British Journal of Radiology (2004) 77, S46-S63
© 2004 British Institute of Radiology
doi: 10.1259/bjr/52620263
Multislice CT in imaging of trauma of the spine, pelvis and complex foot injuries
R Watura, MBBCh, MRCP, FRCR,
M Cobby, MBBCh, MRCP, FRCR and
J Taylor, DCR
Department of Accident and Orthopaedic X-ray, Frenchay Hospital, Beckspool Road, Frenchay, Bristol B16 1JE, UK
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Abstract
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Multislice CT (MSCT) has greatly enhanced the performance of CT scanners and has vastly improved imaging of musculoskeletal trauma. Fast, high resolution scanning is now possible. In our institution, MSCT is an essential part of the imaging of the traumatized patient. The advantages of volume imaging, such as multiplanar reconstructions (MPRs) with near isotropic viewing, three-dimensional imaging and thick slice (wedge) MPRs (mimicking conventional radiographs), enable more accurate assessment of complex anatomical areas such as the spine, pelvis and foot. We discuss the general principles of scanning for musculoskeletal trauma and describe our experience of MSCT of the traumatized spine, pelvis and foot.
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Introduction
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Multislice CT (MSCT) has added advantages for musculoskeletal trauma imaging, including volume imaging, the ability to acquire multiplanar reconstructions (MPRs) with near isotropic viewing, three-dimensional (3D) images and thick slice (wedge) MPRs that mimic conventional radiographs. All these are achieved following a single data acquisition without the need for gantry angulation. We discuss the general principles of MSCT scanning in musculoskeletal trauma, review the current literature and describe our experience using MSCT scanning for imaging trauma of the spine, pelvis and complex injuries of the foot.
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General principles of musculoskeletal MSCT scanning
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Several technical factors need to be taken into account when scanning patients following musculoskeletal trauma. For scanning the limbs or joints, the affected part should ideally be positioned in the centre of the gantry. Owing to detector design, there is more data interpolation at the periphery of the gantry and therefore spatial resolution is greatest at the centre [1].
Motion artefact should, as far as possible, be avoided. The smallest area possible should be irradiated. Artefact from bony structures that do not need to be included in the volume should be avoided.
Use of high peak voltage is recommended for imaging bone. We use 140 kVp for spine and pelvis and 120 kVp for imaging the foot. This increases the likelihood of penetrating the bone and at the same time reduces the total dose to the patient (lower milliampere per second (mAs)) [1].
If articular surfaces are of particular interest, the "principle of obliquity" should be applied. This is described as follows: for the volumetric acquisition required for MPR of a joint, the affected joint should be placed at an oblique angle relative to the gantry to allow for the maximum number of slices to traverse the joint surface. This improves the quality of MPRs. If the joint is parallel to the gantry, fewer slices will traverse the cortical surfaces of the bone [1]. Alteration of gantry angle is not required with MSCT scanning and volume imaging.
To facilitate treatment planning, measurements of joint deformity are often required. It is therefore essential to obtain an adequate field of view, extending the anatomical coverage beyond the specific region of interest to include enough of the adjacent diaphysis and thus allow more accurate measurements to be made [1].
Imaging post-operative patients with metallic implants may present a challenge. Metal causes artefacts such as beam hardening. The metal artefacts depend on the composition of the hardware (titanium produces the least artefacts, cobalt chrome alloys produce the most). Artefact also depends on the geometry of the implant (its thickness and orientation), and is most severe in the direction of the thickest portion of the artefact. Metal artefact also depends on kVp and mAs, reconstruction algorithm and the MPR slice thickness and orientation. Use of high kVp reduces artefact by increasing the likelihood of X-ray penetration. Increasing mAs increases photon flux striking the CT detectors and also reduces artefact but this must be balanced against increased radiation dose. Use of bone or edge enhacement should be avoided as it increases hardware artefact. It is recommended that standard bony or soft tissue reconstruction algorithms are used when imaging patients with dense metal implants. Use of thicker slice widths for MPR reduces metal artefact by averaging pixels [1].
MSCT images are reconstructed using filtered backprojection or reconstruction algorithms. These are referred to as "kernels" for Siemens volume zoom. The higher the kernel number the sharper the image, and the lower the number the smoother the image. We recommend detailed or sharp reconstruction algorithms for MPR.
