Magnetic resonance imaging (MRI) is a type of scan
MRI scans use strong magnetic fields and radio waves to produce a detailed image of the inside of the body. The device that carries out MRI scans is known as an MRI scanner. The scanner consists of a large tube that contains a series of powerful magnets. You lie inside the tube during the scan.
How is an MRI used?
Magnetic resonance imaging (MRI) can be used to look at almost any part of the body.
Due to the way MRI works, an MRI scanner can provide very detailed images of your spinal cord (the long bundle of nerves that runs from your brain down your spine). It is also a very effective way of checking for damage or abnormalities to the soft tissue found in bones and joints, such as:
- cartilage
- tendons
- muscles
- ligaments
How does an MRI work?
Most of the human body is made up of water molecules, which consist of hydrogen and oxygen atoms. At the centre of each hydrogen atom is an even smaller particle called a proton. Protons are very sensitive to magnetic fields.
MRI scanners use powerful magnets. When the powerful magnets that are used in magnetic resonance imaging (MRI) are switched on, all the protons in your body are pulled towards the same direction, in the same way that a magnet can pull the needle of a compass.
The MRI scanner sends radio signals to certain areas of the body which ‘snap' the protons out of position. When this happens, each proton transmits a radio signal that provides information about its exact location in the body.
On its own, a single proton will not provide much useful information, in the same way that a single pixel on a computer screen is essentially just a coloured dot. However, just as millions of pixels can create images, so the radio signals of millions of protons can be collected together and combined to create a detailed image of the inside of the body.
How do I prepare for the scan?
On the day of your magnetic resonance imaging (MRI) scan, you should be able to eat and drink and take any medication as usual.
Due to the strong magnetic fields that are used by the MRI scanner, it is important to remove any metal objects from your body, including:
- watches
- jewellery, such as rings
- piercings, such as ear, nipple and nose rings
- dentures (false teeth)
- hearing aids
- wigs (as some wigs contain traces of metal)
Depending upon which part of your body is being scanned, you may need to wear a hospital gown during the procedure. Otherwise, wear clothing without metal zips, fasteners, buttons, belts or buckles.
As MRI scans are painless, anaesthetic is not usually required. If you are claustrophobic, you can request a mild sedative during the scan to help you relax. If this is the case, inform the radiographer. If you decide to be sedated, you will need to arrange for a friend or a family member to drive you home after the scan because you will be unable to drive for the first 24 hours.
What happens during the scan?
An MRI scanner is a short tunnel which is open at both ends. During the procedure, you lie on a motorised bed, which is moved inside the scanner.
A small receiving device is placed behind or around the part of your body being scanned. You are moved into the scanning tube, either head or feet first, depending on which part of your body is being scanned.
A computer is used to operate the MRI scanner. The computer is located in a different room to the scanner to keep it away from the magnetic field generated by the scanner.
As the radiographer operates the computer, they will also be in a separate room to you. However, you will be able to talk to them, usually through an intercom, and they will be able to see you at all times on a television monitor.
During your scan, a friend or family member may be allowed to stay in the room with you. Anyone who stays in the scanner room with you will be asked the same questions as you about pacemakers and metal objects in their body, and will have to follow the same guidelines about clothing and removing metallic objects.
To avoid the images being blurred, it is very important that you keep the part of your body being scanned still throughout the procedure. Depending on the size of the area being scanned and how many pictures are taken, a typical scan lasts between 15 and 90 minutes.
At certain times during the procedure, the MRI scanner will make a loud knocking noise. You may be given earplugs or headphones to wear. The noise is caused by the magnets in the machine being turned on and off.
After your scan has been completed, you will be moved back out of the scanner.
What happens after the scan?
MRI scans are usually performed as an outpatient procedure, so there is no need for an overnight stay in hospital. Once the scan is over, most people can resume their normal activities immediately.
However, if you have been given a sedative, you will need to be taken home by a friend or relative, and someone should stay with you for the first 24 hours. It is not safe to drive, operate machinery or drink alcohol for 24 hours after you have had a sedative.
As your MRI scan will need to be studied by the radiologist and possibly a number of other related specialists, it is unlikely that the results of your scan will be known immediately.
The radiologist will report their findings to the doctor who arranged the scan, who will discuss the results with you. Unless they are needed urgently, it usually takes a couple of weeks for the results of an MRI scan to come through.
Last reviewed: November 2010