Magnetic Resonance Imaging

Magnetic resonance imaging [MRI, also called nuclear magnetic resonance (NMR)] is receiving more and more attention as we strive to improve the quality of the cross-sectional images of the body so useful in medical diagnosis and treatment. MRI does not expose the patient to potentially hazardous X rays or injected contrast materials such as those employed to obtain computerized axial tomography (

CAT

) scans. The three major components of an MRI system are a huge magnet that can weigh up to

100

tons, a table for transporting the patient into the circular hole in the magnet, and a control center, as shown in [Fig. 1]. The image is obtained by placing the patient in the tube to a precise depth depending on the cross section to be obtained and applying a strong magnetic field that causes the nuclei of certain atoms in the body to line up. Radio waves of different frequencies are then applied to the patient in the region of interest, and if the frequency of the wave matches the natural frequency of the atom, the nuclei will be set into a state of resonance and will absorb energy from the applied signal. When the signal is removed, the nuclei release the acquired energy in the form of weak but detectable signals. The strength and duration of the energy emission vary from one tissue of the body to another. The weak signals are then amplified, digitized, and translated to provide a cross-sectional image such as the one shown in [Fig. 2]. MRI units are very expensive and therefore are not available at all locations. In recent years, however, their numbers have grown, and one is available in almost every major community. For some patients the claustrophobic feeling they experience while in the circular tube is difficult to contend with. Today, however, a more open unit has been developed, as shown in [Fig. 3], that has removed most of this discomfort. Patients who have metallic implants or pacemakers or those who have worked in industrial environments where minute ferromagnetic particles may have become lodged in open, sensitive areas such as the eyes, nose, and so on, may have to use a

CAT

scan system because it does not employ magnetic effects. The attending physician is well trained in such areas of concern and will remove any unfounded fears or suggest alternative methods.