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.


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

MRI is a painless technique that usually takes about 20 minutes. MRI usually is done as an outpatient test in a special scanning area of a hospital or in a scanning facility. You will be asked to remove all metal jewelry and to lie on a scanning table. If a cylindrical scanner is being used, the table will slide into the narrow opening into the MRI cylinder. In an open MRI, the table will slide so that the part of your body being scanned is surrounded by the scanning element. You will need to lie very still during the procedure, and you will periodically hear loud knocking noises as the scanner works. The technicians operating the machine will be in another room. However, they will be able to talk to you through speakers in the machine or through earphones.


If your doctor gave you a sedative or tranquilizer to make you more comfortable during scanning, you may be drowsy after your MRI procedure, and you may not be able 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 MRI facility personnel about when you should call your doctor for the official report.


MRI has minimal risks or side effects in people without implanted metallic or electrical devices.

When To Call A Professional

Because MRI has few known side effects, you probably will not need to call your doctor afterwards, except to get your scan results.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.