Chapter 8

As the millennium has just passed, it is appropriate to review the significant advances in spinal imaging that have occurred in the preceding quarter century. Before then, plain film radiography, conventional tomography, and myelography with either gas or oily material as contrast agents had been the only methods available for imaging abnormalities involving the vertebrae, intervertebral disks, spinal cord, or cauda equina.

By 1975, a nonionic intrathecal contrast agent, metrizamide, was approved for clinical use. Unlike oily agents, nonionic contrast carried negligible risk for arachnoiditis, was absorbable, and thus eliminated the need for its removal from the thecal sac. Secondly, its neurotoxicity was minimal, compared with ionic water-soluble media, which never achieved widespread acceptance in the United States.

In 1977, the introduction of whole-body computed tomography (CT) permitted direct cross-sectional imaging of both spinal and paraspinal structures. However, the margins of the spinal cord could only be reliably demonstrated after the intrathecal administration of water-soluble contrast. This procedure is known as CT myelography (CTM). Because of the greater contrast sensitivity of CT, as compared with plain film myelographic technique, a smaller, less potentially neurotoxic dose of contrast agent could be administered for CTM. Nevertheless, this procedure still requires a lumbar puncture, with its attendant hazards to the patient.

By 1982, magnetic resonance imaging (MRI) became clinically feasible. MRI has proven to be superior to CT because the spinal cord and nerve roots could be visualized directly without the requirement for intrathecal contrast material. Most significantly, the parenchyma of the spinal cord could now be imaged and assessed for intrinsic pathology, such as multiple sclerosis plaques. These lesions may not alter the shape of the spinal cord, and, therefore, would be undetectable by CTM. Secondly, MRI provides multiplanar imaging, including sagittal and coronal orientations, with spatial and contrast resolution equivalent to the axial plane. Lastly, MRI poses no known health risk as it uses only radiofrequency energy, not ionizing radiation as is the case with CT.

Sagittal plane MRI is ideal for extended, rapid evaluation of the entire vertebral column, a procedure facilitated because the spine is arranged in a sagittal plane. Recent improvements in MRI receiver coil design (phased array coil) have provided the capacity to image the entire spine with excellent detail in less than 10 minutes (Fig. 8-1). This is especially helpful in the evaluation for spinal metastases, as these patients are often in pain, and thus have difficulty in remaining motionless for MRI. Newer pulse sequences, including half-Fourier turbo-spin echo (HASTE), can provide interpretable scans in less than 10 seconds, albeit with reduced spatial resolution compared with conventional magnetic resonance studies. HASTE imaging can also suppress some metal-induced artifacts arising from surgical hardware (Fig. 8-2), allowing improved visibility of anatomy or pathology otherwise obscured by these artifacts.

###### Figure 8-1

Midline sagittal ...

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