MRI Scan

What is an MRI scan?

MRI stands for magnetic resonance imaging. An MRI scan uses a strong magnetic field and radio waves to create pictures on a computer of tissues, organs and structures inside your body.

How does an MRI scan work?

Your body contains millions of hydrogen atoms. When you are in an MRI scanner:

• A strong magnetic field 'pulls' on particles called protons which are within the hydrogen atoms. All the protons line up in parallel to the magnetic field. (Normally the millions of protons all lie in random directions.)
• Then, short bursts of radio waves are sent from the scanner into your body. The radio waves knock the protons from their position.
• When the burst of radio waves stops, the protons go back into position. They realign back to being in parallel with the magnetic field.
• As the protons realign, they emit tiny radio signals. These signals are detected by a receiving device in the scanner.
• The receiving device transmits the signals to a computer.

Most of the hydrogen atoms in the body are in water molecules. (Water is made of hydrogen and oxygen - H20.) Each type of tissue has a different water content. Therefore, the strength of the signal emitted from different body tissues varies. The computer creates a picture based on the strength and location of the radio signals emitted from the body. (A different colour or shade for each strength of signal.)

What does an MRI scan involve?

The MRI scanner is like a short tunnel surrounded by a giant circular magnet. You lie on a couch and a 'recieving device' (another smaller magnet) is placed behind, or around, the part of the body being examined. (This detects the tiny radio signals emitted from your body.) The couch then slides into the scanner. When each 'picture' is being taken you need to keep still for a few minutes otherwise the scan pictures may be blurred. The couch is then slid a little further in as several scans are done to obtain pictures of 'slices' of the area of your body being examined.

The scan itself is painless. The whole procedure can take 30-60 minutes, depending on the size of the area being examined and how may 'pictures' are taken. It may be a little uncomfortable lying still on the couch for this time. In some cases an injection of a special contrast dye is given into the bloodstream via a vein on the arm. This helps to give clearer pictures of certain tissues or organs being examined.

Because the computer has to be away from the magnetic field, the operator needs to be in a separate room. However, you can talk to them, usually via an intercom, and you will be observed at all times on a monitor.

The scanner can be quite noisy so you may be given some headphones or earplugs. Some people feel anxious or claustrophobic in the scanner. A mild sedative may be offered if you are particularly anxious. Small children may need a general anaesthetic to keep them still long enough for the pictures to be taken.

What is an MRI scan used for?

An MRI scan can create clear pictures of any part of the body. So it is useful for all sorts of reasons where less detailed tests (such as x-rays) do not give enough information. It is commonly used to get detailed pictures of the brain and spinal cord, to detect abnormalities and tumours.

For example, if you have back pain it may be caused by a disc which is protruding (a 'slipped disc'). A normal x-ray cannot show the disc, only the spinal bones, and so is not very useful. An MRI scan can show the discs clearly, and will show if one is protruding and pressing on a nerve. This helps a surgeon to decide if an operation is needed.

Even torn ligaments around joints can be detected by an MRI scan. So, it is being used more and more following sports injuries.

What preparation do I need to do before an MRI scan?

Usually very little. But there are certain safety procedures as you will be inside a strong magnet. Follow the instructions given to you at the time. For example, you must not have any loose metal on you such as jewellery as this could cause serious problems. Tell the person doing the test if you have a pacemaker fitted or hearing aid. Fixed bits of metal in your body such as an artificial hip or crews used to fix fractures are usually fine but it is best to tell the operator if you have any metal inside your body.

Are there any possible complications?

MRI scans are painless and safe. Unlike x-rays and some other imaging tests, an MRI scan does not use radiation. However:

• Rarely, some people have an allergic reaction to the contrast dye which is sometimes used. This can usually be treated immediately.
• Pregnant women may be advised not to have an MRI scan if it is not urgent. Although the scan is thought to be safe, the long-term effects of strong magnetic fields on a developing baby are not yet known.

What can I expect after the scan?

There are no after-effects form the scan. You can return to your normal activities as soon as the scan is over. However, if you had a sedative for the scan you will need someone to accompany you home. You will not be able to drive until the effect of the sedative wears off.

The pictures from the scan are studied by an x-ray doctor (radiologist) who sends a report to the doctor who requested the scan.

© EMIS and PIP 2006   Updated: February 2006