Magnetic resonance imaging scan
How an MRI scan works
During a magnetic resonance imaging (MRI) scan, you lie in a strong magnetic field and radio-frequency waves are directed at your body. This produces detailed images of the inside of your body.
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 there is an even smaller particle called a proton. Protons are like tiny magnets and are very sensitive to magnetic fields.
When you lie under the powerful scanner magnets, the protons in your body line up in the same direction, in the same way that a magnet can pull the needle of a compass.
Short bursts of radio waves are then sent to certain areas of the body, knocking the protons out of alignment. When the radio waves are turned off, the protons realign and in doing so send out radio signals, which are picked up by receivers.
These signals provide information about the exact location of the protons in the body. They also help to distinguish between the various types of tissue in the body, because the protons in different types of tissue realign at different speeds and produce distinct signals.
In the same way that millions of pixels on a computer screen can create complex pictures, the signals from the millions of protons in the body are combined to create a detailed image of the inside of the body.
How an MRI scan is performed
A magnetic resonance imaging (MRI) scan is a painless procedure that lasts between 15 and 90 minutes, depending on the size of the area being scanned and the number of images being taken.
Before the scan
On the day of your MRI scan you should be able to eat, drink and take any medication as usual, unless advised otherwise.
In some cases, you may be asked not to eat or drink anything for up to four hours before the scan, and sometimes you may be asked to drink a fairly large amount of water beforehand. This will depend on the area being scanned.
When you arrive at the hospital you will usually be asked to fill in a questionnaire about your health and medical history. This will help the medical staff performing the scan be as sure as possible that you can have the scan safely. Read more about who can and can’t have an MRI scan
Once you have completed the questionnaire, you will then usually be asked to give your signed consent for the scan to go ahead.
As the MRI scanner produces strong magnetic fields, it’s important to remove any metal objects from your body, including:
- jewellery, such as earrings and necklaces
- piercings, such as ear, nipple and nose rings
- dentures (false teeth)
- hearing aids
- wigs (some wigs contain traces of metal)
Any valuables can usually be stored in a secure locker.
Depending on which part of your body is being scanned, you may need to wear a hospital gown during the procedure. If you do not need to wear a gown, you should wear clothes without metal zips, fasteners, buttons, underwire (bras), belts or buckles.
Some MRI scans involve having an injection of contrast dye. This makes certain tissues and blood vessels show up more clearly and in greater detail.
It’s possible for contrast dye to cause tissue and organ damage in people with severe kidney disease. If you have a history of kidney disease, you therefore may be given a blood test to determine how well your kidneys are functioning and whether it is safe to proceed with the scan.
An MRI scan is a painless procedure, so anaesthesia (painkilling medication) is not usually required. You can ask for a mild sedative to help you relax if you are claustrophobic. If you would like a sedative, you should ask your GP or consultant well in advance of having the scan.
If you decide to have a sedative during the scan, you will need to arrange for a friend or family member to drive you home afterwards as you will be unable to drive for 24 hours.
General anaesthetic (medication that makes you unconscious) is often used when young children and babies have an MRI scan. This is because it is very important to stay still during the scan, which young children and babies are often unable to do when they are awake.
During the scan
An MRI scanner is a short cylinder that is open at both ends. You will lie on a motorised bed that is moved inside the scanner. You will enter the scanner either head first or feet first, depending on the part of your body being scanned.
In some cases, a frame may be placed over the body part being scanned, such as the head or chest. This frame contains receivers that pick up the signals sent out by your body during the scan and it can help to create a better quality image.
A computer is used to operate the MRI scanner, which is located in a different room to keep it away from the magnetic field generated by the scanner.
The radiographer operates the computer, so 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.
While you are having your scan, a friend or family member may be allowed to stay in the room with you. Children can usually have a parent with them. Anyone who stays with you will be asked whether they have a pacemaker or any other metal objects in their body. They will also have to follow the same guidelines regarding 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 whole of the scan until the radiographer tells you to relax.
A single scan may take from a few seconds to three or four minutes. You may be asked to hold your breath during short scans. Depending on the size of the area being scanned and how many images are taken, the whole procedure will take between 15 and 90 minutes.
The MRI scanner will make loud tapping noises at certain times during the procedure. This is the electric current in the scanner coils being turned on and off. You will be given earplugs or headphones to wear.
You are usually able to listen to music through headphones during the scan if you want to, and in some cases you can bring your own CD of music you would like to listen to.
You’ll be moved out of the scanner when your scan is over.
After the scan
An MRI scan is usually carried out as an outpatient procedure. This means that you will not need to stay in hospital overnight. After the scan, you can resume normal activities immediately.
However, if you have had a sedative, a friend or relative will need to take you home and stay with you for the first 24 hours. It’s not safe to drive, operate heavy machinery or drink alcohol for 24 hours after having a sedative.
Your MRI scan will need to be studied by a radiologist (a doctor trained in interpreting scans and X-rays) and possibly discussed with other specialists. It is therefore unlikely that you will know the results of your scan immediately.
The radiologist will send a report to the doctor who arranged the scan, who will discuss the results with you. Unless they are needed urgently, it usually takes a week or two for the results of an MRI scan to come through.