Advantages of MRI

Major advantages of MRI over other techniques include:

• The possibility of obtaining images in all three planes.

• Superior contrast resolution compared with other modalities, providing a higher detection rate for soft-tissue injuries, particularly the spinal ligaments.

• The possibility of generating myelographic-equivalent images to assess the epidural space for evidence of hematoma, bone fragments, herniations and osteophytic changes without the need for administering contrast.

• Contrast-rich imaging of the spinal cord to allow detection of contusion, hematoma or laceration.

• Reliable prognostic information regarding potential for recovery of function based on the MRI appearance of spinal cord injuries.

• The possibility of imaging the paravertebral vessels without the need for contrast application, providing instead various pulse sequences to impart either positive or negative contrast of the vessels.

• MRI is performed without ionizing radiation and is thus free of side effects.

Fig. 7.1 Normal sagittal T1-weighted SE image of the cervical spine. The image displays the spinal cord with moderate signal intensity, surrounded by CSF with less signal intensity. The vertebral bodies show relatively high signal intensity on account of the fatty marrow. The anterior longitudinal ligament and the ventral portion of the annulus fibrosus appear as structures with low signal intensity (arrows) and are therefore well delineated from the higher signal of the disk and prevertebral tissue. The posterior longitudinal ligament and the annulus on the other hand are, together with the ligamentum flavum, somewhat poorly demarcated due to the low contrast relative to CSF.

A variety of imaging sequences are required to provide an exact portrayal of both normal anatomy and the various pathological changes. Some new sequences have recently been introduced to improve further the diagnostic reliability of the examinations and reduce scanning time.

The following scanning regime is generally employed

Fig. 7.2 Normal sagittal T2-weighted SE image of the cervical spine. The image displays CSF with a very high signal in contrast to the moderate signal intensity of the spinal cord. The signal of the vertebral body is lower than on the T1-weighted image, while the intervertebral disks appear brighter because of the high signal of the nucleus. The posterior longitudinal ligament with the annulus (black arrows) and the ligamenta flava (white arrows) are well delineated due to the adjacent structures, which demonstrate higher signal intensity. The dark line in the center of the spinal cord represents an artifact along the phase-encoding direction (Gibbs or truncation artifact) (with permission Mirvis S.E.: Semin. Musculoskelet. Radiol. 1998, 2 27–43).

Fig. 7.3 Normal axial T1-weighted SE image through the mid-third of the cervical spine. This image shows the spinal cord surrounded by low-signal CSF. The exiting nerve roots (white arrows) are well delineated as they approach the intervertebral foramina (black arrowheads). Blood flowing in the vertebral arteries appears dark (flow void phenomenon; black open arrows). Cortical bone appears uniformly dark (with permission Mirvis S.E.: Semin. Musculoskelet. Radiol. 1998, 2 27–43).

Fig. 7.5 Normal time-of-flight angiography of the cervical vessels. Flowing blood demonstrates a high signal intensity using this two-dimensional GRE time-of-flight technique. The right vertebral artery appears hypoplastic. The horizontal part of the vessel above the loop of the vertebral artery (arrow) shows complete loss of signal due to in-plane signal saturation effects — a typical phenomenon. The suppression of the venous signal is achieved by a saturation pulse applied above the cervical region (with permission Mirvis S.E.: Semin. Musculoskelet. Radiol. 1998, 2 27–43).

Another sequence, the IR sequence, is also recommended to identify ligament injuries and to distinguish osteophytes from bulging or herniated disks.

Recently, three-dimensional volume acquisition of MR data has been introduced and offers the potential of acquiring the entire cervical region in one scanning procedure and reformatting images through any plane with the resolution of a direct two-dimensional scan. The procedure is particularly useful for exact imaging of the foramina, although with agitated patients image quality is very limited (Ross, 1992; Georgy and Hesselink, 1994).

Echo planar imaging (EPI) or other fast imaging sequences can be useful when particularly short acquisition times are required. TSE and fast SE (FSE) sequences are associated with a significant reduction of acquisition time and allow the use of particularly long TR and TE times to achieve more heavily T2-weighted images and improve visualization of the nerve roots within the spinal canal on axial sections (Gundry and Fritts, 1997). TSE sequences also have advantages when imaging patients who have metallic hardware after surgery because metal-induced artifacts appear less pronounced due to decreased magnetic susceptibility effects. However, there is one disadvantage in that these sequences may emphasize CSF motion artifacts (Gundry and Fritts, 1997).

Both GRE and TSE (FSE) sequences are less sensitive in portraying bone-marrow edema than T2-weighted sequences. The combination of TSE (FSE) sequences with fat suppression techniques produces particularly contrast-rich MR myelograms that succeed very well in depicting extradural defects of the nerve roots sleeves. Furthermore, the extent of spinal stenosis can be much better assessed on these images.

Disadvantages of MRI

Disadvantages of MRI include:

• Bone is only poorly demarcated because it contains no hydrogen atoms. Only bone marrow containing fat and blood is depicted as hyperintense. Consequently, only major injuries to the bone are reliably shown on MRI, which should therefore not be considered a sensitive technique for detecting subtle bone injury, particularly injury involving the posterior vertebral elements (Schroeder et al., 1995).

