MRI

A magnetic resonance imaging (MRI) scan is an imaging test that uses powerful magnets and radio waves to create pictures of the body. It does not use radiation (x-rays).

Single MRI images are called slices. The images can be stored on a computer or printed on film. One exam produces dozens or sometimes hundreds of images.

For more information, see the specific MRI topics:

Alternative Names

Magnetic resonance imaging; Nuclear magnetic resonance (NMR) imaging

How the Test is Performed

You may be asked to wear a hospital gown or clothing without zippers or snaps (such as sweatpants and a t-shirt). Certain types of metal can cause blurry images.

You will lie on a narrow table, which slides into a large tunnel-shaped scanner.

Some exams require a special dye (contrast). Most of the time, the dye will be given through a vein (IV) in your hand or forearm before the test. The dye helps the radiologist see certain areas more clearly.

Small devices, called coils, may be placed around the head, arm, or leg, or around other areas to be studied. These help send and receive the radio waves, and improve the quality of the images.

During the MRI, the person who operates the machine will watch you from another room. The test lasts about 30 to 60 minutes, but may take longer.

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How to Prepare for the Test

You may be asked not to eat or drink anything for 4 to 6 hours before the scan.

Tell your health care provider if you are afraid of close spaces (have claustrophobia). You may be given a medicine to help you feel sleepy and less anxious, or your doctor may suggest an open MRI, in which the machine is not as close to the body.

Before the test, tell your provider if you have:

Artificial heart valves
Brain aneurysm clips
Heart defibrillator or pacemaker
Inner ear (cochlear) implants
Kidney disease or dialysis (you may not be able to receive contrast)
Recently placed artificial joints
Vascular stents
Worked with sheet metal in the past (you may need tests to check for metal pieces in your eyes)

Because the MRI contains strong magnets, metal objects are not allowed into the room with the MRI scanner:

Items such as jewelry, watches, credit cards, and hearing aids can be damaged.
Pens, pocketknives, and eyeglasses may fly across the room.
Pins, hairpins, metal zippers, and similar metallic items can distort the images.
Removable dental work should be taken out just before the scan.

How the Test will Feel

An MRI exam causes no pain. If you have difficulty lying still or are very nervous, you may be given a medicine to relax you. Too much movement can blur MRI images and cause errors.

The table may be hard or cold, but you can request a blanket or pillow. The machine produces loud thumping and humming noises when turned on. You can wear ear plugs to help reduce the noise.

An intercom in the room allows you to speak to someone at any time. Some MRIs have televisions and special headphones that you can use to help the time pass.

There is no recovery time, unless you were given a medicine to relax. After an MRI scan, you can resume your normal diet, activity, and medications.

Why the Test is Performed

Having MRIs with other imaging methods can often help your doctor make a diagnosis.

MRI images taken after a special dye (contrast) is delivered into your body may provide extra information about the blood vessels.

A magnetic resonance angiogram (MRA), is a form of magnetic resonance imaging that creates 3-dimensional pictures of blood vessels. It is often used when traditional angiography cannot be done.

Normal Results

A normal result means the body area being studied looks normal.

What Abnormal Results Mean

Results depend on the part of the body being examined and the nature of the problem. Different types of tissues send back different MRI signals. For example, healthy tissue sends back a slightly different signal than cancerous tissue. Consult your health care provider with any questions and concerns.

Risks

MRI does not use ionizing radiation. No side effects from the magnetic fields and radio waves have been reported.

The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions rarely occur. However, gadolinium can be harmful to people with kidney problems who are on dialysis. Tell your provider before the test if you have kidney problems.

The strong magnetic fields created during an MRI can cause heart pacemakers and other implants not to work as well. The magnets can also cause a piece of metal inside your body to move or shift.

References

Wilkinson ID, Graves MJ. Magnetic resonance imaging. In: Adam A, Dixon AK, Gillard JH, et al. eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 6th ed. New York, NY: Churchill Livingstone; 2014:chap 5.

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