Image reconstruction (MPRs and 3D) at increments near or equal to 50% of the slice width is recommended. For 3D imaging, we recommend the use of the smoothest available kernel (filter or algorithm), for example B10. This is because suboptimal or poor quality images are obtained if there is excess noise on the original (axial) slices from which the 3D images are obtained and this is most likely if a bone algorithm is used. Production of two sets of images from the raw data is therefore required if MPR and 3D images are needed.
Handling the large amount of data produced by volume scanning can be a challenge. When imaging large areas such as the lumbar spine, these data sets take a relatively long time to create and manipulate and also take up a comparatively large amount of archive space. In our experience, overlaps of 20% for image reconstruction will give good image quality for most routine scans of these areas and the data produced are proportionately less and therefore more manageable. The large volume of data produced is also a challenge and time consuming for the reporting radiologist. Soft copy (workstation) reviewing and reporting may reduce the time spent handling the large numbers of images. Selected images can then be produced on hard copy to best communicate the pathology to the clinicians.
Hard copy formats depend on the injury complexity and the number of images. We use either 4 x 5 (20) format or 3 x 4 (12) format.
MSCT scanning of patients following musculoskeletal trauma may therefore be summarized as a three phase process: (1) acquisition of raw data (scanning technique); (2) production of appropriate image data; and (3) manipulation of the data sets to produce images.
The advantages and potential disadvantages of MSCT are summarized in Table 1
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Cervical spine
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The reported incidence of cervical spine trauma in head injury patients ranges from 4% to 8% [2, 3]. Prior to the development of helical CT scanning, particularly MSCT, plain films were the only radiological means of imaging the cervical spine to exclude or diagnose injury following blunt trauma. Two main protocols were used: (i) three-view plain radiograph series (anteroposterior (AP), lateral and open mouth); and (ii) five-view series (AP, lateral, open mouth and both obliques) [4]. There are several limitations of plain film radiography for evaluation of injuries. These include poor visualization of areas with overlapping structures such as the dens, craniocervical junction and lower cervical spine. The presence of other pathology such as osteoarthritis or rheumatoid arthritis may interfere with assessment of injuries of the cervical spine. Additional views such as obliques and "Swimmer's" may be necessary, and some views require the injured patient to be moved. The traditional single slice CT scanners also had several disadvantages when imaging the injured cervical spine. Data acquisition was slow, resolution was limited and image reconstruction in additional planes was either not possible, was cumbersome or was time consuming and of poor quality. MSCT is now widely used to complement plain radiography for examination of the cervical spine following blunt trauma and there are an increasing number of publications in the literature on the subject of MSCT for cervical spine trauma. The positive contribution of MSCT for examination of this complex region is now well recognized [4–14]. Crim et al [6] reviewed the literature on the clearance of the cervical spine in multitrauma patients. According to their findings, lateral radiographs detect 60–70% of cervical spine fractures but some fractures are not visible even on three views. Sensitivity of plain films may be increased by attention to several subtle features. CT detects 97–100% of fractures but accuracy for detection of ligamentous injury is not documented. MRI is highly sensitive in detection of ligamentous injury [6].
Many of the injuries demonstrated at CT and not identified by plain radiography may not be clinically significant. It is therefore imperative that radiologists and spine surgeons have criteria for distinguishing between injuries requiring stabilization and those that do not. Daffner [14] proposed a new classification of cervical vertebral injuries into two categories, major and minor. Injuries were classified as major if the following radiographic and/or CT criteria were present: displacement of more than 2 mm in any plane, wide vertebral body in any plane, wide interspinous/interlaminar space, wide facet joints, disrupted posterior vertebral body line, wide disc space, vertebral burst, locked or perched facet joints (unilateral or bilateral), "hanged man" fracture C2, dens fracture, or type III occipital condyle fracture. All other fractures were considered minor.
In our institution, conscious patients with possible spinal injury initially undergo three-view plain radiography of the cervical spine (AP, lateral and odontoid peg). Supplementary views such as "Swimmer's" are carried out where deemed feasible or safe, if required. MSCT is carried out if plain radiographs are abnormal or inadequate, to evaluate suspicious areas or injuries demonstrated on plain radiographs or if there remains clinical concern. The indications for MSCT of the cervical spine are summarized in Table 2.
Intubated trauma patients requiring a head CT scan also require "clearing of the cervical spine". The diagnostic quality of plain radiography (especially for the craniocervical junction) for the intubated patient is often inadequate due to artefact from anaesthetic and other equipment. We carry out MSCT of the craniovertebral junction to C3 at the same time as the head scan on these patients.
