Myelography utilizes intrathecal contrast material to demonstrate the contours of the spinal cord and spinal roots. It can be accomplished either by conventional fluoroscopy or by CT. By injecting water-soluble radiopaque contrast through an LP needle and then placing the patient in the Trendelenberg position, the entire spinal subarachnoid space can be visualized (Fig. 2-2A–C). The procedure is almost as harmless as the LP except for cases of complete spinal block, in which high concentrations of contrast near the block can cause pain and regional myoclonus. Iophendylate (Pantopaque), a formerly used fat-soluble dye, is still approved by the FDA but is now employed only in special circumstances (visualizing the upper level of a spinal canal lesion that completely obstructs the flow of water soluble dye). If iophendylate is left in the subarachnoid space, particularly in the presence of blood or inflammatory exudate, it may incite arachnoiditis of the spinal cord and brain. MRI, because of its ability to clearly demonstrate intrathecal structures has largely supplanted contrast myelography as discussed in a later section.
Risks of CT
The primary risk of CT is radiation exposure, and overexposure can have clinical consequences ranging from relatively benign alopecia to leukomalacia and neoplasia. The interested reader should refer to FDA guidelines on the subject (http://www.fda.gov/Radiation-emittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX-Rays/ucm115317.htm). Given the need for repeated CT examinations in certain patients, tracking of total radiation exposure may be advisable and may find greater use in the future. CT is usually deferred during pregnancy unless the mother’s health is at imminent risk (i.e., following trauma). The potential harm to a fetus from radiation depends on gestational age and total absorbed dose. It is noteworthy that the fetal radiation dose from maternal cranial CT is lower than from maternal pelvic CT.
The risks of contrast infusion include allergic reactions and nephropathy, which is most often transient and reversible, but can be more severe in patients with underlying renal dysfunction. Intravenous contrast in generally withheld if the glomerular filtration rate (calculated GFR) is less than 30 mL/min/1.73 m2; if GFR is 30 to 60, hydration and, discontinuation of potentially nephrotoxic medications should precede the administration of contrast, particularly nonsteroidal anti-inflammatory agents, cisplatin-containing chemotherapy and aminoglycoside antibiotics. Repeated infusions of contrast should be done cautiously.
Many engineers, mathematicians, and physicists made contributions to the technology of nuclear MRI, and a Nobel Prize was awarded to Lauterbur and Mansfield for its development. MRI provides images in any plane, and it has the advantage over CT in using nonionizing energy and providing higher resolution views, and improved contrast between different structures within the nervous system. For visualization of many neurologic lesions, MRI is the preferred procedure.