Magnetic resonance imaging (MRI) is a non-invasive 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. For example, healthy tissue sends back a slightly different signal than cancerous tissue.
Single MRI images are called slices. The images can be stored on a computer or printed on film.
MRI can easily be performed through clothing. However, because the magnet is very, very strong, certain types of metal can cause significant errors, called artifacts, in the images.
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 metal fasteners (such as sweatpants and a t-shirt).
You will be asked to lie on a narrow table, which slides into the middle of the MRI machine. If you have a fear of confined spaces (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.
Certain exams require that a special dye (contrast) be given before the test. The dye is usually given through an intravenous line (IV) in your hand or forearm. The contrast helps the radiologist see certain areas more clearly.
During the MRI, the person who operates the machine will watch you from a room next door. Several sets of images are usually need, each taking from 2 to 15 minutes. Depending on the areas being studied and type of equipment, the exam may take 1 hour or longer.
How to prepare for the test
An MRI can be performed immediately after other imaging studies. Depending on the area of interest, the patient may be asked to fast for 4 - 6 hours prior to 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 can not receive an MRI and should not enter an MRI area.
If you have any of the following metallic objects in your body, you should not get an MRI:
- Inner ear (cochlear) implants
- Brain aneurysm clips
- Certain artificial heart valves
- Older vascular stents
- Recently placed artificial joints
You will be asked to sign a consent form that says you do not have any of these items in your body.
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 of the strong magnets, certain metallic objects are 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 prior to the scan.
- When the MRI magnet is turned on, pens, pocketknives, and eyeglasses may fly across the room. This can be dangerous, so such items are not allowed into the scanner area.
How the test will feel
An MRI exam causes no pain. Some people may become anxious when 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. Ear plugs are usually given to help reduce the noise.
An intercom in the scanner allows you to speak to the person operating the exam 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 sedation was necessary. After an MRI scan, you can resume your normal diet, activity, and medications.
Why the test is performed
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, which creates three-dimensional pictures of blood vessels. It is often used when traditional angiography cannot be done.
What the risks are
There is no ionizing radiation involved in MRI, and there have been no documented significant side effects of the magnetic fields and radio waves used on the human body to date.
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.
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.
For more information, see the specific MRI topics:
Grainger RC, Allison D, Adam, Dixon AK. Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. Orlando, Fl: Churchill Livingstone; 2001:101-136.
Goldman L, Ausiello D. Cecil Textbook of Medicine, 22nd ed. Philadelphia, Pa: WB Saunders; 2004: 29.