The following examples demonstrate some of the advantages of MSCT. In the first example, the lower cervical spine could not be visualized at plain film radiography (Figure 1a). MSCT was carried out and demonstrated injuries that were not diagnosed on the plain radiographs. A fracture of the posterior inferior corner of C6 was present (Figure 1b). A unilateral facet dislocation of C6 on C7 was shown (sagittal image; Figure 1c) and a fracture of the lateral mass of C6 was clearly demonstrated on the coronal images (Figure 1d). The second example demonstrates the value of MSCT for evaluation of injuries seen on plain films and also for diagnosis of other injuries that may be present. Fracture of C7 was seen on the lateral plain radiograph (Figure 2a). MSCT, however, demonstrated additional fractures not seen on plain radiographs (Figures 2b–f). MSCT was particularly useful in the diagnosis and evaluation of the fragment retropulsed into the cervical canal (Figures 2b and 2c). In the third example, a fracture of the dens was suspected on the plain radiographs of a patient with rheumatoid arthritis but MSCT provided clear confirmation of the injury (Figures 3a–e). MSCT demonstrated additional fractures of C1.
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Figure 1. (a) Plain lateral radiograph of the cervical spine. Only five vertebral bodies are visualized. (b) Sagittal image showing fracture avulsion of the posterior inferior corner of C6. (c) Sagittal CT scan on the same patient showing unilateral perched facet (left) C6 on C7. (d) Coronal image, same patient, showing fracture of the lateral mass of C7.
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Figure 2. (a) Sagittal plain radiograph of the cervical spine. There is an acute kyphotic angulation at C6/7 and a fracture of C7 with anterior wedging due to collapse of the superior endplate. There is a separated bone fragment from the superior endplate of C7. The C6/7 disc space is widened posteriorly. No other fractures were demonstrated on plain film. (b) Sagittal multislice CT (MSCT) showing fractures demonstrated on the plain radiograph, but in addition there is a retropulsed fragment of the posterior superior part of C7 into the canal. (c) Axial MSCT image of the fractured C7 vertebra showing the retropulsed fragment posteriorly displaced into the canal. (d) Sagittal MSCT image showing fracture of the inferior facet of C6. This was not diagnosed on the plain film. (e) Coronal MSCT showing fractures of the uncovertebral processes of C7. (f) Coronal MSCT image showing additional burst fracture at C6 not demonstrated on the plain radiograph.
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Figure 3. (a) Lateral plain radiograph of the upper cervical spine in a rheumatoid patient following trauma. A fracture of the posteriorring of C1 is clearly demonstrated. There is also a possible fracture across the base of the dens but this area is not as well visualized. (b) Open mouth view of the dens. There is suspicion of a fracture through the base of the dens. (c) Sagittal multislice CT (MSCT) image confirms an undisplaced type II dens fracture. (d) Coronal MSCT scan of the dens clearly demonstrating type II fracture through the base of the dens. (e) A fracture through the anterior part of the C1 ring is demonstrated on the axial MSCT image. This was not demonstrated on any of the plain radiographs.
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MRI also has a role in the evaluation of cervical spinal injury [15]. We carry out MRI for suspected soft tissue or ligamentous injury and for evaluation of damage to the cervical spinal cord. MRI in such cases is superior to MSCT. Figure 4a demonstrates abnormal prominence of the pre-vertebral soft tissues in a traumatized patient. MSCT did not demonstrate a fracture (Figure 4b). MRI was conducted to evaluate the ligaments and soft tissues (Figure 4c). This confirmed extensive haemorrhage in the pre-vertebral soft tissues and also posterior to the cervical spine, consistent with extensive ligament and soft tissue injury. A linear bright signal across the C5 vertebral body was consistent with recent injury or fracture.
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Figure 4. (a) Plain lateral radiograph of the cervical spine showing marked prominence of the pre-vertebral soft tissues following trauma. Pre-vertebral haemorrghage is suspected. There are degenerative changes at C5/6 with narrowing of the disc space and sclerosis at the endplates. (b) Sagittal multislice CT showing osteophytes at C6/7 and disc space narrowing consistent with degenerative changes. No fracture is identified and the pre-vertebral soft tissues are not adequately visualized. (c) A sagittal T2 weighted MR image with fat suppression showing bright signal within the pre-vertebral soft tissues with central areas of low signal. Bright signal is also present posteriorly and between the spinous processes. The appearances are consistent with haemorrghage and oedema due to soft tissue and ligament injury (anteriorly and posteriorly). There is linear signal across the C5 vertebral body in keeping with marrow haemorrghage or a fracture.
