The ABC's of MRI

Magnetic Resonance Imaging

Information for Patients*
-What is magnetic resonance imaging (MRI)?
-What is MRI used for?
-Is MRI safe?
-How to prepare for the MRI examination.
-What is the MRI examination like?

What is magnetic resonance imaging (MRI)?
MRI, or magnetic resonance imaging, is a means of "seeing" inside of the body in order for doctors to find certain diseases or abnormal conditions. MRI does not rely on the type of radiation (i.e., ionizing radiation) used for an x-ray or computed tomography (CT) scan. The MRI examination requires specialized equipment that uses a powerful, constant magnetic field, rapidly changing local magnetic fields, radiofrequency energy, and dedicated equipment including a powerful computer to create very clear pictures of internal body structures.

During the MRI examination, the patient is placed within the MR system or scanner. The powerful, constant magnetic field aligns a tiny fraction of subatomic particles called protons that are present in most of the body's tissues. Radiofrequency energy is applied to cause these protons to produce signals that are picked up by a receiver within the scanner. The signals are specially characterized using the rapidly changing, local magnetic field and computer-processed to produce images of the body part of interest.

What is MRI used for?
MRI has become the preferred procedure for diagnosing a large number of potential problems in many different parts of the body. In general, MRI creates pictures that can show differences between healthy and unhealthy tissue. Doctors use MRI to examine the brain, spine, joints (e.g., knee, shoulder, wrist, and ankle), abdomen, pelvic region, breast, blood vessels, heart and other body parts.

Is MRI safe?
To date, over 200 million patients have had MRI examinations. Every year, approximately 10 million patients undergo MRI procedures. MRI has been shown to be extremely safe as long as proper safety precautions are taken. In general, the MRI procedure produces no pain and causes no known short-term or long-term tissue damage of any kind.

The powerful magnetic field of the scanner can attract certain metallic objects known as 'ferromagnetic' objects, causing them to move suddenly and with great force towards the center of the MR system. This may pose a risk to the patient or anyone in the way of the object. Therefore, great care is taken to prevent ferromagnetic objects from entering the MR system room. It is vital that you remove metallic objects in advance of an MRI exam, including watches, jewelry, and items of clothing that have metallic threads or fasteners.

MRI facilities have screening procedures that, when carefully followed, will ensure that the MRI technologist and radiologist knows about the presence of metallic implants and materials so that special precautions can be taken (see below). In some unusual cases the examination may be canceled because of concern related to a particular implant or device. For example, if an MRI is ordered, it may be canceled if the patient has a ferromagnetic aneurysm clip because of the risk dislodging the clip from the blood vessel. Also, the magnetic field of the scanner can damage an external hearing aid or cause a heart pacemaker to malfunction. If you have a bullet or other metallic fragment in your body there is a potential risk that it could change position, possibly causing injury.