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MRI scanning is the most advanced diagnostic technique. This article outlines what safety precautions are required and how the scan is performed.

What is it?

Magnetic resonance imaging, or MRI, is the most advanced method of diagnostic imaging. It uses a high strength magnet and radio waves to produce highly detailed cross-sectional images of any part of the body. Unlike CT scanning, MRI can produce images in any desired plane, and no x-rays are used.

What is it used for?

MRI is used for imaging most parts of the body, including the brain, spine, joints, limbs, chest, abdomen and pelvis. Newer uses of MRI include scanning arteries and veins (MR angiogram) and bile ducts (MR cholangiogram).

Can anyone undergo MRI?

Due to the strong magnetic field, people with certain implanted devices cannot safely undergo MRI. For example, patients with cardiac pacemakers, cochlear implants, neurostimulator devices or certain kinds of aneurysm clips cannot be scanned. Metallic fragments in the eyes are not safe, and if there is a possibility of metal in the eye, an x-ray picture of the eye is necessary prior to having the MRI scan.

Many implants are safe for scanning, such as joint replacements and vascular stents. In some instances, it is advisable to wait at least 6 weeks after surgery before having a scan. Providing you have no implants that preclude you from having MRI, there are no known harmful effects from undergoing a scan. Scanning in pregnancy is also considered to be safe, however, scanning in early pregnancy is avoided if possible. Prior to undergoing your scan, you will be asked to complete a safety checklist to ensure MRI is safe for you.

Before you arrive

In general, no special preparation is needed. Patients having a head scan should not wear eye make-up, as it may degrade the image quality. In certain types of abdomen or pelvic scanning, you may be asked to fast or avoid caffeine prior to the scan to minimise bowel activity. A friend or family member may accompany you into the scan room, but they will need to complete the safety checklist first. You may wish to bring a CD along so that you can listen to music of your choice during the scan.

What happens?

When you arrive in the MRI suite for your scan, a staff member will explain the procedure to you and ask you to fill in the safety checklist. Some personal items may not be taken into the scan room, such as keys, credit cards, watch and hearing aids. A special locker or safe storage space is usually provided for these items. Depending on what body part you are having scanned, you may be asked to change into a gown for the scan, unless you are wearing clothes with no metal.

In the scan room, you will be placed onto a padded table. Depending on the body part being scanned, a device known as a coil will be placed upon or around that part, to improve the detail shown on the scan. For example when the knee is being scanned, a cylinder known as a knee coil is placed around the knee. The table is then moved into the centre of the magnet. The centre of the magnet is a tunnel, open at each end. Most of your body will be within the tunnel. During the scan you will wear headphones, and you may communicate with the technologist. Music may be played for you during the scan.

Some degree of claustrophobia may be experienced by up to 20% of patients. If necessary, a sedative agent may be given prior to the scan. If you think you will be claustrophobic, it is a good idea to advise the booking staff of this when making your appointment.

During scanning, a rhythmic tapping noise will be heard. These noises last from a few seconds to a few minutes. The technologist will tell you how long each part of the scan will take - it is important to be still during this scan time. Sometimes an injection of a contrast agent, or "dye" is necessary. This is a paramagnetic substance that helps to light up certain parts of the anatomy and some abnormalities, allowing for greater diagnostic accuracy. Side effects from this are uncommon, but may include nausea and vomiting. Serious allergic reactions are rare. The contrast agent is rapidly removed from the body by the kidneys.

The total time you are in the scan room depends on what kind of scan you are having; typically 30 to 60 minutes should be allowed.

After the scan

The images will be printed and the radiologist will study them and issue a report. The report is usually available to the referring doctor within 24 -48 hours; or immediately in urgent cases.

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