What is magnetic resonance imaging (MRI)?

Nancy Ferrari

Senior editor, Harvard Health

Magnetic resonance imaging (MRI) is a diagnostic technique that uses the electromagnetic properties of hydrogen molecules to collect information about organs and other tissues. A computer then converts this information into an image. MRI signals can give an image of a single slice of any part of the body, much like a slice of bread in a loaf. Usually, images are created of several “slices” of an organ or part of the body. The computer also can combine these slices into three-dimensional (3-D) images.

Because water molecules are especially sensitive to the forces used in this technique, MRI scans are very good at showing differences in water content between different body tissues. This is particularly helpful in distinguishing healthy tissue from cancer.

Before undergoing an MRI, your doctor will ask whether you have any electronic or metal medical devices in your body, such as a heart pacemaker or any metallic clips, pins, or screws. These precautions are necessary because the MRI magnet is so powerful that it could interfere with a pacemaker or displace some implanted clips.

MRI is a painless technique that usually takes about 45 minutes to a little over an hour. Most MRI scanners require you to lie inside a narrow cylinder. This can make some people feel anxious and claustrophobic. If you tend to feel anxious in tight places, ask your doctor for medication to help you relax during the procedure. A new type of MRI scanner, called an open MRI, is more comfortable for some people because it is open on all sides.

The procedure itself is very low-risk. Ionizing radiation, which is the type used to generate an x-ray or CT scan, carries some risk because too much exposure to this form of radiation can potentially damage a person’s genetic material. MRI does not involve ionizing radiation, so there is no danger with single or repeat exposures.

MRI is usually done as an outpatient test in a hospital or in a scanning facility. If a cylindrical scanner is being used, the table will slide into the machine’s narrow opening. In an open MRI, the table will slide so that the part of your body being scanned is surrounded by the scanning element, or the machine will move over you as you lie on the table. You will need to lie very still during the procedure, and you will periodically hear loud knocking noises as the scanner works. The technologist operating the machine will be in another room, but you will be able to talk to him or her thoughout the procedure.

If your doctor gave you a sedative or tranquilizer beforehand, you may be drowsy after your MRI procedure and unable to drive safely. Have a friend or family member take you home.

Your MRI scan will be read by a specialist who will tell your doctor the results. Ask when you should call your doctor for the official report.

Originally published March 2009; last reviewed May 5, 2011.

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