the ankle do not require CT. However, in complex intraarticular fractures of the distal tibia, the added information provided by spiral CT and volume-rendered imaging may assist the clinician in triaging the patient to immediate surgery or later, definitive arthroplasty (25,26) (Figs 21, 22). Talar and calcaneal injuries can also be easily evaluated (Figs 23, 24), and 3D mapping can be done for preoperative planning. We perform a single data acquisition in a plane parallel to the foot or two data acquisitions including one with direct coronal imaging; the choice depends on the site of injury and the preference of the referring orthopedic surgeon.
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Figures 21, 22. (21) Anterior (a) and superior (b) volume-rendered spiral CT images obtained through a cast show a comminuted, impacted fracture of the distal tibia (arrows in a). (22) Lateral (a) and anterior (b) volume-rendered spiral CT images show a Salter II fracture of the distal tibia (arrows in a, solid arrow in b) with considerable anterior and lateral displacement and angulation of distal fracture fragments. The anterior view also clearly shows an associated fibular injury (open arrow in b).
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Figures 21, 22. (21) Anterior (a) and superior (b) volume-rendered spiral CT images obtained through a cast show a comminuted, impacted fracture of the distal tibia (arrows in a). (22) Lateral (a) and anterior (b) volume-rendered spiral CT images show a Salter II fracture of the distal tibia (arrows in a, solid arrow in b) with considerable anterior and lateral displacement and angulation of distal fracture fragments. The anterior view also clearly shows an associated fibular injury (open arrow in b).
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Figures 21, 22. (21) Anterior (a) and superior (b) volume-rendered spiral CT images obtained through a cast show a comminuted, impacted fracture of the distal tibia (arrows in a). (22) Lateral (a) and anterior (b) volume-rendered spiral CT images show a Salter II fracture of the distal tibia (arrows in a, solid arrow in b) with considerable anterior and lateral displacement and angulation of distal fracture fragments. The anterior view also clearly shows an associated fibular injury (open arrow in b).
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Figures 21, 22. (21) Anterior (a) and superior (b) volume-rendered spiral CT images obtained through a cast show a comminuted, impacted fracture of the distal tibia (arrows in a). (22) Lateral (a) and anterior (b) volume-rendered spiral CT images show a Salter II fracture of the distal tibia (arrows in a, solid arrow in b) with considerable anterior and lateral displacement and angulation of distal fracture fragments. The anterior view also clearly shows an associated fibular injury (open arrow in b).
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Figure 23. Ankle fracture in a patient with a history of a fall from a height of several meters. Lateral volume-rendered spiral CT image obtained through a cast shows a comminuted fracture of the calcaneus (arrows). The subtalar joint appears to be intact.
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Figure 24a. Superior (a) and posterior (b) volume-rendered spiral CT images obtained through a cast show the extent of a comminuted calcaneal fracture (arrow), which was the result of a three-story jump from a burning building. The orientation of the fracture fragments seen on these views is helpful for presurgical planning, and the intraarticular nature of the fracture (arrow in b) is best seen in the posterior projection.
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Figure 24b. Superior (a) and posterior (b) volume-rendered spiral CT images obtained through a cast show the extent of a comminuted calcaneal fracture (arrow), which was the result of a three-story jump from a burning building. The orientation of the fracture fragments seen on these views is helpful for presurgical planning, and the intraarticular nature of the fracture (arrow in b) is best seen in the posterior projection.
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Infection
An increasing number of examinations are being performed for the evaluation of known or suspected musculoskeletal infection. This increase is due to the high prevalence of intravenous drug use in our patient population and to the growing population of immunocompromised patients, which includes persons with acquired immunodeficiency syndrome, transplant recipients, renal dialysis recipients, and patients with underlying malignancies (27).
Spiral CT with volume rendering is valuable in detecting infection and in determining which compartments are involved (subcutaneous tissue, fascia, muscle, bone) and the extent of the process. This information is needed for patient triage (medical vs surgical management) and for monitoring the response to surgical or antibiotic therapy.
Iodinated contrast material is necessary for defining the extent of disease. Abnormal muscle generally enhances less than normal muscle, and the disease process is thereby accentuated. However, rim enhancement of an abscess is not uncommon in these cases (28). Definition of the vascular anatomy is also helpful in these cases, and 3D CT angiography may be performed as needed.
Osteomyelitis.—Spiral CT with volume rendering can be used to evaluate cortical bone and associated soft-tissue masses in suspected osteomyelitis of the spine (Fig 25). Infection of the sternoclavicular joint is most common among patients with the human immunodeficiency virus and intravenous drug users (Fig 26), although we have seen cases in patients with neither of these risk factors (29). In patients with osteomyelitis of these and other regions, extension of infection and inflammation into adjacent soft tissues is not uncommon. CT can be used to assess the extent of disease and monitor the response to therapy.
