Magnetic resonance imaging (MRI)

MRIs use magnetic fields and radio waves to create three-dimensional images and two-dimensional cross-sections of the body. The images produced by MRI are similar to an X-ray, but show more information about the soft tissues of the body for a more detailed picture. This picture more clearly defines subtle abnormalities in the anatomy of the heart and great vessels.

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MRI machines have tubes that are big enough for a patient to fit inside. Radio waves and a strong magnetic field produced by electrical currents traveling through coils within the shell of the tube are directed toward the body during an MRI scan. The body responds by emitting weak energy signals that are detected by the MRI machine. A computer converts these signals into three-dimensional images that can be manipulated to show any two-dimensional cross-section.

Before the test you must remove all accessories and clothing containing metal, including jewelry, metal fasteners, dentures, wigs, and hearing aids. Metal objects may interfere with the magnetic field of the MRI, affecting the quality of the image.

An MRI exam is painless and generally lasts between 30 and 60 minutes. The procedure involves lying on a table that slides into the tube. Movement can blur the images, so it is important to remain still. Often patients will be offered headphones or earplugs to help block out the loud tapping and thumping sounds made by the machine. The space inside the tube is quite narrow, and some patients may feel claustrophobic.

Magnetic resonance angiography (MRA) is one of the newer uses of MRI. For this test, a contrast agent, akin to a dye, is injected into the arm or leg through an intravenous (IV) line. The dye makes it easier to visualize the heart and blood vessels. One advantage of the MRA is that it uses contrast dyes that are less likely to cause allergic reactions than those used during computed tomography (CT or CAT Scans).

There are no known risks from the magnetic fields and radio waves produced by MRI machines. However, people who have pacemakers, cardioverter defibrillators, or other metal-containing objects inside their bodies cannot get MRIs. Also, the affects of magnetic fields on fetuses are not well understood, so getting an MRI may not be advisable for pregnant women.