Among imaging methods, MRI is unique in that it does not depend on ionizing radiation, as does the conventional x-ray examination. The basis of MRI is to direct radio waves at protons, particles that help make up the nucleus of hydrogen atoms. When this is done in a strong magnetic field generated by a large magnet that surrounds the patient's body, the protons are alternately "excited" and "relaxed," emitting signals that are processed by a computer program to form images. Because protons are most abundant in the hydrogen atoms of water (the "H" in H20), MRI images depict differences in the content and distribution of water in various tissues. With MRI, different types of tissue within the same body structure are clearly displayed in fine anatomic detail. In the spine, for instance, fatty tissue, cerebrospinal fluid, and the central portion of the material making up the intervertebral disks contain considerable water, more than is found in bone, cartilage, and nerve tissue. MRI is well suited to detecting conditions that increase the amount of fluid, such as tumors, inflammation, and infection.
A typical MRI exam includes two to six imaging sequences, which produce sectional views or "slices" of the spine in different planes: left to right, front to back, upper to lower. The sections are often about a quarter-inch apart, providing a detailed look at the tissues making up the spinal column. The images may be stored in a computer and subsequently viewed on screen, or they may be printed on film much like a conventional x-ray. Depending on the location of symptoms, you probably will have only part of your spine imaged: the cervical (neck) portion, the thoracic (chest) spine, or the lumbar (lower) spine.