MRI is highly sensitive to soft tissues, especially for edema.⁸^–¹⁰ MRI is the best method for visualizing the spinal cord. Compression or deformity of the spinal cord, edema, and hemorrhage are visualized well with MRI. It is also excellent for evaluating the intervertebral discs (Fig. 171-1). For example, detection of an acute disc disruption may alter plans for the surgical approach.¹¹ Cases have been reported in which the reduction of a dislocation worsened symptoms because of further herniation or displacement of disc material. MRI permits detection of such herniation before surgery. Ligament disruption can occasionally be directly visualized with MRI, especially the anterior and posterior longitudinal ligaments. Some evidence suggests that the extent of ligament disruption may correlate with the risk of instability in cervical spine dorsal element fractures.

FIGURE 171-1 Traumatic disc and ligament disruption. Sagittal fast inversion-recovery MRI shows disruption of both anterior and posterior longitudinal ligaments at C5-6 and disc disruption with traumatic disc herniation and ventral and dorsal soft tissue edema. The findings were confirmed at surgery.

The soft tissues around the vertebra are also visualized with MRI. Extensive edema can serve as a marker for acute injury and the need for further evaluation. Deep, interspinous edema in the setting of acute trauma may be an indication of high risk of instability due to flexion injury. Tears or stretching of the anterior and posterior longitudinal ligaments are especially concerning for instability. MRI can be helpful in a variety of clinical situations in which the combination of clinical and initial imaging findings is ambiguous or nondefinitive. For example, degenerative changes are very common in the cervical spine and can make detection of acute fractures difficult. Although CT can help to identify fractures, subluxation or chronic deformity can still be a challenge. A negative MRI in such situations, showing no evidence of any significant nearby soft tissue edema, makes acute injury unlikely as a cause of subluxation. MRI can be especially helpful when clinical assessment is limited, such as in the obtunded or intubated patient.¹²

Unexpected worsening of neurologic status after spine trauma can be due to a variety of factors. MRI may reveal such causes as epidural hematoma, disc herniation, or spinal cord edema from infarction.

The limitations of MRI mentioned earlier can be overcome in most cases. Monitoring is essential in the acutely injured patient. MRI-compatible monitoring equipment is available. The patient must be screened for the presence of metallic devices or metal within the body that would preclude MRI. Although standard spine coils may not be usable with a cervical collar, other coils can still permit diagnostic images. Faster imaging techniques continue to be developed for MRI, and it is often not necessary or appropriate to use the same sequences for acute trauma patients that would be used for evaluation of degenerative disc disease, for example. The specific sequences to be used can be tailored to the clinical situation.

Although it is impossible to specify MRI parameters that should be used because of the great variety of manufacturers, machines, and software available, some broad principles apply. A T2-weighted sequence is important for detecting edema. Fast-spin echo imaging, in which multiple echoes are acquired during each pulse sequence, is nearly always used in standard spine imaging. Such sequences are much faster than spin echo sequences and produce excellent signal-to-noise and high-quality images. Fast-spin echo sequences can be excellent for visualizing the spinal cord, for example. However, it is important to note that fat remains very bright on such sequences, even with T2-weighting. Therefore, if adjacent soft tissue edema is to be demonstrated, different sequences must be used that suppress the signal from fat (Fig. 171-2). A fat saturation pulse can be added to fast-spin echo imaging sequences. Alternatively, inversion recovery sequences (short-tau inversion recovery [STIR]) accomplish the same effect of heavy T2 weighting and fat suppression.

FIGURE 171-2 Fat suppression MRI for soft tissue evaluation. MRI was performed on a young man with myelopathic symptoms after a motor vehicle accident. A, Sagittal fast-spin echo T2-weighted image shows slight subluxation at C6-7, but edema from soft tissue injury is difficult to identify because fat also remains bright. B, On a sagittal fast-spin echo inversion-recovery image the fat is now dark, but edema in the dorsal soft tissues, the marrow space of upper thoracic vertebral bodies, and prevertebral space is very conspicuous. Anterior and posterior longitudinal ligaments at C6-7 are stretched (arrows).

