MRI (magnetic resonance imaging) is a painless test used to view the inside of the body without using X-rays. It uses a large magnet, safe, low-energy radio waves and a computer to produce 2- or 3-dimensional pictures. Radio waves are passed into your body and are absorbed by some of the tissues, which in turn retransmit the radio waves. The magnet is turned on and off, and reads the radio waves you are giving off. The computer picks up this information and generates a picture. Diseased tissue gives off a different signal from healthy tissue and the machine detects this.
MRI is used when simpler and less expensive tests have failed to give a diagnosis. It provides a detailed picture of any part of the body and having an MRI scan may mean that some of the less pleasant tests do not have to be done.
MRI scans of the brain, soft tissues or chest areas don’t usually require any special preparation. For scans of the abdomen and pelvis, you may be asked to restrict food and drink for a few hours before the test.
The magnetic field used in MRI scanning can affect some metal objects. You should tell the doctor who arranges the test if you have any internal metal devices or implants in your body, such as metal clips on blood vessels in your brain, a pacemaker or implanted defibrillator, a surgically implanted joint/pin, artificial heart valves, an intrauterine contraceptive device (IUD), or any metal foreign bodies in your eyes or elsewhere. Metal objects such as these can be a safety hazard or affect the MRI images. You will also be asked to remove all jewellery, watches and hearing aids, and to take out any removable dental work.
Small children and infants may be given a general anaesthetic to keep them completely still. They should not eat or drink anything for 6 hours before the test. The doctor will discuss the procedure with you and answer any questions. You will be asked to sign a consent form allowing the test to be done.
You will be asked to change into a hospital gown and to lie on a narrow table. The table will then slide you into a tunnel. You will be in the tunnel (and room) by yourself, but you can talk to the staff at any time through an intercom. There is usually a panic button that you can press if you feel very claustrophobic or unwell. Fans will circulate air through the tunnel.
The table doesn’t move during the test and nothing will touch you. The machine makes knocking noises, which can be quite loud, as it takes the pictures, but most MRI machines have earphones through which music will be played to distract you. You may be given an injection of a contrast medium or dye that makes the pictures easier to read; this will be discussed before the procedure. You may be asked to hold your breath while some images are being taken.
Once enough images have been obtained, the table will slide out of the tunnel, and you can go and get dressed. The scan may take from 15 to 60 minutes.
You can resume normal physical activity and return to work as soon as the test is finished.
The results will be sent to the doctor who referred you for the test, and a follow-up appointment will be made for you to discuss them.
MRI itself has no known harmful effects for most people. The effects of MRI scanning on a developing fetus are unknown. You should tell your doctor if you are pregnant, or think you may be pregnant.
This test can be slightly uncomfortable for people who are claustrophobic. If you think you could not lie still and relax you should tell your doctor before you are sent for the test. Sometimes people who have had an injection of contrast dye may have an allergic reaction to the dye and this will be treated with medication at the time.
Talk to your doctor. Contact the hospital or radiology centre where you are having the MRI.
Last Reviewed: 19 August 2009