Magnetic resonance imaging (MRI) is a noninvasive way to take pictures of the body.

Unlike x-rays and computed tomographic (CT) scans, which use radiation, MRI uses powerful magnets and radio waves. The MRI scanner contains the magnet. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's.

The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals.

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:

• Abdominal MRI
• Chest MRI
• Cranial MRI
• Heart MRI
• Lumbosacral spine MRI
• Spine MRI

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

You will lie on a narrow table, which slides into the middle of the MRI machine. If you fear confined spaces (have claustrophobia), tell your doctor before the exam. You may be prescribed a mild sedative, or your doctor may recommend an "open" MRI, in which the machine is not as close to the body.

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

Some exams require a special dye (contrast). The dye is usually given before the test through a vein (IV) in your hand or forearm. The dye helps the radiologist see certain areas more clearly.

During the MRI, the person who operates the machine will watch you from another room. Several sets of images are usually needed, each taking 2 - 15 minutes. Depending on the areas being studied and type of equipment, the exam may take 1 hour or longer.

Depending on the area being studied, you may be asked not to eat or drink anything for 4 - 6 hours before the scan. Other preparations are usually not needed.

The strong magnetic fields created during an MRI can interfere with certain implants, particularly pacemakers. Persons with cardiac pacemakers cannot have an MRI and should not enter an MRI area.

You may not be able to have an MRI if you have any of the following metallic objects in your body:

• Brain aneurysm clips
• Certain artificial heart valves
• Inner ear (cochlear) implants
• Recently placed artificial joints
• Some older types of vascular stents

Tell your health care provider if you have one of these devices when scheduling the test, so the exact type of metal can be determined.

Before an MRI, sheet metal workers or any person that may have been exposed to small metal fragments should receive a skull x-ray to check for metal in the eyes.

Because the MRI contains a magnet, metal-containing objects such as pens, pocketknives, and eyeglasses may fly across the room. This can be dangerous, so they are not allowed into the scanner area.

Other metallic objects are also not allowed into the room:

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

An MRI exam causes no pain. Some people may become anxious inside the scanner. If you have difficulty lying still or are very anxious, you may be given a mild sedative. Excessive 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 the person operating the scanner 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 need sedation. After an MRI scan, you can resume your normal diet, activity, and medications.

Combining MRIs with other imaging methods can often help the doctor make a more definitive diagnosis.

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

An MRA, or magnetic resonance angiogram, is a form of magnetic resonance imaging, that creates three-dimensional pictures of blood vessels. It is often used when traditional angiography cannot be done.

Results are considered normal if the organs and structures being examined are normal in appearance.

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.

MRI contains no ionizing radiation. To date, there have been no documented significant side effects of the magnetic fields and radio waves used on the human body.

The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. The person operating the machine will monitor your heart rate and breathing as needed.

MRI is usually not recommended for acute trauma situations, because traction and life-support equipment cannot safely enter the scanner area and the exam can take quite a bit of time.

People have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.

Beller GA, Kramer CM. Nuclear cardiology and computed tomography. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 54.

Wilkinson ID, Paley MNJ. Magnetic resonance imaging: basic principles. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. New York, NY: Churchill Livingstone; 2001:chap 5.