MRI scan (magnetic resonance imaging scan)

Magnetic resonance imaging (MRI) is a type of scan that uses strong magnets and radio waves to produce very detailed two- and three-dimensional pictures of the inside of your body — in fact, the images produced are the most detailed of any type of scan available today. It does not use ionising radiation as X-rays do.

It’s suitable for every part of the body, including bones, soft tissues (such as blood vessels, ligaments and muscles) and the brain. This means it can help diagnose, plan treatment for and monitor many different medical conditions. Doctors who specialise in analysing MRI scans are called radiologists.

MRI contrast

An MRI contrast agent is a special dye used to improve the visibility of certain tissues on an MRI scan. If your doctor recommends that you have an MRI scan with a contrast agent, the agent will usually be injected into a vein in your body. The most common MRI contrast agents contain gadolinium and have been in use since 1983. Since this time, millions of patients worldwide have received gadolinium-based contrast agents before having an MRI scan.

Your doctor may recommend an MRI scan to examine different parts of your body including:

• Bones, joints and muscles — to help diagnose injuries or abnormalities affecting your joints, particularly your back, knee or hip and to detect conditions such as arthritis or tendon tears
• Brain and spinal cord — an MRI brain scan can look for brain tumours, possible causes of a headache, abnormal tissue growth and to assess damage after a stroke
• Breasts — a breast MRI can help detect breast cancer and is used alongside a mammogram to screen women at high risk
• Heart and blood vessels — a cardiac MRI scan captures images of your heart, heart valves and blood vessels to diagnose heart defects or disease or to assess damage after a heart attack
• Internal organs, including the liver, womb and prostate gland — to detect diseases, cysts or tumours
• Pelvis — to help diagnose the cause of pelvic pain in women, such as endometriosis or fibroids, or to investigate potential womb abnormalities in women with fertility problems

You can usually eat, drink and take your medications as usual on the day of your MRI scan. However, in some cases, depending on the part of your body being scanned, your doctor may ask you not to eat or drink anything for up to four hours before your MRI scan or conversely, they may ask you to drink a large amount of water just before your MRI scan.

When you arrive for your MRI scan, you will be given a questionnaire to complete about your medical history and health. This ensures your healthcare team are aware of any health issues you're facing so they can complete your scan safely. You will then need to sign a consent form to confirm that you agree to have an MRI scan.

You will be asked to remove all metal objects from your body as the MRI machine uses strong magnets. You will therefore need to remove:

• hearing aids
• jewellery eg bracelets, earrings and necklaces
• piercings eg piercings in your ears, nose and nipples
• dentures
• watches
• wigs — certain wigs have trace amounts of metal

Depending on the part of your body being scanned, you may be asked to change into a hospital gown. If you do not need to wear a hospital gown, you will need to wear clothes that do not contain any metal parts eg metal belts, buckles, buttons, fasteners or zips, or bras with an underwire.

Anaesthesia and sedatives

MRI scans are painless so you will not need any anaesthesia. However, babies and very young children may be given a general anaesthetic before their MRI scan as they can’t usually lie still enough during the scan.

If you are feeling very anxious about your MRI scan or are claustrophobic, make sure you talk to your doctor well before the day of your scan about having a sedative to help relax you.

If you have a sedative during your MRI scan, you will not be able to drive for 24 hours and will therefore need someone to take you home.

An MRI machine has a large cylinder containing a strong magnet, which you lie inside. When it’s turned on, it creates a magnetic field that sends out radio waves. These radio waves affect the water molecules that are in all of your cells.

Water molecules are made up of hydrogen and oxygen atoms. The radio waves affect the hydrogen atoms, specifically an even smaller particle in the centre of the hydrogen atoms called a proton. The protons move slightly so that they all point in the same direction. When the machine is switched off, the protons move back to their original positions and give off signals that are detected by the MRI machine.

The information from these millions of signals can be converted into very detailed images of the part of your body being scanned. Depending on which type of tissue the protons are in, they move back to their original positions at different speeds and give off slightly different signals — this is why different tissues can be distinguished on the images produced by an MRI scan.

