Magnetic resonance imaging involves using strong magnetic fields and radiofrequency (RF) energy to produce images based on the hydrogen content (primarily water) of body tissues.

Magnetic resonance imaging relies on the magnetic properties of atoms. When exposed to an external magnetic field, atoms tend to align uniformly within that field. RF energy is then directed at the atoms, knocking them out of this alignment and causing them to spin. When the RF pulse is discontinued, the atoms realign themselves with the magnetic field, emitting RF energy as a signal. The response time of this signal varies according to the density of atoms in the tissue. A computer can process this signal and display the results as a high-resolution image. Different substances (e.g. bone, neurons, body fluids) are represented as different brightnesses in this image.

Like X-ray imaging techniques (e.g. computed tomography or CT), MRI can "see through" the body. MRI has relatively less spatial resolution than CT, but better contrast.

Since the MRI scanner includes a strong magnet, it is important that patients undergoing MRI remove all metal objects such as jewelry or watches. Additionally, patients with metal embedded in the body (e.g. aneurysm clip, pacemaker, shrapnel, etc.) should not undergo MRI; the magnet can rip the metal object free, causing bleeding and even death.

Otherwise, MRI has no known risks and eliminates the danger from radiation in imaging techniques employing X-ray (e.g. CT scanning). The magnetic fields and RF pulses are imperceptible to the patient. The procedure does involve lying within the confines of the scanner, which some patients (especially those with claustrophobia) find uncomfortable. Open MRI scanners have now been developed to alleviate this problem.

Further Reading:

Illustrated Guide to Diagnostic Tests, 2nd edition. Springhouse Corporation, Springhouse PA, 1998

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