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.
Fig. 1: Magnetic resonance imaging equipment.
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.
Fig. 2: Magnetic resonance image
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.
Fig. 3: Magnetic resonance imaging equipment (open
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.