• Currently, an MRI scan of the spine requires somewhat more time than comparable CT studies because of longer data acquisition. However, new imaging acquisition sequences are rendering MRI almost as fast or even faster than CT.

• Exact monitoring requires MR-compatible supervisory systems that can be used without restriction in the magnet room and without creating radiofrequency noise, which interferes with the image acquisition process (Shellock et al., 1993, 1995).

• The development of non-magnetic neck braces, patient transport systems and remote telemetric monitoring systems now allow MR examination of patients with acute spinal injury, in the majority of cases. However, patients with certain types of ferromagnetic intracranial aneurysm clips, cochlear implants and pacemakers must still be excluded from MR scanning and should be kept away from the magnetic field. Furthermore, patients with metal foreign bodies in the immediate vicinity of vital soft-tissue structures such as the spinal cord, nerve roots, or the orbita are at risk while undergoing MR examination, especially when these metallic foreign bodies show no scar-tissue formation within the tissue (Shellock, 1993). If there is any uncertainty whether the patient has in fact been exposed to metallic foreign bodies, e.g. in the case of welders, then radiographic screening should be performed prior to MR examination.

• It may be assumed that, due to severe claustrophobia, 3 % of patients can only be examined under sedation (Katz et al., 1994).

Indications for the Use of MRI in the Presence of Spinal Trauma

The use of MRI in spinal trauma has clearly increased with improved technology, especially with regard to reduced screening time as well as the introduction of MR-compatible devices for patient positioning and monitoring:

• Today MRI is indicated in the evaluation of all patients with a neurological deficit after spinal injury, especially to the cervical region, when permitted by the patient’s overall clinical state and in the absence of any absolute contraindications.

• MRI is particularly indicated in patients with an incomplete or progressive neurological deficit related to spinal trauma.

• Patients with a complete neurological deficit should also undergo MRI assessment if decompression is being considered in an attempt to promote nerve root recovery.

• A further indication for MRI of the spine is given in patients with myelopathy or radiculopathy after spinal trauma in whom previous radiological or CT studies have been negative and in cases where the level of the neurological deficit does not correlate with clinical evidence of location.

• MRI is also useful in deciding the need for internal fixation and the most appropriate surgical approach because MRI best assesses the level or levels of potential mechanical instability secondary to ligament injury, or the presence of spinal-cord compression from epidural processes such as disk herniation or reactive osteophytes.

   MRI and Soft-Tissue Injuries

Fig. 7.6 Contusion of the cervical cord. The sagittal T2-weighted image shows a hyperflexion fracture of C5 vertebral body with loss of anterior height. The high signal intensity in the vertebral bodies of C5 and C6 is due to marrow edema and hemorrhage, indicating that both vertebral bodies have been injured. The increased signal within the spinal cord at the levels C4 and C5 (arrows) is an indication of contusion-related edema. In addition, a prevertebral edema is apparent, recognizable by the bright signal anterior to the spine along C2–C6.

Fig. 7.8 Contusion of the cervical cord. The sagittal T2-weighted SE image in a patient who had sustained cervical spine injury displays increased signal intensity in the cord at C3/C4 level due to cord edema (open arrow). A hyperextension mechanism is assumed to be the cause and is indicated by the signal interruption of the anterior longitudinal ligament and annulus fibrosus at the C3/C4 level (arrow). Note the marked prevertebral edema (with permission Mirvis S.E.: Semin. Musculoskelet. Radiol. 1998, 2 27–43).

Fig. 7.9 Contusion of the cervical cord. T2-weighted TSE sequence in a patient with a teardrop fracture at the C5 level secondary to a hyperflexion trauma. The image demonstrates extensive cord edema in the C4–C6 segment (white arrows). There is also a circumscribed structure with reduced signal intensity recognizable at the C5 level, compatible with intramedullary hemorrhage associated with deoxy- and methemoglobin components (arrowheads). Moderate prevertebral edema.

Fig. 7.11 Spinal cord injury with development of edema and hemorrhage. Sagittal T2-weighted image in a patient with a bursting fracture of C5. The image shows a high signal intensity of the cord edema and a low signal in the center of this edematous zone (white arrow) indicating hemorrhage at the C4/C5 level. There is also marrow edema recognizable at C5 and C6 in the region of the vertebral bodies. Extensive prevertebral edema is also present. Retropulsion of C5 has resulted in cord injury. The posterior longitudinal ligament has been separated from the posterior cortex of C6 and is shown as a dark epidural line (arrowhead) (with permission Mirvis S.E.: Semin. Musculoskelet. Radiol. 1998, 2 27–43).

Fig. 7.12a, b Spinal cord injury with extensive hemorrhage. 28-year-old patient with tetraplegia secondary to a C5 teardrop fracture. The images show decreased signal intensity in the center of the spinal cord due to intracellular methemoglobin and deoxyhemoglobin with surrounding peripheral edema. The GRE sequence is more sensitive to field inhomogeneity created across the intact erythrocyte membranes and producing larger areas of signal loss. Edema is present in the C4/C5 disk and anterior C5 body:

a Sagittal T2-weighted SE image.

Types of spinal cord injury according to Kulkarni et al.