An MRI scan is a type of test that creates detailed pictures of the inside of your body. It uses a powerful magnet and radio waves and is often used to diagnose or monitor a condition. MRI stands for magnetic resonance imaging.
What is an MRI scan used for?
An MRI scan can be used to look at almost any part of your body. It can detect many problems such as inflammation, tumours or a torn ligament. The pictures created by an MRI scan can help to show up differences between healthy and unhealthy tissue.
In particular, an MRI scan can be used to diagnose or monitor:
- conditions that affect your brain, spine and nervous system, such as stroke and multiple sclerosis
- heart conditions such as valve problems
- problems with your blood vessels, such as narrowing of your arteries or risk of rupture (bursting)
- liver or kidney disease
- conditions or injuries that affect your bones and joints, such as arthritis or sports injuries
- bowel conditions such as Crohn’s disease or ulcerative colitis
Some MRI scans use a special dye called a contrast medium to create more detailed pictures. The dye is injected into your body, usually through a vein in your arm.
The images are taken in thin ‘slices’ through your body from various directions without you having to move. An MRI scan can often show things that can’t be seen on an X-ray or in other tests such as an ultrasound scan. For more information on this, see our FAQ: What’s the difference between an MRI scan and CT scan? below.
Who can have an MRI scan?
An MRI scan has lots of benefits but it isn’t suitable for everyone. The magnetic field from an MRI scanner affects some metals, including those used for surgical clips or pins. If you’ve had surgery that left metals inside your body, it’s important to tell your radiographer (a healthcare professional who specialises in imaging). This is because the magnets can make the metal move, which could damage nearby tissues.
The magnetic field from the MRI may also affect electronic implants such as pacemakers. This may mean the devices don’t work properly or may heat up, which could be dangerous. You can’t usually have an MRI scan if you have:
- a heart pacemaker or defibrillator (a device that keeps your heart rhythm regular)
- an inner-ear hearing aid (cochlear implant)
- an aneurysm clip (a metal clip on an artery in your brain)
If you’re pregnant, you won’t usually be offered an MRI scan, especially during the first three months. Doctors don’t know for sure if the magnetic field of an MRI could affect your unborn baby. But if you’re pregnant or think you may be, tell your radiographer before your MRI appointment. If you do need to have an MRI scan while you’re pregnant, the scan will be done in a way that reduces any risk to your baby.
If you’re claustrophobic, tell your doctor when they suggest you have an MRI scan. You may find it difficult to have an MRI scan because the machine is shaped like a tunnel. For more information, see our FAQ: Will my claustrophobia stop me having an MRI scan? below.
Preparing for an MRI scan
Before the day of your MRI scan
Before having an MRI scan, you should be able to eat, drink and take any medicines as usual. But your radiographer will give you specific instructions.
If you wear stick-on medicine patches, you may need to take these off before having an MRI scan, so take a spare one to the hospital. This is because some patches contain metal and may heat up during the scan.
On the day of your MRI scan
At the hospital, your radiographer will ask you to complete and sign a safety questionnaire. You’ll be asked if you’re pregnant or have any medical conditions. You’ll also be asked if you have any metal or electronic implants because these may cause problems during your MRI scan.
Your radiographer will go through a checklist of metal items that you may have inside your body. Having something metallic in your body doesn’t necessarily mean you can’t have an MRI scan. But your radiographer does need to know, so they can make the right safety decisions.
Before you go into the scanning room, you’ll probably be asked to undress down to your underwear and put on a hospital gown. You’ll probably be asked to remove your:
- underwired bra (which usually contains metal)
- jewellery (including piercings)
- metal hairclips
- dentures with metal parts
- glasses and hearing aids if you wear them
- make-up, especially eye make-up, which can contain metal fibres
Tell the radiographer if you have any tattoos, especially if they’re on the part of your body that’s being scanned. Your tattoo may need to be covered with a cold compress or ice pack during the scan. This is because tattoos can heat up if they contain any metal.
Don’t take any electronic or metal items, such as your keys, mobile phone or credit cards, with you into the scanning room. A friend or relative may be able to stay with you during the scan. They’ll also have to leave any metal or electronic items behind, and complete and sign a safety questionnaire.
Young children and people with claustrophobia may be offered a sedative or a general anaesthetic before an MRI scan. This means they’ll be drowsy or asleep during the procedure, which will help them to lie still.
Before you have an MRI scan with a dye
If you’re having an MRI scan that uses a dye (contrast medium), your doctor may need to inject this before or during your scan. Where it’s injected will depend on which part of your body needs scanning and why. You may need a blood test before your scan to check the dye is right for you.
You may not be able to have the dye if your kidneys aren’t working well. This may be because you:
- have diabetes or heart failure
- are very dehydrated
- are taking medicines that affect your kidneys
A very small number of people may be allergic to the dye. Tell your radiographer before your scan if you have any allergies.
What are the alternatives to an MRI scan?
If an MRI scan isn’t suitable for you, you may be able to have other tests that create pictures of the inside of your body. These include:
- X-ray – this uses radiation to create a picture
- ultrasound – this uses sound waves to create a picture
- computerised tomography (CT) scan – this uses X-ray equipment and computer software to create pictures
Your doctor will discuss your options and recommend the best test for you.
What happens during an MRI scan?
You’ll usually have your MRI scan and go home on the same day.
When you go for your MRI scan, your radiographer will ask you to lay on the scanner table. The table slides into the cylinder-shaped MRI scanner, which contains a magnet. Your position may be adjusted using foam pads. Your radiographer will make sure you’re comfortable and any equipment around you is in a safe place.
You’ll be told before your scan how long it’s likely to take. You’ll usually need to lie still for about 20 minutes, sometimes with your hands at your sides. Your scan may take longer than this depending on how much of your body is being scanned. You’ll be encouraged to close your eyes and relax as much as possible.
Making the MRI scan more comfortable
Your radiographer will operate the MRI scanner from behind a window. They may ask you to hold your breath at certain times during the scan. They’ll be able to see and hear you during the scan, and you can talk to them if you need to. An MRI scanner is very noisy and makes knocking or drilling sounds. This is the sound of the magnetic fields changing during the scan.
To make an MRI scan more comfortable for you, your radiographer will:
- talk to you regularly to reassure you and remind you to be as still as possible
- show you how to use the buzzer if you need to talk to them
- give you earplugs or headphones to wear or may play background music for you to listen to if you wish
An MRI scan probably won’t increase your body temperature but you may feel some warmth in the area of your body that’s being scanned. If you feel burning or discomfort in any part of your body during the test, tell your radiographer straight away.
What to expect afterwards
After your MRI scan, you’ll usually be able to go home when you feel ready. You should be able to go back to your normal routine straightaway.
A radiologist (a doctor who specialises in using imaging methods to diagnose medical conditions) will examine your MRI scans. The results will then be sent to the doctor who requested the test. You’ll have a follow-up appointment to find out the results.