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Protocol for scanning the cervical spine
We obtain a lateral topogram of the cervical spine with the patient lying supine, neck hyperextended slightly (where possible), head secure in the head cradle and arms at the sides with shoulders relaxed. No breath-holding is required. The scanning (data acquisition) protocols are summarized in Table 3. Data sets of 1 mm slice widths with 0.8 mm increments are reconstructed on a bony (very sharp B70) algorithm. These are used to produce MPRs. The reconstruction protocols are detailed in Table 4. Similar data sets using a very smooth algorithm (any window) are also reconstructed. These are used to produce 3D images if required (see Table 4).
Intubated patients undergoing MSCT head scan are scanned on a 1 mm collimator protocol (this allows for all appropriate reconstructions for brain, skull and facial injuries). The scan area is extended caudally to include C1 and C2. Data sets are reconstructed as above. The reconstruction protocols are detailed in Table 4
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Thoracic and lumbar spine
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Although both the thoracic and lumbar regions of the spine are vulnerable to injury during trauma, each region has unique anatomic and biomechanical features that result in different characteristic patterns of injury. MSCT is invaluable for the evaluation of these structures and associated injuries of the thoracolumbar spine. The upper thoracic vertebrae are particularly difficult to demonstrate clearly by plain radiography. MSCT is superior to plain films for delineating extension of fracture fragments into the spinal canal [16–18]. MRI is used to assess the spinal cord and surrounding soft tissues, including the presence of epidural haematoma. Marrow oedema may also indicate bone injury at other levels.
Most patients at risk of thoracolumbar spine injury after blunt trauma also undergo abdominal and pelvic CT to exclude intra-abdominal injuries. Gestring et al [18] conducted a study to determine whether a lateral CT scanogram and axial CT views would provide adequate imaging to allow for evaluation of the thoracolumbar junction and lumbar spine and therefore eliminate the need for conventional screening computed lumbar spine radiographs. 71 patients were scanned. All abnormalities identified by plain radiographs were seen using CT plus scanogram. Eight transverse process and two spinous process fractures not seen at computed lumbar spine radiography were identified using CT and scanogram. The authors concluded that CT and scanogram protocol outperformed computed lumbar spine radiographs in detecting fractures.
Wintermark et al [19] conducted a prospective study to determine whether MSCT can replace conventional radiography and can be performed alone in severe trauma patients for depiction of thoracolumbar spine fractures. Mean sensitivity and interobserver agreement for detection of unstable fractures were, respectively, 97.2% and 0.951 for MSCT and 33.3% and 0.368 for conventional radiography. The median times to perform a conventional radiographic and MSCT examination were 33 min and 40 min, respectively. Effective radiation doses at conventional radiography of the spine and thoracoabdominal MSCT were 6.36 mSv and 19.42 mSv, respectively. The costs of conventional radiography and MSCT were, respectively, US $145 and $880 per patient. The authors concluded that MSCT is a better examination for depicting spine fractures compared with conventional radiography, can replace plain radiography and can be performed alone in patients who have sustained severe trauma.
Replacing plain films with MSCT as suggested by Wintermark et al [19] would not only have resource implications but would also significantly increase radiation exposure to patients, the vast majority of whom will have normal spines that are adequately examined by plain film radiographs.
For suspected thoracic and lumbar spinal injuries we perform standard AP and lateral radiographs followed by MSCT to better demonstrate and evaluate any abnormalities or suspicious areas identified on plain radiographs. Ideally at least two vertebrae above and two vertebrae below the level of injury should be included in the scan. Indications for MSCT of the thoracic and lumbar spine are summarized in Table 2.
The examples shown in Figures 5a–g demonstrate the value of MSCT for evaluating the injured thoracic and lumbar spine, particularly the use of MPRs to assess injuries of complex structures such as the facet joints and the posterior elements. MSCT (Figure 5d) demonstrated lateral subluxation and rotation of L2 (coronal plane) with a fracture of the inferior right corner of the vertebral body. Fracture of the left lamina of L3 was shown, as was disruption and widening of the right L2/3 facet joint (Figure 5e). MRI was used to evaluate the canal and neural structures pre-operatively (Figure 5g). This demonstrated canal narrowing due to a retropulsed fracture fragment from the posterior superior aspect of L3. The fragment was compressing the nerve roots of the cauda equina.
