What is magnetic resonance imaging (MRI)?

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 provide an image of a single slice of any part of the body, similar to a slice of bread in a loaf. Typically, images are created of multiple “slices” of an organ or body part. The computer can also combine these slices into three-dimensional (3-D) images.

Because water molecules are particularly 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 you undergo an MRI, your doctor will ask whether you have any electronic or metal medical devices in your body, such as: B. a pacemaker or metallic clips, pins or screws. These precautions are necessary because the MRI magnet is so strong that it could interfere with a pacemaker or displace some implanted clips.

MRI is a painless technique that usually takes about 45 minutes to just over an hour. Most MRI scanners require you to lie in a narrow cylinder. This can make some people feel anxious and claustrophobic. If you tend to feel anxious in tight spaces, ask your doctor about medications 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 used to produce X-rays or CT scans 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 from single or repeated exposure.

MRI is usually performed as an outpatient test in a hospital or scanning facility. When a cylindrical scanner is used, the table slides into the narrow opening of the machine. With an open MRI, the table slides so that the part of your body being scanned is surrounded by the scanning element, or the machine moves above you while you lie on the table. You will need to lie very still during the procedure and you will periodically hear loud tapping noises as the scanner works. The technician operating the machine is in another room, but you can speak to him or her throughout the procedure.

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

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

Originally published March 2009; last checked on May 5, 2011.