Magnetic resonance imaging (MRI) is a big magnet that takes advantage of the electromagnetic qualities of the body’s cells to create images. To detect cancer cells, a dye must be injected into a woman’s vein prior to the MRI procedure. The dye travels through the body and is picked up most rapidly by lesions, like cancerous tumors, that have lots of blood vessels. These lesions can then be seen on the three-dimensional picture of the breast produced through MRI.

Annual MRI versus annual mammogram

It’s important to understand the difference between using a test to screen for breast cancer and using a test to diagnose breast cancer. For a test to be a good screening tool, it must be both inexpensive and good at finding asymptomatic people who have a disease. To determine a test’s value as a screening tool, researchers assess its “sensitivity” and “specificity.” A highly-sensitive test produces few false negatives (women told they don’t have cancer when they really do). A highly-specific test produces few false positives (women told they have cancer when they really don’t). For a test to be a good diagnostic tool, it must aid in the diagnosis of a problem—like a lump—once it has already been detected. Mammograms, two-dimensional X-rays that produce pictures of the soft tissue within the breast, are regularly used for both screening and diagnostic purposes. Screening mammograms are recommended for all women 50 and over. In this instance, mammography is used to “screen” large numbers of asymptomatic women for signs of breast cancer. A diagnostic mammogram is performed if a woman finds a lump or has another breast complaint and her doctor wants to better understand what the problem is. In this instance, the mammogram is used to help “diagnose” the problem. The types of pictures an MRI provides make it a good tool to get the sense of a size of a lesion that has been found with mammography. But it’s not a good screening test. The problem is that MRI is not highly specific—it all too often results in a false positive result, which leads to unnecessary worry and medical testing. That’s because benign lesions, like fibroadenomas, also have lots of blood vessels, which means they can easily be mistaken for tumors. Further, because MRI only spots lesions that have lots of blood vessels, it may miss the slow-growing cancers that have only a small number of blood vessels. Currently no governmental or health organization recommends that MRI be used for breast cancer screening. For high-risk women, there may be some advantages to MRI screening. For that reason, the American Cancer Society recommends that all high-risk women receive an annual MRI screening in addition to mammography. But for most women, even though it’s not perfect, mammography remains a better screening tool than MRI.

How do radiologists perform a breast MRI?

Magnetic resonance imaging (MRI) uses a magnetic field and radio waves to create a composite, 3D image of your breast. A breast MRI is much more complicated to perform than a mammogram. That’s one reason why your insurance may not cover the test, unless you are high risk or your doctor has requested that the MRI be done to further explore something suspicious seen on a mammogram. You should plan on the MRI taking at least an hour, if not longer. You should tell the MRI center if you’re premenopausal, as many facilities prefer to schedule a premenopausal woman’s MRI during days seven to 14 of her menstrual cycle. (Due to fluctuation in hormones, this is often the best time to have an MRI.) You should also tell the MRI center if you have a cardiac pacemaker or other electronic device implanted in your body or if you’re pregnant or think you may be pregnant. (MRI generally isn’t recommended in these circumstances.) Because MRI uses magnetic fields to create an image of the breast, you will need to remove any metal you are wearing, such as hearing aids, dentures, jewelry, watches, and hairpins, which might be attracted to the magnetic field the MRI creates. You will probably be wearing a cloth or paper covering, but if you are allowed to keep your pants on, be sure to remove any coins or credit cards from your pockets, as they, too, can be attracted to the magnet. In addition, the magnet could erase information encoded on scanner strips on your credit or ID cards. Before going into the MRI machine, a nurse will put a small catheter into your arm. Once the procedure gets underway, the nurse will inject gadolinium—a contrast agent that allows abnormalities in the breast to be seen on the MRI—into the catheter. You will then be asked to lie still for about an hour while the scanner creates its image. This can be difficult to do, but it’s necessary for the picture to be clear. The MRI machine has a large, round opening; it looks like a big tube. You will be asked to lie on your stomach on a padded table. Your breasts will fit into a hollow area of the table that contains coils that detect the magnetic signals that create the breast images. Once you have been positioned properly, the radiologist will move the entire table into the opening of the machine. This can be hard for people who have trouble being in a small, confined space. To remedy this, many doctors will have their patients take a mild sedative, like lorazepam (Ativan), 30 minutes to an hour before the MRI. (If you have a sedative, you will not be able to drive after the MRI, so be sure to bring along a friend or family member who can assist you.) When the MRI machine is turned on, it creates a magnetic field around you, and radio waves are directed at your body. There aren’t any known harmful effects from exposure to the magnetic fields or radio waves. You may hear loud tapping or thumping sounds while you are in the MRI machine. Some facilities give you earplugs or headphones with music to reduce the noise. (If you have a sedative, these noises will probably be less bothersome.) Insurance companies don’t always cover MRI, and they are expensive—$1,000 or more. Before having a breast MRI, always check with your insurance company to see whether you’ll be covered.

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