An MRI scan uses magnets and radiowaves to produce detailed images of the inside of the body. It can help to diagnose and monitor many different medical conditions.
You will meet the health professional carrying out your procedure to discuss your care. It may differ from what is described here as it will be designed to meet your individual needs. Details of the procedure may also vary from country to country.
This information was published by Bupa's health information team and is based on reputable sources of medical evidence. It has been peer reviewed by Bupa doctors. The content is intended for general information only and does not replace the need for personal advice from a qualified health professional.
An MRI scan uses radiowaves and a magnetic field to create images of the inside of your body. The images that an MRI scanner takes are very detailed. They are taken in thin 'slices' through your body and can be taken from any direction. They show your bones, soft tissues such as muscle, skin, nerves and blood vessels, and organs such as your brain and heart. An MRI scanner can take pictures of most parts of your body.
Because an MRI scanner takes very detailed pictures of the body, it can often show things that aren't seen on an X-ray or in other tests such as an ultrasound.
You may have an MRI scan to find out the cause of symptoms such as pain, joint or muscle stiffness, swelling or weight loss. You may also have an MRI scan if you have been diagnosed with a condition such as cancer, where your doctor needs detailed information so you can make decisions about what treatment is best.
Not everyone can have an MRI scan. The magnetic field from the scan affects some metals and can cause any metal inside your body to move. This is why it's important to tell your radiographer (radiologic technologist; a health professional trained to perform imaging procedures) if you have any metal in your body. The kinds of metal you might have in your body include:
- heart pacemaker, metal valve or defibrillator (a device that keeps your heart rhythm regular)
- an inner ear hearing aid (cochlear implant)
- aneurysm clip (a metal clip on an artery)
- joint replacements or large metal implants
- an intra-uterine contraceptive device or coil
- shrapnel or gunshot wounds
- a body piercing
- metal fragments anywhere in your body
- some medication patches
- tattoos that have been done using a metallic ink
This isn't a complete list - your radiographer will go through a safety checklist with you before the scan.
Not all metals are affected by the scanner, for example some artificial joints like hips and knees aren't magnetic. Metal stitches or shrapnel pieces that have been inside your body for a long time may also not be affected by the scanner.
If you're pregnant then you wouldn't usually be offered an MRI scan, particularly in the first three months of pregnancy, unless it's absolutely necessary. If you are, or think you might be pregnant, tell your radiographer before your MRI appointment.
Other investigations that produce images of the inside of your body include the following.
- X-ray. An X-ray uses radiation to produce an image.
- Ultrasound. This uses sound waves to produce an image.
- Computerised tomography (CT) scan. This is a test that uses X-rays to create three-dimensional images of part of your body.
Talk to your doctor about which procedures are most suitable for you.
Most MRI scans don't need any special preparation. However, if you're having a scan of your abdomen (tummy) or your pelvis, you may be asked to drink extra fluids before the scan. This is to help your stomach and bowel show up better on the scan. Your radiographer will tell you if you should drink before your scan.
If you have claustrophobia, you should tell your radiographer before the day of your scan. You may be able to have a sedative to help you relax during the scan, but this will need to be arranged in advance.
Young children may also be offered a sedative or a general anaesthetic. This means they will be drowsy or asleep during the procedure.
You will need to have an MRI scan in a hospital, but you will usually be able to go home on the same day.
At the hospital, you will probably be asked to take off your outer clothes and put on a hospital gown. Take off any clothes that contain metal, for example zips, clasps or underwiring (in bras). You should also take off any jewellery, or metal objects such as hairclips, your watch or dentures with metal parts. Don't take any electronic or metal items, such as your keys, mobile phone or credit cards, with you into the scanning room.
You may be able to ask for a friend or relative to stay with you during the scan. He or she will also have to leave any metal or electronic items behind.
What happens during an MRI scan
An MRI scan usually takes between 20 and 30 minutes to complete. However, some scans may last for an hour. You will be asked to lie on your back on a table, which slides inside a cylinder-shaped machine. The radiographer may use pillows or straps to adjust your position and help you to stay still.
Usually the part of your body being scanned is placed in the middle of the scanner. The machine is open ended so you won't be completely enclosed at any time. The tunnel in the scanner is just over one metre (about four feet) long.
Your radiographer will operate the scanner from behind a window and will be able to see and hear you during the scan. You will be given a call button to hold during the scan, and you can press this button to talk to the radiographer in an emergency. He or she will let you know what is happening during the scan and what you need to do before and during the examination.
The MRI scanner makes loud knocking or buzzing sounds throughout the scan. You may be given earplugs or headphones to wear, and you may be able to listen to music during the scan.
It can take several minutes for each image to be taken, so it's important to lie very still and breathe gently. Your radiographer may ask you to hold your breath at certain times during the scan.
Sometimes a special dye (contrast medium) is used during the scan to produce more detailed images. If you have kidney problems, tell your radiographer when booking the MRI scan appointment. The dye is injected into a vein in your hand or arm. You may feel a warm sensation after the dye is injected, which lasts for a short time. A very small number of people may be allergic to the dye. Tell your radiographer before your scan if you have any allergies.
What to expect afterwards
You will usually be able to go home when you feel ready.
If you have a sedative during the scan, it can temporarily affect your co-ordination and reasoning skills, so you must not drive, drink alcohol, operate machinery or sign legal documents for 24 hours afterwards. If you're in any doubt about driving, contact your motor insurer so that you're aware of their recommendations, and always follow your doctor's advice. You will also need to arrange for someone to drive you home. Try to have a friend or relative stay with you for the first 24 hours after your scan.
A radiologist (a doctor who specialises in using imaging methods to diagnose medical conditions) will examine your MRI scans. The results will be sent to the doctor who requested your test.
MRI scans are commonly performed and generally safe. However, in order to make an informed decision and give your consent, you need to be aware of the possible side-effects and the risk of complications of this procedure.
Side-effects are the unwanted but mostly temporary effects you may get after having the test. You may feel slightly claustrophobic and uncomfortable from being inside the scanner. If you're worried about this, talk to your radiographer.
Complications are when problems occur during or after the procedure. Most people aren't affected. An MRI is a very safe test and there are no known complications or side-effects from the magnetic field used during the scan.
It's possible to have an allergic reaction to the contrast medium used, though this is very unlikely. If you have any itching or shortness of breath during the scan, tell your radiographer immediately. Medicines are available to treat the allergic reaction.
The exact risks are specific to you and will differ for every person, so we haven't included statistics here. Ask your radiographer to explain how these risks apply to you.
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