Magnetic resonance imaging

Other names include MRI, magnetic resonance (MR), and nuclear magnetic resonance (NMR) imaging.

What does it show?

Like CT scans, MRI creates cross-section pictures of your insides. But MRI uses strong magnets instead of radiation to make the images. An MRI scan can take cross-sectional slices (views) from many angles, as if someone were looking at a slice of your body from the front, from the side, or from above your head. MRI creates pictures of soft tissue parts of the body that are sometimes hard to see using other imaging tests.

MRI is very good at finding and pinpointing some cancers. An MRI with contrast dye is the best way to see brain tumors. Using MRI, doctors can sometimes tell if a tumor is not cancer (benign) or cancerous (malignant).

MRI can also be used to look for signs that cancer may have spread (metastasized) from where it started to another part of the body.

MRI images can also help doctors plan treatment such as surgery or radiation therapy.

Special MRI machines, now available in some hospitals, are designed just for looking inside the breast. This is called an MRI with dedicated breast coils. Breast MRI is recommended along with mammograms to look for breast cancer in women at high risk for breast cancer. At this time MRI is not used by itself to detect breast cancer early. (If you think you may be high risk for breast cancer and want more information on breast MRI as part of breast cancer early detection, call 1-800-227-2345.) Breast MRI can also be used in women who have already been diagnosed with breast cancer to better determine the actual size of the cancer and to look for any other cancers in the breast.

How does it work?

An MRI scanner is a cylinder or tube that holds a very strong magnet weighing several tons. As you lie on a table within the tube, the device surrounds you with a powerful magnetic field. The magnetic force causes the nuclei (centers) of hydrogen atoms in your body to line up in one direction. Once the atoms are lined up, the MRI machine gives off a burst of radiofrequency waves. These waves cause the hydrogen nuclei to change direction. When they return to their original position, they give off certain signals that the scanner detects. Hydrogen nuclei in the body tissues change direction in different ways. A computer takes the signals from these changes and converts them into a black and white picture.

Contrast materials can be put in through a vein to improve the quality of the image. Once absorbed by the body, these agents speed up the rate at which tissue responds to the magnetic and radio waves. As a result, the signals produce stronger, clearer pictures.

How do I get ready for the test?

You don’t need a special diet or preparation before an MRI.

Some tests require use of a contrast material before imaging. If contrast is to be used, you may have an intravenous (IV) catheter put in a vein in your arm.

If being in an enclosed space such as the MRI scanner concerns you, you might need to take medicine to help you relax. Talking with the technologist or a patient counselor, or getting a tour of the MRI machine before the test can be helpful. You will be in the exam room alone, but the technologist will be able to see and hear what’s going on. You will also have a call button in case you need to speak with the technologist. In some cases, you can arrange to have the test done with an open MRI machine that allows more space around your body (see the next section).

Before the test, you usually will be asked to undress and put on a gown or other clothes without zippers or metal. Be sure to remove all metal objects, like hair clips or jewelry. Before the scan, the MR technologist will ask you if you have any metal in your body, such as surgical clips or staples, pacemakers, artificial joints, metal fragments, tattoos, permanent eyeliners, and so on. Some metallic objects will not cause problems, but others can. You may need to have an x-ray to check for metal objects if there is any doubt.

What is it like having the test?

MRI scans are usually done on an outpatient basis in a hospital or clinic. You will lie down on a narrow, flat table. The technologist may use straps or pillows to make you comfortable and help keep you from moving. The table then slides into a narrow cylinder. The part of your body that is being looked at will be in the center of the cylinder.

The test is painless, but you have to lie still inside the cylinder with its surface a few inches from your face. You may be asked to hold your breath or keep very still during certain parts of the test. The machine may make loud, thumping and whirring noises, much like the sound of a washing machine, as the magnet switches on and off. Some facilities let you wear earplugs or headphones to block this noise out during testing.

Special machines that are less restrictive may be easier for some people. These open MRI machines replace the narrow cylinder with a larger ring. This design lessens the banging sound and the feeling of lying in an enclosed space. But the device does not create as strong a magnetic field. Although open MRI technology is improving, the pictures may not be as clear or detailed as they are with standard MRI. Sometimes, this may require retesting on a standard MRI machine.

You may be given contrast material in a vein or have to swallow it. The contrast material used for an MRI exam is called gadolinium. Let the technologist know if you have any kind of allergies.

Breast MRI uses a special machine that only does this test, and contrast material is often used. You have to lie inside a narrow tube, face down, on a platform specially designed for the procedure. The platform has openings for each breast that allow them to be scanned without being compressed. The platform contains the sensors needed to capture the MRI image. It is important to remain very still throughout the exam.

How long does it take?

MRI scans can take between 45 and 60 minutes and sometimes up to 2 hours. After the test, you may be asked to wait while the pictures are checked to see if more are needed.

What are the possible complications?

People can be hurt in MRI machines if they take metal items into the room or if other people leave metal items in the room.

Some people become very uneasy and even panic when lying inside the MRI scanner.

Some people react to the contrast material. Such reactions include:

• Nausea
• Pain at the needle site
• Headache that develops a few hours after the test is over
• Low blood pressure leading to a feeling of lightheadedness or faintness (this is rare)

Be sure to let your health care team know if you have any of these symptoms.

Gadolinium, the contrast material used for MRI, can cause a special complication when it’s given to patients on dialysis or who have severe kidney problems, so it is rarely given to these patients. Your doctor will discuss this with you if you have severe kidney problems and need an MRI with contrast.

What else should I know about this test?

• People who are overweight may have trouble fitting into the MRI machine.
• Some kinds of surgical implants that contain metal or are affected by magnets may cause problems due to the strong magnetic field. If you have a pacemaker, surgical clips, staples, an implanted pump, or other device with metal, you may not be able to have an MRI. Not all metal implants are a problem. Your health care team will ask you questions about these before the MRI test.
• If you have an implanted intravenous (IV) catheter or port, your doctor will need to decide whether you should have an MRI.
• If you have tattoos or permanent makeup (such as eyeliner), let the technologist know so that they can take the needed precautions and ensure the best results.
• The use of MRI during pregnancy has not been well studied. MRI is usually not done in the first 12 weeks of pregnancy unless there is a strong medical reason to use it.
• Do not bring credit cards or other items with magnetic scanning strips with you into the exam room – the magnet could wipe out the information stored on them.
• MRI costs more than a CT scan.
• MRI does not expose you to radiation.
• Many hospitals and imaging centers do not have dedicated breast MRI equipment available. It is important that MRIs done for breast cancer screening in high risk women be done on dedicated breast MRI equipment at facilities that can also perform an MRI-guided breast biopsy. Otherwise, the entire scan will need to be repeated at another facility when the biopsy is done.