Spine and Spinal Cord Surgery

Spring 2001
Volume 12, Number 1

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Imaging is playing an increasingly important role in the care of spinal and peripheral nerve disorders. X-ray, fluoroscopy, computed tomography (CT), and magnetic resonance imaging (MRI) all play an important role in the imaging work up. Further advances will see imaging at the forefront not only in the detection of spinal disease, but as an integral part of the treatment process.

Introduction. The widespread availability of cross-sectional imaging modalities has had a dramatic impact on spinal imaging. Prior to CT, plain film myelography was the only means of depicting intraspinal pathology by depicting compression of the thecal sac and nerve root sleeves. CT scanning provided excellent bony imaging in cross section, but still required myelographic injection for identification of thecal sac contents. The high contrast resolution of MRI allows depiction of the soft tissues within the spinal canal, making MRI the modality of choice for detailed assessment of most intraspinal pathology. Further improvements continue to expand the role of imaging in a variety of clinical circumstances.

Patients with back pain. Low back pain is the second most common complaint among patients presenting to primary care physicians, and 4 of every 5 people experience low back pain at some point in their lives.1 Although advanced imaging has had a huge impact on the evaluation of patients with back pain and/or radiculopathy, many questions remain to be answered.

When to image. The timing of imaging evaluation in patients with nonspecific back pain can be problematic. Because the vast majority of patients with back pain will have resolution of symptoms, imaging is generally not indicated at initial presentation. For persistent symptoms, constitutional symptoms worrisome for tumor or infection, or if fracture is a concern, imaging is performed more promptly. Even in patients with radiculopathy, the indications for surgical versus conservative management are difficult to specify, making it difficult to justify routine imaging. Further complicating the issue is the high prevalence of imaging abnormalities, such as disc protrusions, in an asymptomatic population. Thus, a certain percentage of findings in symptomatic patients will merely be coincidental findings, unrelated to symptomatology. Despite these limitations, MRI is valuable to identify pathologic changes, to direct surgical intervention, and can be useful as a predictor of successful conservative therapy.

Which modality. MRI has achieved widespread acceptance for spine imaging, but plain radiographs remain a viable screening study and can detect degenerative or chronic changes in many cases. CT scanning is the best modality to visualize bony structures for fracture, osteophyte and other osseous changes but has limited visualization of the contents of the thecal sac. Myelograhic contrast injection improves visualization of the thecal sac, allowing imaging quality comparable to MRI for patients unable to tolerate MRI or for confirmation of MRI findings. MRI has superior soft tissue resolution for detection of abnormalities of the disc, thecal sac contents, bone marrow, and paraspinous tissues. (Figure 1)

Figure 1. Spectrum of disc disease in a single patient. IA. Sagittal MRI T2-weighted. IB-D. Axial proton-density images. MRI accurately depicts disc, vertebral marrow, and thecal sac contents. In this patient, L3-4 shows normal disc signal and morphology (1B). At L4-5, there is annular bulging (1C) and at L5-S1 a central disc protrusion is present (1D).
Fig 1A	Fig 1B
Fig. 1C	Fig 1D

Open MRI scanners typically have lower magnetic field strength compared to conventional MR scanners, which leads to significantly decreased signal to noise. One advantage of open imaging systems is the ability to perform imaging with varied patient positioning. Kinematic studies in the spine performed with flexion and extension can demonstrate position dependent changes in size of the neural foramina, ligamenta flava, and spinal canal.2

Future directions. An improved understanding of the pathologic basis for back pain syndromes will allow more reliable correlation between imaging findings and patient symptoms. As gradient strength and processing speeds improve, faster imaging studies, with higher resolution and decreased artifact will result. Because of the high prevalence of back pain, the feasibility of limited, rapid MRI for screening patients has been considered.

Bar Imaging will also play an even greater role in spine intervention. Currently, real time image guidance is primarily limited to CT or fluoroscopy used during needle placement for percutaneous procedures. Typically, intraoperative image guidance is based on an image dataset acquired preoperatively, or with cumbersome Xray guidance. Improved imaging technology will result in direct intraoperative imaging with real-time image guidance. Minimally invasive procedures will dominate as imaging replaces direct visualization of structures during surgical intervention. Once image guidance is perfected, surgical intervention itself can be steered by the imaging data, leading to robotic interventions.

Trauma. Evaluation and treatment of cervical spinal trauma has rapidly progressed as imaging has improved. Imaging plays a vital role in assessment of trauma patients. The initial assessment requires rapid screening of critically ill patients for spinal injury. Generally, plain film evaluation is the initial screening study. The continued refinement of spiral CT scanners, including multidetector scanners, has allowed rapid, accurate detection of spinal injuries. Studies have shown that in patients with severe trauma spiral CT of the entire cervical spine is an acceptable screening exam, preferably in conjunction with plain film assessment. The combination of spiral CT and plain film will detect cervical fractures with a sensitivity approaching 100%.3 Patients with suspected ligamentous injury can be evaluated with flexion/extension radiographs. Unconscious or uncooperative patients may need MRI assessment to detect subtle ligamentous injury.