Edema within the bone marrow is also well demonstrated with MRI. This appears as low signal (dark) on T1-weighted images, replacing the normally bright signal from fat in the marrow and high signal on fat-suppressed T2-weighted images. Acute fractures, especially those that involve the vertebral body, cause marrow changes, but bone contusions that do not result in cortical bone disruption can also result in marrow edema. Over time, marrow signal intensity usually returns to an appearance close to that of normal vertebral body marrow. Thus, signal intensity in marrow of a compressed vertebral body that matches that of adjacent, normal vertebrae provides evidence for a chronic rather than an acute injury.

The sensitivity of MRI for acute soft tissue injury depends on several factors. Edema resolves over several days; the precise time course has not been defined but clearly depends on the severity of the original injury. In our experience, soft tissue edema associated with an acute cervical spine injury is likely to be less extensive on MRI in the setting of axial load injuries. Presumably, this is due to less stretching and tearing of the soft tissues. For example, a minimally displaced Jefferson burst fracture may result in very little soft tissue edema. When these limitations are understood, however, MRI can play a very useful adjunct role in assessing major acute cervical spine trauma.

Motion Radiography Studies

Radiographs of the spine in different positions (i.e., flexion and extension views in a lateral position) are excellent for evaluating the stability of the spine in a delayed or chronic setting (Fig. 171-3). Such studies have significant limitations in the acute setting, however. Most importantly, they pose major risks if the spine is in fact unstable. Complications of flexion-extension radiographs are rare but well known. If motion studies are to be undertaken, it is highly desirable that the patient be fully alert, cooperative, and capable of controlling or stopping the motion. If flexion and extension are performed on an obtunded or comatose patient, performing the study under fluoroscopy should improve the safety. The motion can be immediately stopped as soon as subluxation or abnormal movement is visualized. However, data on the safety and accuracy of performing motion radiography on an obtunded patient in the setting of acute injury are very limited. Such studies are often time-consuming, and visualization of the cervicothoracic junction is frequently difficult.

FIGURE 171-3 Instability on flexion film. Flexion lateral radiograph obtained on a delayed basis after cervical spine trauma shows focal kyphosis and dorsal widening. Alignment in neutral position was normal.

A second limitation in the acute setting is a high incidence of muscle spasm or guarding. From one quarter to one third of patients with acute cervical spine injury may have nondiagnostic results because there is inadequate movement of the neck to assess stability.¹³ Delayed studies, several weeks after trauma and after muscle spasm has subsided, with the patient cooperative and in control of neck motion, remain the gold standard for evaluating the stability of the cervical spine.

Myelography

Because of the availability of MRI, myelography has a very limited role in the evaluation of spine trauma. There are occasional situations in which patency of the spinal canal must be assessed and MRI is not possible.

Imaging Findings

Cervicocranial Junction and Upper Cervical Spine

Upper cervical spine injuries can be multiple, complex, and difficult to identify on imaging.

C1 Fractures

The Jefferson burst fracture of C1 is a relatively common injury. CT can demonstrate the multiple fractures of the ring of C1 and the extent of displacement (Fig. 171-4). Plain films usually show prevertebral soft tissue swelling. Some components of the fractures can be visible on plain radiographs, more so if the fractures are displaced. The lateral masses of C1 are likely to be displaced laterally if the transverse ligament is disrupted. Total displacement of greater than 7 mm as seen on an odontoid view has been suggested as a guideline to the presence of transverse ligament rupture.¹⁴ The transverse ligament can be visualized directly on MRI, and fluid signal in place of the expected low signal intensity of the ligament is evidence of rupture.¹⁵

FIGURE 171-4 Jefferson fracture. Patient fell from a ladder onto the head. CT shows multiple fractures of the ring of C1.

The imaging findings just described apply to adults. In at least the first 4 years of life, the lateral masses of C1 often project lateral to the lateral margins of C2 on an AP view.¹⁶ Because Jefferson fractures are uncommon in children, CT may be necessary to demonstrate such fractures in children.

Isolated fractures of the dorsal arch of C1 can occur with hyperextension injuries (Fig. 171-5). In such cases, prevertebral soft tissue swelling would likely be absent, and the dorsal arch fracture can be seen on a lateral radiograph. Another type of hyperextension injury at C1 is an avulsion at the ventral caudal portion of the ventral arch, at the attachment of the atlantoaxial ligament. In this case, the fracture is visible on the lateral view, and focal prevertebral soft tissue swelling is usually present.