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Figure 5. (a) Plain radiograph (anteroposterior film) showing reduced height of L3 following fracture and widening of the right L2/3 facet joint space. (b) Plain radiograph (lateral) demonstrating fracture of L3. There is collapse of the superior endplate of the vertebral body with anterior wedging. (c) Sagittal multislice CT (MSCT) showing burst fracture of L3 with multiple fragments anteriorly and posteriorly and a fracture of the lamina of L3 not identified on the plain film. (d) Coronal MSCT demonstrates burst fracture of L3 but also fracture of the inferior right corner of L2. The L2 vertebral body is laterally translocated and rotated anticlockwise relative to L3. (e) Coronal MSCT showing disrupted right L2/3 facet joint and fracture of the left lamina of L3 as a result of fracture/rotation of L2 on L3. (f) Three-dimensional MSCT clearly demonstrating fracture of the inferior endplate of L2 and burst fracture of L3. (g) Sagittal T2 weighted MRI showing compression of the cauda equina nerve roots by the retropulsed posterior superior fracture fragment of L3. (h) Sagittal T2 weighted MR image showing severe narrowing of the spinal canal by the retropulsed posterior superior fracture fragment of L3.
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Protocol for scanning the thoracic and lumbar spine
A lateral tomogram is obtained with the patient lying supine, arms positioned comfortably above the head and legs supported by a cushion. Scan acquisition protocols are summarized in Table 3. Reconstruction protocols are detailed in Table 4.
We have occasionally been requested to review MSCT images to exclude significant spinal injury in patients who had undergone CT scanning of the chest, abdomen and/or pelvis for assessment of internal organ injuries but who had not been scanned specifically to exclude spinal injuries. In such cases, we extract additional data sets from the original scans as follows: (i) reduce the field of view to the area of interest, e.g. thoracic or lumbar spine; (ii) reconstruct 3 mm slice widths on 1.5 mm increments using a sharp kernel (B70). The axial 3 mm slices can then be viewed for bony injury. If no bone injury is seen, wedge (thick slice) sagittal and coronal MPRs are produced on hard copy (Figures 6a and 6b). These mimic plain film radiographs and are available to view by the referring clinicians in a more familiar format. The main advantage of wedge MPRs is the absence of overlying soft tissues. The main disadvantage is the loss of bony detail and the possibility of missing pathology if the entire anatomical area of interest is not included (very thick slice widths, e.g. 70 mm). If the soft copy review demonstrated abnormality or fracture, MPRs may be produced from the existing data but, if required (e.g. complex injury), a repeat scan specifically for the thoracic and lumbar spine may be necessary.
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Figure 6. (a) Normal sagittal thick slice multiplanar reconstruction (MPR) of the thoracolumbar spine mimicking lateral plain radiograph. (b) Coronal thick slice MPR of the thoracolumbar spine mimicking anteroposterior radiograph.
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Pelvis
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Acetabular and pelvic fractures are frequently complex and the precise pathological anatomy is not easily demonstrated by routine radiographs [20]. Plain radiographs alone have limited sensitivity for detection of pelvic fractures compared with CT [21, 22]. Fractures of the sacrum and acetabulum are most easily overlooked [20, 21]. CT is also useful for detecting intra-articular fragments, fragment interposition, marginal impaction and occult pelvic ring fractures [23]. When CT is combined with radiographs of the pelvis, the sensitivity for fracture detection is increased [24]. 3D imaging enables complex pelvic anatomy to be better visualized without having to move or rotate the patient and reduces the need for additional X-ray views and radiation exposure. Hard copy printouts of 3D reconstructions provide additional and more accessible information for the orthopaedic surgeons to enable surgical planning. 3D images are not a substitute for good plain radiographs and analysis of axial CT images and other reformats [24].
