About an MRI scan

Magnetic resonance imaging (MRI) is a type of scan that uses strong magnetic fields and radio waves to produce detailed images of the inside of the body.

An MRI scanner is a large tube that contains powerful magnets. You lie inside the tube during the scan.

An MRI scan can be used to examine any part of the body.

The results of an MRI scan can be used to:

  • help diagnose conditions
  • plan treatments 
  • assess how effective previous treatment has been

Before an MRI scan

On the day of your MRI scan, you should be able to eat, drink and take any medication as usual, unless you're advised otherwise.

In some cases, you may be asked not to eat or drink anything for up to 4 hours before the scan. Other patients may be asked to drink quite a large amount of water beforehand. This depends on the area being scanned.

When you arrive for your scan, you'll be asked to fill in and sign a questionnaire about your health and medical history. This helps to ensure that it's safe for you to have the scan.

As the MRI scanner uses strong magnetic fields, it's important to remove any metal objects from your body. These include:

  • watches
  • jewellery, like earrings and necklaces
  • piercings, like ear, nipple and nose rings
  • dentures (false teeth)
  • hearing aids
  • wigs (some wigs contain traces of metal)
  • hairpins
  • medicine patches (like nicotine or hormone patches)
  • glucose monitors
  • tethered or patch insulin pumps

You should avoid bringing valuables along with you to your MRI scan. However, any valuables you do bring 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 don't need to wear a gown, you should wear clothes without metal zips, fasteners, buttons, underwire (bras), belts or buckles.

Is an MRI scan safe?

An MRI scan is a painless and safe procedure. Extensive research has been carried out into whether the magnetic fields and radio waves used during MRI scans may pose a risk to the human body. No evidence has been found to suggest there's a risk, which means MRI scans are one of the safest medical procedures available.

Some patients occasionally experience a tingling sensation or feel hot from being in the MRI scanner. These effects only last a short while and should ease as soon as the scan is over.

You're given a squeeze alarm which can alert the Radiographers if you have any concerns during your scan.

MRI scans may not be recommended for patients with certain implants or foreign bodies. If you have any metal in your body, you should tell the person referring you for the scan,. This can then be looked into before your scan.

Read more about who can and can't have an MRI scan.

Contrast agents

Some MRI scans involve having an injection of a contrast agent. This makes certain tissues and blood vessels show up more clearly and in greater detail.

Sometimes the contrast agent can cause side effects, like:

  • feeling or being sick
  • a skin rash
  • a headache
  • dizziness

These side effects are usually mild and don't last very long.

It's also possible for contrast agents to cause tissue and organ damage in people with severe kidney disease. You may be given a blood test to determine how well your kidneys are functioning and whether it's safe to go ahead with the scan.

You should let the staff know if you have a history of allergic reactions or any blood clotting problems before having the injection.

Anaesthesia and sedatives

An MRI scan is a painless procedure, so anaesthesia (painkilling medication) isn't usually needed.

If you're claustrophobic, you can ask for a mild sedative to help you relax. You should ask your GP or consultant well before having the scan.

If you decide to have a sedative during the scan, you'll need to arrange for a friend or family member to drive you home afterwards, as you won't be able to drive for 24 hours.

Babies and young children may be given a general anaesthetic before having an MRI scan. This is because it's very important to stay still during the scan, which babies and young children are often unable to do when they're awake.

What happens during an MRI scan?

An MRI scanner is a short cylinder that's open at both ends. You'll lie on a flat motorised bed that moves inside the scanner.

In some cases, a frame may be placed over the body part being scanned, like the head or chest. This frame contains receivers that pick up the signals sent out by your body during the scan. This can help to create a better quality image.

Depending on the part of your body being scanned, you'll be moved into the scanner either head or feet first.

Who operates an MRI scanner?

The MRI scanner is normally operated by a Radiographer, who is trained in carrying out imaging investigations. They control the scanner using a computer. This is in a different room to keep it away from the magnetic field generated by the scanner.

You'll be able to talk to the Radiographer through an intercom. They'll also be able to see you throughout the scan via a television monitor and a viewing window.

During the scan

At certain times during the scan, the scanner will make loud tapping noises. This is the electric current in the scanner coils being turned on and off. It can be extremely loud and patients have often compared it to standing immediately next to roadworks.

You'll be given earplugs or headphones to wear to help you feel more comfortable.

You're 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.

To avoid the images being blurred, it's very important to keep your whole body still throughout the entire scan.