Once spinal injury has been detected, imaging workup shifts from a screening study to a diagnostic evaluation of degree of injury and to plan possible intervention. Spiral CT with axial imaging and coronal and sagittal reconstructed images accurately displays 3-dimensional bony anatomy. (Figure 2) Associated canal or for aminal compromise can be evaluated. In patients with evidence of cord injury, MRI can easily detect injury to the cord substance or compression of the cord by disc, bone, or epidural hemorrhage, which would require immediate decompressive surgery.

Fig 2. Spiral CT in trauma. Axial (2A) images demonstrate fracture through C2 at the base of the odontoid process. However, sagittal reconstructed images (2B) are needed to clearly depict the degree of subluxation at C1-2.
Fig 2A	Fig 2B

Following cord injury, MRI is currently used to evaluate the status of the cord and to look for possible cord tethering or syrinx formation. In the future, as more sophisticated methods of neural protection and axonal regrowth are employed, MRI will play an increasingly important role in monitoring spinal cord injury patients. Diffusion weighted MRI shows promise for early detection of Wallerian degeneration in cord injury, and may be useful in following response to axonal regrowth strategies.4

Spinal Tumors. Neoplastic involvement of the spinal canal may be the result of tumor invasion from the adjacent vertebral column, blood borne or CSF spread of tumor directly to the the cal sac, or from primary tumors of the contents of the canal.

Evaluation of the vertebrae for tumor can be accomplished with technetium bone scan, plain radiography, CT, or MRI. In patients with compression fractures, initial reports indicated a role for diffusion imaging to distinguish benign from pathologic fractures. Further experience has shown limited specificity of diffusion imaging in differentiating these entities. Routine MRI with and without gadolinium contrast often will allow differentiation of benign from malignant fractures, although occasionally follow up exam will be necessary to demonstrate interval healing of benign fractures or progressive disease in malignancy.5 (Figure 3)

Fig 3. Benign insufficiency fractures. Sagittal T1 image (3A) shows loss of vertebral height at multiple vertebral levels in the thoracic and lumbar spine. Fat-saturated T1 post Gadolinium image (3B). Despite the abnormal enhancement seen at the T11 level, the normal marrow signal in the remaining levels is consistent with multiple benign fractures, with enhancement in the acute fracture.
Fig 3A	Fig 3B

Tumors within the spinal canal require imaging with MRI. MRI can easily distinguish tumors within the cord (intramedullary) from tumors within the thecal sac but outside the cord (intradural, extramedullary) and tumors outside of the dural sac (extradural). MRI with gadolinium will establish which compartment a tumor resides in, and will demonstrate the relationship to adjacent neural elements. The imaging characteristics of the tumor, including evidence of hemorrhage, enhancement, and associated cysts, will all help in arriving at a differential diagnosis. The vast majority of cord tumors are of glial origin, either ependymoma or astrocytoma. Intradural, extramedullary tumors are usually neurofibromas or schwannomas, or occasionally tumor implants from seeding of the CSF. Extradural tumors are often the result of spread from the adjacent bone, often by metastatic disease, lymphoma, or hemangioma.6

Advanced MR techniques, such as functional MR and MR spectroscopy are now employed for pre and postoperative evaluation of patients with intracranial neoplasms, but these techniques have been slow to develop in the spine. The unique anatomy of the spinal cord, with the close proximity of the bony canal throughout, challenges the ability of the MR scanner to maintain the field uniformity necessary for these exams. In the future, improved or higher field magnets may solve these problems.

Peripheral Nervous System. Peripheral nervous system imaging has lagged behind the tremendous advances in CNS imaging. However, with improved microsurgical procedures for repair of damaged nerves, an impetus has arisen for high resolution imaging of peripheral nerves to depict sites of nerve injury and assess results of interventions. Electrophysiologic studies can detect nerve injury with high sensitivity, but they lack specificity, and are unable to anatomically display the site of injury. Concurrent with this need for improved detection of peripheral nerve lesions, MR surface coil technology has advanced to allow high resolution imaging of these structures. Using phased array surface coils, current imaging techniques provide reliable images of peripheral nerves measuring at least 2 to 3 mm in diameter. These include nerves of the brachial and sacral plexus and major nerves in the upper and lower extremities.7 In addition to direct imaging evaluation of the peripheral nerves, MR can detect early changes of denervation in the muscle groups innervated by the injured nerve.

Common indications for peripheral nerve imaging include evaluation of patients with suspected compressive neuropathy or entrapment syndromes, and evaluation of involvement by mass lesions, such as a nerve sheath tumor, metastasis, lymphoma, or desmoid tumor. Patients with traumatic nerve injury may benefit from imaging to identify the site of injury and from postoperative MR to assess response to therapy.7

Conclusion. Advances in spinal imaging will continue in concert with improved technology and better understanding of spinal diseases and their treatment. Further technologic innovation will add to the armamentarium of neuroimagers and allow accurate depiction of spinal structure and function. Imaging will extend from the pre and postoperative realm, to play a pivotal intraoperative role, helping to redefine treatment of spinal disease.

Address comments and questions to:

David H. Dungan, MD
Swedish Medical Center
Department of Radiology
501 E. Hampden Avenue
Englewood, CO 80113

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