Wedegatner et al [25] assessed the usefulness of MSCT in the detection and classification of pelvic and acetabular fractures. Routine radiographs and MSCT of the pelvis with sagittal and coronal multiplanar images were performed in 50 patients with clinically suspected pelvic or acetabular fractures. The images were evaluated by a trauma surgeon and a radiologist in terms of fracture detection, classification and visualization. The AO classification of fractures was used. MSCT detected additional fractures in 17% of patients (two acetabular and five posterior pelvic rim fractures) not seen on conventional radiographs. In three other patients, MSCT changed the fracture classification by delineating the full extent of the fracture. MPR visualized seven acetabular and four sacral fractures better than axial source images. They concluded that MSCT of the pelvis is required for correct fracture detection. Jurik et al [26] investigated the total effective radiation dose from spiral CT and conventional radiography of the pelvis with regard to fracture classification. Their results showed an 11% lower calculated effective dose following spiral CT compared with the dose obtained by actual measurement using the Alderson phantom, but the result for conventional radiography was 68% higher.
All patients admitted to our institution with a suspicion of injury to the bony pelvis are investigated with a series of plain films (AP, inlet and outlet and Judet views where necessary) followed by MSCT if fractures or suspicious areas are demonstrated. Patients with normal plain films do not require MSCT. Table 2 is a summary of indications for pelvic MSCT scan.
The following are examples of the application of MSCT for imaging the traumatized pelvis. Figure 7a demonstrates fracture of the acetabulum on a plain radiograph. Figures 7b and 7c are sagittal and axial MSCT images of the same pelvis demonstrating the fracture and the separated posterior column fragment. The fragment is clearly demonstrated on the 3D images, both transluscent (Figure 7d) and surface shaded (Figure 7e). A Judet type view of the pelvis is demonstrated (Figure 8a) showing sacral, acetabular and pubic rami fractures. The MPRs and 3D images of the pelvis can be produced and manipulated in any desired plane to suit the reporting radiologist or the orthopaedic surgeon. The 3D images (Figures 8c and 8d) are particularly useful for treatment planning.
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Figure 7. (a) Oblique plain radiograph showing posterior column fracture of the acetabulum with a large separated fragment. (b) Sagittal multislice CT (MSCT) reconstruction of the acetabulum showing the separated posterior column fragment. (c) Axial MSCT showing acetabular fractures. (d) Radiolucent three-dimensional (3D) image of the pelvis in the sagittal plane showing fracture of the posterior column with a large separated fragment. (e) 3D image of the pelvis showing posterior column fracture.
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Figure 8. (a) "Judet view" transluscent three-dimensional (3D) image of the pelvis demonstrating bilateral sacral fractures, fractures of the right acetabulum posterior wall and fracture of the left pubic rami. (b) 3D image of the pelvis showing bilateral sacral fractures, bilateral pubic rami fractures and fracture of the right transverse process of L5. (c) Pelvic inlet 3D reconstruction to demonstrate the pelvic ring and to assess displacement of the fractured left hemipelvis.
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Protocol for scanning the pelvis
Patient lying supine, arms comfortably on the chest or above the head and lower legs supported. No breathing command is required. Raw data acquisition is summarized in Table 5. Image data sets of 1 mm width slices on 0.8 mm increments on both sharp kernel (B70) and very smooth kernel (B10) are acquired. Reconstruction protocols are summarized in Table 4.
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Imaging the spine and pelvis of the polytrauma patient (4-slice scanner)
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For the polytrauma patient requiring a scan of the chest, abdomen or pelvis to exclude visceral injury, the normal soft tissue (chest, abdomen and pelvis) protocols should be adapted to acquire diagnostic bone images. This avoids irradiating the patient twice with two separate scan protocols for bone and then soft tissues if there is known bony trauma, fracture, or if injury needs to be excluded by MSCT. For example, in chest trauma patients who also require assessment of the thoracic spine by MSCT, the mAs should be increased from 85 mAs (chest scan protocol) to 125 mAs (acceptable for bone imaging). The collimation should be reduced from 2.5 mm (chest protocol) to 1 mm (suitable for bone imaging). This adaptation will create data sets from which reconstructions for chest (soft tissues) and the thoracic spine may be derived. For the bone images, the field of view should be reduced to the area of interest, a sharp filter should be applied and 1 mm thin slices with 0.8 mm increment data sets should be reconstructed to create a volume of data from which MPRs can then be produced.
For imaging the bony pelvis in the polytrauma patient, the soft tissue (e.g. abdomen and pelvis) scan protocol should be adapted. Intravenous contrast medium will most likely have been injected. This does not appear to cause any problems with image quality. The scan protocol is adapted by dividing the scan into two sections. The abdomen scan (diaphragm to iliac crests) is scanned with 2.5 mm collimation, then the pelvis scan (iliac crests to lesser trochanters) is scanned with 1 mm collimation. Routine axial soft tissue images of the pelvis for viscera are then acquired from the data set in addition to bony 1 mm slices on 0.8 mm increments for MPRs.
