Magnetic Resonance Imaging (MRI)

What Is It?

Magnetic resonance imaging (MRI) is a diagnostic technique that uses a magnetic field to produce pictures of structures inside the body.

During an MRI, your body is in a very strong magnetic field. The MRI machine also uses pulses of radio waves. The machine creates an image based on the way hydrogen atoms in your body react to the magnetic field and the radio waves. 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 MRI's 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 important in detecting tumors and in checking for abnormalities in the body's soft tissues, such as the brain, spinal cord, heart and eye.

What It's Used For

MRI scans have many uses. They can help to determine when a stroke occurred, support a diagnosis of multiple sclerosis, and identify problems of the brain and spinal cord that may not be seen on a computed tomography (CT) scan. They are used to detect cancerous tumors in many organs, including the brain, spinal cord, lung, liver, bone, prostate and uterus. An MRI scan also can help to distinguish between a tumor and benign (noncancerous) fibrocystic disease in women's breasts and to pinpoint cancers in women who have very dense breast tissue or breast implants.

Preparation

Because MRI uses a strong magnetic field that can move metal objects, you cannot have an MRI scan if you have a metallic implant such as a pacemaker or implanted pump, or if you have an artificial joint, implanted metal plates or screws, or metal surgical clips. You also may need to avoid MRI scans if you have a hearing aid, metal monitoring device, or certain types of tattoos. Always check with your doctor before the procedure.

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.

MRI scanners also make loud knocking sounds. Usually the technician will offer ear plugs or earphones so you can listen to listen to music or the radio during the test. You also will be reassured that you can press a special signal button if you are feeling claustrophobic and want to stop the scan.

How It's Done