How is an MRI scan performed?

The scan is usually done as an outpatient procedure, which means that the patient can go home after the test. During the scan it is important to lie completely still.

For this reason it might be necessary to give a child an anaesthetic before they are tested.

Since you are exposed to a powerful magnetic field during the MRI scan, it is important not to wear jewellery or any other metal objects.

It is also important for the patient to inform medical staff if they use electrical appliances, such as a hearing aid or pacemaker, or have any metal in their body such as surgical clips, but orthopaedic metalware such as artificial hips or bone screws is not normally a problem.

MRI scan and CT scan

With an MRI scan it is possible to take pictures from almost every angle, whereas a CT scan only shows pictures horizontally.

There is no ionizing radiation (X-rays) involved in producing an MRI scan. MRI scans are generally more detailed, too. The difference between normal and abnormal tissue is often clearer on the MRI scan than on the CT scan.

How does an MRI scanner work?

The patient lies inside a large, cylinder-shaped magnet. Radio waves 10,000 to 30,000times stronger than the magnetic field of the earth are then sent through the body. This affects the body's atoms, forcing the nuclei into a different position. As they move back into place they send out radio waves of their own. The scanner picks up these signals and a computer turns them into a picture. These pictures are based on the location and strength of the incoming signals.

Our body consists mainly of water, and water contains hydrogen atoms. For this reason, the nucleus of the hydrogen atom is often used to create an MRI scan in the manner described above.

Importance of MRI

i. MRI is painless and has no known side effects.

ii. It usually reveals more details than other imaging modalities. Usually there is a clear distinction between healthy and diseased tissue on MRI.

iii. Harmful effects of radiation are avoided.

Problems with MRI

It may not be possible, or safe, to have an MRI scan if you have any of these items:

- Cardiac pacemaker
- Surgical clips in your head (particularly aneurysm clips)
- some artificial heart valves
- Electronic inner ear implants (bionic ears)
- Metal fragments in your eyes
- Electronic stimulators
- Implanted pumps

Let the MRI Unit know well before your appointment if you have any of these. Experienced MRI staff will have to discuss the exact implant or metal with you to decide if it is safe to perform the scan.

Deciding which implants cannot be scanned takes special knowledge and experience. Please do not try to guess, and don’t just rely on your doctor to determine if we can scan you.

Before the scan you will be asked a series of questions to check that it is safe for you to enter the scan room.

People with dental fillings and bridges, hip and knee replacements, and tubal ligation clips can all be scanned safely. The Radiographers will want to know about these things to minimise the effect they have on your images.

What happens during the MRI scan?

A radiographer performs the scan. He or she explains the procedure to you and answers your questions. Once you are inside the scan room, the radiographer helps you to lie on a comfortable couch. This moves you into the centre of the scanner. The scanner is open at both ends. It is important that you keep still during the scan, as movement causes blurring of the pictures. The radiographer can see you throughout the examination. If, at any time, you want to contact the radiographer, just squeeze the buzzer that we give you.

Sometimes we give you a small injection of contrast agent (a colourless liquid) and/or buscopan (a muscle relaxant). These help to show parts of the body more clearly and improve the scan. The whole procedure can take up to 2 hours but most patients are on the scanner for about 45 minutes.

Is there any preparation for my MRI scan?

Very little preparation is needed for your scan. You should continue with any medication and you can eat and drink normally. If you are having a head or neck scan, please do not wear make up or mascara as this can interfere with the scan. You may wear a wedding ring for your scan but you should remove all other jewellery.

When you arrive for your scan, we may ask you to wear a hospital gown. You are welcome to bring your own dressing gown to wear over this gown while you are waiting for your scan. We will also ask you to remove all metallic items such as keys, coins, hairgrips, and hearing aids as well as other items such as watches or swipe cards with magnetic strips. There are lockers to store your valuables.

Please give the key to the radiographer as it is magnetic.

What is an MRI scan?

Magnetic Resonance Imaging (MRI) combines a powerful magnet and radio waves with a computer to produce pictures of any part of the body. The detail in these pictures allows the doctor to see soft tissues such as muscle, fat and internal organs as well as bone.

The MRI scan uses magnetic and radio waves, meaning that there is no exposure to X-rays or any other damaging forms of radiation.