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Mid foot and calcaneum
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Complex foot injuries such as calcaneal fractures, tarsal injuries and fracture dislocations such as the Lisfranc injury are well demonstrated by MSCT [27]. Preidler et al [27] compared the diagnostic accuracy of conventional radiography, CT and MRI in patients with hyperflexion injuries of the foot. Accuracy for detection of bony and ligamentous changes were compared. 45 patients were included in the study. Conventional radiographs revealed 33 metatarsal and 20 tarsal fractures. Eight patients had tarsometatarsal (Lisfranc joint) malalignment. CT showed 53 metatarsal and 41 tarsal fractures. Joint malalignment (Lisfranc) was evident in 16 patients. MRI revealed 41 metatarsal fractures and 18 metatarsal bone bruises. Tarsal bones were fractured at 39 sites and there were nine tarsal bone bruises. Joint malalignment was evident in 16 patients, with Lisfranc ligament being erupted in 11 of the 16. The authors concluded that conventional radiographs including weight-bearing views were not sufficient for routine diagnostic work-up of patients with acute hyperflexion injuries in the foot and that CT should serve as the primary imaging technique for such patients.
MSCT has been used in the classification and pre-surgical assessment of complex fractures of the calcaneum [28–31]. MPR allows for accurate and detailed evaluation of fracture fragments, displacement, comminution and therefore appropriate treatment [31]. Complex injuries including subtalar joint dislocation can be assessed [32]. Bibbo et al [32] reviewed the records of all their subtalar joint dislocations over a 3-year period. Nine cases were identified. Plain films diagnosed a subtalar joint dislocation in all patients. CT scan identified additional injuries missed on the initial plain radiographs in all of the nine patients. Injuries diagnosed only after obtaining CT included: fracture of the medial aspect of the posterior talar facet (n=3), comminution of the posterior talar facet (n=1), fracture of the medial talar process (n=2), bone fragments in the sinus tarsi (n=3), fracture of the posterior talar process (n=1), avulsion of the sustentaculum tali (n=1), cuboid fracture (n=1) and second metatarsal base fracture (n=1).
Figures 9a–c demonstrate comminuted fracture of the calcaneum and the position of the main fracture fragments. Figure 10b demonstrates Lisfranc fracture with diastasis at the first and second metatarsals and the separated Lisfranc fragment.
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Figure 9. (a) Coronal multiplanar reconstruction (MPR) demonstrating comminuted fracture of the calcaneum and the main fracture fragments (sustentaculum, body and lateral fragment. (b) Axial MPR of comminuted calcaneal fracture demonstrating three main fracture fragments. (c) Sagittal MPR of comminuted calcaneal fracture.
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Figure 10. (a) Plain radiograph showing Lisfranc fracture. There is malalignment between the second metatarsal and the middle cuneiform indicating a dislocation at the tarsometatarsal joint. (b) Axial multislice CT showing Lisfranc fracture. Malalignment of the second tarsometatarsal joint is identified. There is associated avulsion fracture of the adjacent first cuneiform.
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Table 2 is a summary of indications for MSCT of the foot and calcaneum.
Protocol for scanning the foot and calcaneum
The patient lies supine with the unaffected limb bent at the knee to exclude it from the scan plane. The limb to be scanned is placed at 45° to the horizontal to avoid artefact from the tibia and fibula. Acquisition of the raw data set is described in Table 6. Image data sets of 1 mm slice width and 0.8 mm increment using sharp kernel (B70) and very smooth kernel (B10) are reconstructed from the raw data. Image reconstruction protocols are summarized in Table 4.
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Conclusion
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Radiological imaging has a crucial role in the evaluation of the acutely traumatized patient. Imaging should be fast, efficient and accurate to allow timely and appropriate patient management. MSCT has increased the speed, efficiency and accuracy of computed tomographic imaging in musculoskeletal trauma. Added advantages such as MPRs and 3D imaging have greatly enhanced the ability to evaluate complex anatomical areas such as the spine, pelvis and foot. The results are communicated to the surgeon in an accessible form.
Received for publication June 27, 2003.
Revision received January 22, 2004.
Accepted for publication February 12, 2004.
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Abstract
Introduction