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Figure 26. Osteomyelitis in a patient with a history of intravenous drug abuse. Anterior volume-rendered spiral CT image shows osteomyelitis with erosion of the proximal right clavicle and the manubrium (arrow). The infection was due to S aureus.
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Soft-Tissue and Muscle Abscess.—Patients with acquired immunodeficiency syndrome or a history of intravenous drug use have an increased prevalence of soft-tissue abscesses, which often have subtle clinical manifestations (Fig 27). Use of contrast-enhanced spiral CT optimizes lesion detection during the preequilibrium phase, even in patients with poor tissue planes. The extent of involvement is well demonstrated on multipla–nar and 3D images; these images are useful in surgical planning, especially when the area of involvement is extensive (Fig 28).
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Figure 27a. Soft-tissue abscess in a patient with a history of drug use and shoulder pain. Lateral (a) and posterior (b) volume-rendered spiral CT images obtained after administration of intravenous contrast material show a large abscess that involves the right shoulder. The full extent of the abscess is seen, including involvement of the supraspinous (arrowheads), infraspinous (solid arrow), and teres minor (open arrow) muscles. Blood cultures were positive for S aureus.
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Figure 27b. Soft-tissue abscess in a patient with a history of drug use and shoulder pain. Lateral (a) and posterior (b) volume-rendered spiral CT images obtained after administration of intravenous contrast material show a large abscess that involves the right shoulder. The full extent of the abscess is seen, including involvement of the supraspinous (arrowheads), infraspinous (solid arrow), and teres minor (open arrow) muscles. Blood cultures were positive for S aureus.
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Figure 28a. Soft-tissue abscesses in a 33-year-old woman with systemic lupus erythematosus, which was being treated with steroids. The patient presented with shoulder and arm pain, and the steroid dosage was increased to treat the suspected flare of the disease. Anterior (a) and inferior (b) volume-rendered spiral CT images obtained after administration of intravenous contrast material show multiple hypoattenuating, rim-enhancing abscesses in the right arm and chest wall (arrows). These abscesses, which track through the arm musculature, are seen to be interconnected. Group A streptococci were cultured from the abscesses.
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Figure 28b. Soft-tissue abscesses in a 33-year-old woman with systemic lupus erythematosus, which was being treated with steroids. The patient presented with shoulder and arm pain, and the steroid dosage was increased to treat the suspected flare of the disease. Anterior (a) and inferior (b) volume-rendered spiral CT images obtained after administration of intravenous contrast material show multiple hypoattenuating, rim-enhancing abscesses in the right arm and chest wall (arrows). These abscesses, which track through the arm musculature, are seen to be interconnected. Group A streptococci were cultured from the abscesses.
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Tumors
For primary bone tumors, plain radiography remains the mainstay of lesion detection and differential diagnosis. Although MR imaging has become the leading modality for evaluating the extent of bone and soft-tissue neoplasms, many studies, including the Radiology Diagnostic Oncology Group trials (30), have demonstrated that CT is nearly as efficacious. CT remains superior to MR imaging in the detection of cortical destruction and lesion calcification.
Spiral CT with volume rendering is useful in defining the full extent of primary (Fig 29) and metastatic (Fig 30) bone tumors (1). Although bone scintigraphy is an excellent screening study for metastases, cross-sectional imaging with CT is much more specific when symptoms are localized to a particular anatomic region. Soft-tissue masses are best evaluated with the addition of intravenous contrast material.
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Figure 29. Anterior volume-rendered spiral CT image shows a large lytic lesion involving the superior aspect of the sacrum (arrow). No matrix was seen. This finding proved to be a giant cell tumor. On the basis of the CT appearance, the differential diagnosis would also include metastatic disease, chordoma, and neurofibroma.
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Anatomic information gained from 3D CT can be useful in planning oncologic therapy, whether it be surgery or radiation therapy (31) (Fig 31). Three-dimensional images are especially valuable in anatomically complex areas such as the ribs, pelvis (Fig 32), shoulder (Fig 33), and spine (Fig 34) and have proved very useful in communicating information to referring clinicians. Follow-up scans allow monitoring of the response to therapeutic interventions.
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Figures 31-34. (31) Posterior volume-rendered spiral CT image shows a sclerotic lesion within the left aspect of the transitional L5 vertebral body (arrows). The study was performed for preoperative planning. The lesion represented a Ewing sarcoma, which appeared to be localized to this vertebral body. (32) Anterior volume-rendered spiral CT image shows a destructive, midline lesion of the sacrum (arrow). The lesion was a biopsy-proved chordoma. (33) Superior volume-rendered spiral CT image shows extensive replacement and destruction of the scapula by advanced multiple myeloma. (34) Lateral volume-rendered spiral CT image shows a mixture of sclerotic (thin arrows) and lytic (thick arrow) lesions within the lumbar spine, findings consistent with the patient's known lymphoma. The superior end plate of the L3 vertebral body has collapsed (top thin arrow).
