Summary of Key Points
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Imaging the spine after trauma should be rapid, accurate, and performed in concordance with the primary and secondary trauma survey.
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High-quality computed tomography imaging of the cervical spine is the initial imaging of choice for patients with suspected cervical spine trauma.
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Magnetic resonance imaging is particularly suited for evaluation of ligamentous injury, spinal cord compression, and spinal cord injury.
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Computed tomography angiograms may be performed for patients with blunt or penetrating cervical spine trauma and suspected vertebral artery injury.
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Newer imaging techniques and specific imaging sequences can be tailored to provide clinically relevant information on both spine stability and neural injury.
Imaging of the spine is an essential part of the early management of the polytrauma patient. Most major trauma centers have a variety of imaging modalities available for acute spinal trauma. The choice of imaging modality is determined by the need to evaluate bony, ligamentous, soft tissue, or neural elements. In the acute setting, the choice of imaging should provide a rapid and accurate assessment regarding spinal stability, as well as guide clinical management. It is important to recognize that the availability of rapid imaging techniques does not replace a good clinical examination, and imaging should supplement and confirm findings of the primary and secondary trauma survey.
This chapter reviews the various imaging modalities available for acute spine trauma, their individual merits and drawbacks, important imaging findings, as well as future imaging techniques.
Imaging Modalities for Acute Spinal Trauma
A number of imaging tools are available for evaluating a patient with suspected acute spinal trauma. Current guidelines for radiographic assessment of the trauma patient provide evidence-based recommendations for initial radiologic evaluation, though subsequent studies need to be individualized based on each patient’s cardiopulmonary stability, additional injuries, comorbidities, and neurologic examination.
Plain Radiographs
Plain radiographs continue to be the most rapid technique to evaluate spinal alignment and bony anatomy. With the increasing use of computed tomography (CT) imaging, the role of plain radiographs has diminished in the acute setting. However, for select trauma patients this may be the most suitable imaging modality available.
In cases of cervical spine trauma, lateral, anteroposterior, and odontoid views can provide a quick assessment for fractures, dislocations, and malalignment. Lateral views can detect fracture-dislocations and subluxations from the occipitocervical junction to the lower cervical spine. Important anatomic correlates on lateral cervical radiographs are shown in Figure 125-1 . These images also provide subtle markers of spine trauma such as prevertebral soft tissue swelling and widening of the interspinous spaces, both of which could represent ligamentous injury. The major limitations of lateral x-rays include evaluation of the uppermost and lowermost regions of cervical spine, as well as the dorsal elements. In patients with a short neck and prominent shoulders, swimmer’s view can be used to obtain transaxillary views of the C7-T1 junction, though this often provides only limited visualization of the bony anatomy. Nondisplaced facet, laminar, or spinous process fractures are often not seen on lateral radiographs. Anteroposterior views of the cervical spine can detect rotational as well as translational subluxations and dislocations. Open-mouth odontoid views can detect odontoid fractures and fracture-dislocations, though these images can be difficult to obtain in the intubated and obtunded patient. For thoracolumbar injuries, lateral radiographs can detect loss of alignment and vertebral body fractures, though inadequate visualization due to poor penetration of x-rays can be a problem.
Dynamic radiographs can provide valuable information on spinal stability in patients with cervical spine trauma. In the acute setting, patient cooperation and muscle relaxation play a key role in obtaining these images in a safe and effective manner. It is preferable not to obtain these images in the obtunded patient, even if fluoroscopic guidance is available. Due to the potential risks of inducing or worsening spinal cord compression, it is preferable that these studies are performed in the subacute setting when the patient is alert and cooperative.
The sensitivity of plain radiographs in detecting cervical spine injury is low, varying from 37.5% to 64%. However, for select patients, plain radiographs have a role in providing rapid initial assessment of the bony anatomy. Current guidelines for radiographic assessment of patients with acute cervical trauma recommend that plain radiographs (lateral, anteroposterior and odontoid views) be used when high-quality CT imaging is not available.
Computed Tomography
CT imaging is the current standard of imaging for patients after acute trauma, particularly for the evaluation of the cervical spine with a sensitivity of 90% to 100%. Current guidelines recommend high-quality cervical spine CT imaging for awake, symptomatic, as well as obtunded trauma patients. In patients with head injuries and those who are obtunded, CT imaging of the cervical spine is often combined with imaging of the head. The availability of multidetector CT scanners allows for rapid image acquisition producing high-quality axial images to exclude unstable cervical spine injuries. Sagittal, and coronal reconstructions, which are now routine, provide excellent visualization of alignment and fracture pattern. In addition, three-dimensional reformatted images can better define complex fractures. Some of the limitations of CT imaging include poor visualization of neural structures, motion artifacts, and the requirement for the patient to be supine, which may pose a problem in certain hemodynamically unstable polytrauma patients.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is not routinely used for initial evaluation of the trauma patient. MRI is usually performed after initial clinical and radiologic (CT or x-rays) evaluations require further delineation of neural structures: the spinal cord, nerve roots, or brachial plexus. MRI also provides better definition of intervertebral disc herniation as well as soft tissue and ligament injuries.
MRI detects pre- and paravertebral soft tissue swelling, which often indicates traumatic injury to the spine. High signal intensity in the anterior and posterior longitudinal ligaments as well as interspinous ligaments can indicate potential ligamentous injury and instability, particularly in the case of cervical spine injuries. Short-tau inversion recovery (STIR) sequences produce fat-suppressed images that better delineate ligament and soft tissue injury. T2-weighted MRI is ideal for demonstrating spinal cord edema, hemorrhage, or compression. High signal intensity on T2-weighted images indicates edema within the spinal cord, whereas a focus of acute hemorrhage has low signal intensity on T2-weighted images and gradient echo images.