What to expect during your MRI scan

For some types of MRI scan, you’ll need an injection of a contrast agent (a special dye), which shows up certain tissues more clearly.

You may be given earplugs or headphones to wear during your scan. You will lie down on a motorised bed that will move into the tunnel part of the MRI machine — the tunnel is open at either end, is well-lit and has a fan to help you feel comfortable. You will either move into the tunnel head-first or feet-first, depending on which part of your body is being scanned, to ensure the part that needs scanning is in the middle of the MRI machine.

An MRI technician will talk to you via a two-way intercom and you can talk back. They will make sure you feel comfortable before the scan starts. You will need to lie as still as possible but you can ask to stop the scan if you need to move or feel very anxious.

A single scan can be captured quite quickly, taking between a few seconds to three to four minutes. For short scans, the MRI technician may ask you to hold your breath. During your scan you will hear noises made by the MRI machine working; these may include banging, beeping, clanging, clicking, tapping and whirring.

Once your MRI scan is complete, you will be moved out of the MRI machine.

If you were given a contrast agent before your scan, the intravenous catheter used to administer the dye will be removed. Most contrast agents contain gadolinium and it is very rare to have an allergic reaction to it. However, if you develop a rash, hives or shortness of breath, let your MRI technician know immediately if you are still at the MRI department. If you have left the hospital, call your doctor or return to the hospital immediately.

If you were given a sedative, you may need to stay at the hospital for a short time until you are fully awake and alert. You will need someone to take you home as you will not be able to drive for 24 hours. You should also not drink alcohol or operate heavy machinery for 24 hours after having a sedative. It is not advised to sign any legal documents during this time either and you should have someone stay with you for 24 hours after your sedative.

An MRI scan is usually an outpatient procedure so you can go home on the same day as your scan. Unless you have had a sedative, you can immediately return to your usual activities.

An MRI scan takes between 15 and 90 minutes depending on the part of your body being scanned. You can go home shortly afterwards.

A radiographer (someone trained to perform scans) will carry out your MRI scan. Afterwards, a radiologist (a doctor trained to read MRI scans) will examine the images. A report will be sent to the doctor who requested your scan, this can sometimes take a few days. Your radiographer will tell you when you can expect to get your results.

An MRI scan is painless and very safe as there are no known side effects from exposure to the magnetic fields or radio waves. Some people may feel claustrophobic, but our experienced team can help you relax.

Potential risks or complications may include:

• A reaction to the contrast agent eg a rash, dizziness, headaches or sickness — this is very rare and usually mild but in extreme cases can cause a severe reaction that needs emergency treatment
• Nephrogenic systemic fibrosis specifically in people with severe kidney disease who receive high doses of the MRI contrast agent
• If you have kidney disease, you may need to have blood tests first to determine if it is safe for you to have a contrast agent during your MRI scan. MRI scans aren't usually recommended for pregnant women.

At Spire Healthcare, we’re careful to weigh up the benefits and risks of any MRI scan and discuss it with you if you have any concerns.

Can you have an MRI scan if you have a metal implant?

You may not be able to have an MRI scan if you have any metal implants or fragments in your body. You should therefore let your healthcare team know if you have any of the below implants:

• Aneurysm clips
• Artificial heart valves, pacemaker, vascular stent or stent-graft
• Cochlear implants
• Dorsal column stimulators or neurostimulators
• Implanted drug infusion device eg insulin pump
• Metallic implants or prostheses eg an artificial joint

You should also let your healthcare team know if you have:

• Bullet wounds or shrapnel
• Tattoos — some tattoos contain trace amounts of metal in the ink used although in most cases tattoos are safe in an MRI machine
• Worked as a sheet metal worker — you may have metal shavings in your eyes that you are unaware of

An MRI scan captures the details of your tissues differently from other scans. However, in some cases, an alternative scan can be used, such as an arthrography, bone scan, CT scan or ultrasound scan.