A magnetic resonance imaging (MRI) scan of the breast is a noninvasive method to create detailed pictures of the breast and surrounding tissues. It may be done in combination with mammography or ultrasound. However, it is not a replacement for mammography.

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.

See also:

• Breast self-exam
• Breast ultrasound
• Mammography

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 your stomach with your breasts hanging down into cushioned openings. The narrow table slides into the middle of the MRI machine.

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 type of equipment, the exam may take 30 minutes to 1 hour.

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

Before the test, tell the radiologist if you currently undergo dialysis, as this may affect whether you can have IV contrast.

If you fear confined spaces (have claustrophobia), tell your doctor before the scan. You may be given a medicine to help you feel sleepy or less anxious, or your doctor may recommend an "open" MRI, in which the machine is not as close to the body.

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.

MRI provides detailed pictures of the breast. It also provides clear pictures of parts of the breast that are difficult to see clearly on ultrasound or mammogram.

Breast MRI may also be performed to:

• Check for more cancer in the same breast or the other breast after breast cancer has been diagnosed
• Distinguish between scar tissue and tumors in the breast
• Evaluate a breast lump (usually after biopsy)
• Evaluate an abnormal result on a mammogram or breast ultrasound
• Evaluate for possible rupture of breast implants
• Find any cancer that remains after surgery or chemotherapy
• Guide a biopsy (rare)
• Screen for cancer in women at very high risk for breast cancer (such as those with a strong family history)
• Screen for cancer in women with very dense breast tissue

An MRI of the breast can also show:

• Blood flow through the breast area
• Blood vessels in the breast area

Results depend on 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.

Abnormal results may be due to:

• Breast cancer
• Cysts
• Leaking or ruptured breast implants

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.

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.

Breast MRI is more sensitive than mammogram, especially when it is performed using contrast dye. However, breast MRI may not always be able to distinguish breast cancer from noncancerous breast growths. This can lead to a false positive result.

MRI also cannot pick up tiny pieces of calcium (microcalcifications), which mammogram can detect.

A biopsy is needed to confirm the results of a breast MRI.

Saslow D, Boetes C, Burke W, Harms S, Leach MO, Lehman CD, et al. American Cancer Society guidelines for breast screening with MRI as an adjunct to mammography. CA Cancer J Clin. 2007;57:75-89.

Lehman CD, DeMartini W, Anderson BO, Edge SB. Indications for breast MRI in the patient with newly diagnosed breast cancer. JNCCN. 2009;7:193-201.

Morris EA. Diagnostic breast MR imaging: current status and future directions. Radiol Clin N Am. 2007;45:863-880.

Liberman L. Breast MR imaging in assessing extent of disease. Magn Reson Imaging Clin N Am. 2006;14:339-349.

Sukumvanich P, Borgen P. Diseases of the breast. In: Rakel RE, Bope ET, eds. Conn's Current Therapy 2008. 60th ed. Philadelphia, Pa:Saunders Elsevier;2008:chap 260.