Magnetic resonance imaging (MRI) is a technique used in medicine for producing images of tissues inside the body. Physicians use these images to diagnose certain diseases, disorders, and injuries.

MRI is an important diagnostic tool because it enables physicians to identify abnormal tissue without opening the body through surgery. MRI does not expose the patient to radiation, unlike tests that use X rays. Also, MRI lets physicians see through bones and organs. MRI is safe for most people. But MRI uses a powerful magnet and so cannot be used on people with metal implants, such as pacemakers or artificial joints.

An MRI unit consists mainly of a large cylindrical magnet, devices for transmitting and receiving radio waves, and a computer. During the examination, the patient lies inside the magnet, and a magnetic field is applied to the patient's body. The magnetic field causes nuclei in certain atoms inside the body to line up. Radio waves are then directed at the nuclei. If the frequency of the radio waves equals that of the atom, a resonance condition is produced. This condition enables the nuclei to absorb the energy of the radio waves. When the radio-wave stimulation stops, the nuclei return to their original state and emit energy in the form of weak radio signals. The strength and length of these signals--and therefore the kind of image produced--depend on various properties of the tissue. A computer translates the signals into highly detailed cross-sectional images.

Excerpt from the "Magnetic resonance imaging (MRI)" article, The World Book Encyclopedia © 1999