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Figures 31-34. (31) Posterior volume-rendered spiral CT image shows a sclerotic lesion within the left aspect of the transitional L5 vertebral body (arrows). The study was performed for preoperative planning. The lesion represented a Ewing sarcoma, which appeared to be localized to this vertebral body. (32) Anterior volume-rendered spiral CT image shows a destructive, midline lesion of the sacrum (arrow). The lesion was a biopsy-proved chordoma. (33) Superior volume-rendered spiral CT image shows extensive replacement and destruction of the scapula by advanced multiple myeloma. (34) Lateral volume-rendered spiral CT image shows a mixture of sclerotic (thin arrows) and lytic (thick arrow) lesions within the lumbar spine, findings consistent with the patient's known lymphoma. The superior end plate of the L3 vertebral body has collapsed (top thin arrow).
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Figures 31-34. (31) Posterior volume-rendered spiral CT image shows a sclerotic lesion within the left aspect of the transitional L5 vertebral body (arrows). The study was performed for preoperative planning. The lesion represented a Ewing sarcoma, which appeared to be localized to this vertebral body. (32) Anterior volume-rendered spiral CT image shows a destructive, midline lesion of the sacrum (arrow). The lesion was a biopsy-proved chordoma. (33) Superior volume-rendered spiral CT image shows extensive replacement and destruction of the scapula by advanced multiple myeloma. (34) Lateral volume-rendered spiral CT image shows a mixture of sclerotic (thin arrows) and lytic (thick arrow) lesions within the lumbar spine, findings consistent with the patient's known lymphoma. The superior end plate of the L3 vertebral body has collapsed (top thin arrow).
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Figures 31-34. (31) Posterior volume-rendered spiral CT image shows a sclerotic lesion within the left aspect of the transitional L5 vertebral body (arrows). The study was performed for preoperative planning. The lesion represented a Ewing sarcoma, which appeared to be localized to this vertebral body. (32) Anterior volume-rendered spiral CT image shows a destructive, midline lesion of the sacrum (arrow). The lesion was a biopsy-proved chordoma. (33) Superior volume-rendered spiral CT image shows extensive replacement and destruction of the scapula by advanced multiple myeloma. (34) Lateral volume-rendered spiral CT image shows a mixture of sclerotic (thin arrows) and lytic (thick arrow) lesions within the lumbar spine, findings consistent with the patient's known lymphoma. The superior end plate of the L3 vertebral body has collapsed (top thin arrow).
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Postoperative Imaging
When the results of plain radiography fail to answer the clinical question in a postoperative orthopedic patient, cross-sectional imaging with CT or MR imaging is generally attempted. In such patients, CT is often markedly limited secondary to extensive streak artifact from implanted hardware and MR imaging is limited by susceptibility artifact.
Spiral CT with volume rendering is often able to compensate for streak artifact, and studies are usually quite successful despite the presence of metal plates, pins, or prostheses (Figs 35 –37). For this reason, volume-rendered spiral CT has become our modality of choice for postoperative cross-sectional imaging in orthopedic patients.
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Figures 35-37. (35) Complex pelvic injury reduced intraoperatively. Spiral CT with 3D reconstruction was performed to determine if the reduction was successful. Despite extensive metal artifact due to plates and screws in the pubic symphysis, both iliac crests, and the sacrum, superior (a) and inferior (b) volume-rendered spiral CT images show successful reduction of the fractures. Specific detail, especially that of the left sacral fracture (arrows in a), is well defined. (36) Anterior volume-rendered spiral CT image (transparent mode) shows a pin inserted through an impacted fracture of the left femoral neck. The study was performed to confirm accurate placement of the pin, which is well seen on this image. No streak artifact from the pin is seen. (37) Anterior volume-rendered spiral CT image shows a failed right hip prosthesis, which is superiorly displaced relative to the osseous acetabulum. Several cerclage wires are broken. A slight streak is visible, but osseous detail in the region of the prosthesis failure can still be seen.
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Figures 35-37. (35) Complex pelvic injury reduced intraoperatively. Spiral CT with 3D reconstruction was performed to determine if the reduction was successful. Despite extensive metal artifact due to plates and screws in the pubic symphysis, both iliac crests, and the sacrum, superior (a) and inferior (b) volume-rendered spiral CT images show successful reduction of the fractures. Specific detail, especially that of the left sacral fracture (arrows in a), is well defined. (36) Anterior volume-rendered spiral CT image (transparent mode) shows a pin inserted through an impacted fracture of the left femoral neck. The study was performed to confirm accurate placement of the pin, which is well seen on this image. No streak artifact from the pin is seen. (37) Anterior volume-rendered spiral CT image shows a failed right hip prosthesis, which is superiorly displaced relative to the osseous acetabulum. Several cerclage wires are broken. A slight streak is visible, but osseous detail in the region of the prosthesis failure can still be seen.
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Abstract
INTRODUCTION