The use of MRI for the initial evaluation of trauma patients is limited largely by long scanning times. Although monitoring is available in many MRI suites, it is not advisable to keep trauma patients for extended periods of time on account of pain and hemodynamic instability. The use of a few specific sequences with appropriate coils can reduce scanning time. Current guidelines for the use of MRI in acute spine trauma are based on limited available data. A meta-analysis suggests MRI has a sensitivity of 100% and a specificity of 95% for cervical spine injuries. A normal MRI within 48 hours in obtunded and unevaluable patients with normal cervical spine CT or radiographs may be used to guide the need for cervical immobilization.
Novel Imaging Techniques: Diffusion Tensor Imaging
Diffusion tensor imaging (DTI) of the spinal cord is an evolving field directed at improving the ability to estimate neural integrity after spinal cord injury. DTI is based on characterization of microstructure by measuring the directional diffusion of water molecules within neural tissues. Measurement of DTI indices such as fractional anisotropy within the spinal cord at the level of spinal cord injury as well as rostral to the level of injury show changes consistent with axonal injury ( Fig. 125-2 ). To date, only moderate correlation has been demonstrated between these indices and neurologic function in the acute setting. With refinement, these measurements may be utilized to estimate neurologic function after acute spinal cord injury, particularly in patients who cannot undergo a complete neurologic exam due to sedation, spinal shock, or associated injury. Faster image acquisition and standardized automated postprocessing is necessary to establish DTI as a clinical tool for acute spinal cord injury.
Imaging Findings for Specific Spine Injuries
The following section provides a brief description of common spine injuries in acute spine trauma.
Craniocervical Junction and Upper Cervical Spine
Anteroposterior visualization of the upper cervical spine requires either an open-mouth odontoid view or a coronal CT reconstruction. Normal studies typically demonstrate symmetrically articulating occipital condyles and C1 lateral masses, with the odontoid well centered between the lateral masses of C1. The articular surfaces of occipital condyles and the C1 superior facets should be equidistant to each other. Plain radiographs utilize a variety of reference lines for screening occipitocervical ligamentous injuries ( Table 125-1 ).
| Reference Line | Definition |
|---|---|
| Powers ratio | The ratio (AB/CD) of distance from the basion (A) to the dorsal arch of the atlas (B) and the distance from opisthion (D) to the anterior arch of the atlas (C). A value > 1 is suggestive of atlanto-occipital dislocation. |
| X-line method | The first line from basion to the axis-spinolaminar junction should intersect C2 and the second line from opisthion to the posteroinferior corner of the axis body should intersect the C1. A deviation from these interactions is considered abnormal. |
| Harris lines (BDI and BAI) | Basion axis interval (BAI): distance between basion and a dorsal axial line upward from the dorsal vertebral body of C2. A normal value is between 6 mm to 12 mm. Basion dense interval (BDI): distance between basion and the tip of the dens. A value >12 mm is considered abnormal. |
| Condylar gap method | The distance between occipital condyle and the superior articular facet of the atlas. A value > 2 mm is considered abnormal. |
Occipitocervical Dislocation
The direction of displacement of the cranium relative to cervical spine forms the basis of classification of occipitocervical dislocation. Type I injuries (anterior) involve anterior displacement of the occiput over atlas, whereas type II injuries (longitudinal) include distraction injuries with vertical displacement, and type III injuries (posterior) involve posterior displacement of the occiput.
Plain lateral radiographs are the initial screening modality used to calculate the Powers ratio and Harris lines (see Table 125-1 ). The bony landmarks to determine the Powers ratio can be difficult to identify, whereas Harris lines are easier to perform and less sensitive to the direction of dislocation. Studies support the use of the basion dense interval (BDI) (with a cutoff of 10 mm) on CT scans and the occipital condyle–C1 interval (with a cutoff ≥ 2 mm considered abnormal) on plain radiographs as the diagnostic tests of choice.
Occipital Condyle Fracture
An open mouth odontoid view can visualize a fracture of the occipital condyles, but thin-slice CT with image reconstruction is a superior imaging modality. Type I condyle fractures are compression fractures from excessive axial loading and produce comminution of occipital condyle. Type II fractures are basilar occiput fractures that extend into the occipital condyle due to a shear mechanism. The occipital condyle can be partially attached to the base of the cranium and in such cases occipitocervical stability is maintained. Type III fractures result from avulsion, an outcome of lateral bending and forced rotation. Due to ligamentous injury, type III are the condyle fractures most likely to be unstable.
C1 Fractures
Fractures of C1 constitute 2% to 13% of all acute cervical spine fractures and are often bilateral, nondisplaced, and stable. The Jefferson burst fracture is a relatively common and unstable fracture ( Fig. 125-3 ). It occurs due to axial loading that causes breaks in the anterior and posterior arches and disruption of the transverse ligament. Lateral films show prevertebral swelling due to hemorrhage and a widened predental space between the anterior arch of C1 and odontoid. A predental interval > 3 mm in adults and > 5 mm in children is considered abnormal. The odontoid view identifies a bilateral offset of the lateral masses of C1 relative to C2. The rule of Spence states that a lateral overhang > 6.9 mm of C1 relative to C2 in odontoid view is used to identify a transverse atlantal ligament rupture. The number should be adjusted to 8.1 mm owing to radiographic magnification. CT can demonstrate multiple fractures of the ring of C1 and the extent of displacement. MRI, though, is more sensitive for evaluating the transverse atlantal ligament.
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