Modern MRI scanners have a wider tunnel, which helps reduce claustrophobia. If you're claustrophobic you should tell the Radiographer. They'll be able to support you during your scan. Going into the scanner feet first may be easier for claustrophobic patients, although this isn't always possible.

How long does an MRI scan take?

A single scan may take a few seconds or 3 to 8 minutes. You may be asked to hold your breath during short scans.

The total scan lasts 15 to 90 minutes, depending on the size of the area being scanned and how many images are needed.

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 you won't need to stay in hospital overnight.

After the scan, you can resume normal activities immediately. 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.

Getting your MRI scan results

Your MRI scan needs to be studied by a Radiologist (a doctor trained in interpreting scans and X-rays). It may also need to be discussed with other specialists. This means it's unlikely you'll get the results of your scan immediately.

The Radiologist will send a report to the doctor who arranged the scan. They'll discuss the results with you.

It usually takes 1 to 2 weeks for the results of an MRI scan to come through, unless they're needed urgently.

How does an MRI scan work?

Most of the human body is made up of water molecules. These consist of hydrogen and oxygen atoms.

At the centre of each hydrogen atom is an even smaller particle called a proton. Protons are like tiny magnets and are very sensitive to magnetic fields.

The MRI scanner has powerful magnets. This means that when you lie inside it, the protons in your body line up in the same direction. This is similar to when a magnet pulls the needle of a compass. You will not be able to feel this happening.

Short bursts of radio waves are then sent into the body, knocking the protons out of alignment. When the radio waves are turned off, the protons realign. This sends 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 show the difference between types of tissue in the body. This is because the protons in different types of tissue realign at different speeds and produce distinct signals.

Who can have an MRI scan?

Magnetic resonance imaging (MRI) is very safe and most people are able to have the procedure.

However, there are some instances where an MRI scan may not be recommended.

Before having an MRI scan, you should tell medical staff if:

  • you think you have any metal in or on your body
  • you're pregnant or breastfeeding
  • you have had a previous allergic reaction to a contrast agent
  • you have known problems with your kidneys

You should tell the person who is referring you for an MRI scan if you have any metal in your body. You must also tell them if you're pregnant. This will allow the Radiology department to check whether it's safe for you to have your MRI scan before you arrive for your appointment.

Pregnancy

There's no evidence to suggest MRI scans pose a risk during pregnancy. However your specific case will be reviewed by the clinical team looking after you to decide whether an MRI scan is right for you.

The Radiographers will change the settings on the scanner to make it as safe as possible for you and your baby.

Contrast agents, which are sometimes given as part of the scan, will only be given if a Radiologist decides this is right for you.

Metal implants or fragments

Having something metallic in your body doesn't necessarily mean you can't have an MRI scan. However, it's important that the medical staff carrying out the scan are aware of it.

They can decide on a case-by-case basis if there are any risks. They'll also decide if further measures need to be taken to ensure the scan is as safe as possible. For example, it may be possible to make it safe for a pacemaker or defibrillator to be scanned when certain conditions are met.

You may need to have an X-ray if you're unsure about any metal fragments in your body.

Examples of metal implants or fragments that you should make your Radiographer aware of before being scanned include:

  • a pacemaker – a small electrical device used to control an irregular heartbeat
  • an implantable cardioverter-defibrillator (ICD) – a similar device to a pacemaker that uses electrical shocks to regulate heartbeats
  • metal plates, wires, screws or rods – used during surgery for bone fractures
  • a nerve stimulator – an electrical implant used to treat long-term nerve pain
  • a cochlear implant – a device similar to a hearing aid that's surgically implanted inside the ear
  • a drug pump implant – used to treat long-term pain by delivering painkilling medication directly to an area of the body, such as the lower back
  • brain aneurysm clips – small metal clips used to seal blood vessels in the brain that would otherwise be at risk of rupturing (bursting)
  • metallic fragments in or near your eyes or blood vessels (people who do welding or metalwork for a living have a higher risk of this)
  • prosthetic (artificial) metal heart valves
  • penile implants – used to treat erectile dysfunction (impotence)
  • eye implants – like small metal clips used to hold the retina in place
  • an intrauterine device (IUD) – a contraceptive device made of plastic or copper that fits inside the womb
  • artificial joints – like those used for a hip replacement or knee replacement
  • dental fillings and bridges
  • tubal ligation clips – used in female sterilisation
  • surgical clips or staples – used to close wounds after an operation
  • tattoos and permanent make-up
  • foreign bodies like bullets or shrapnel
  • breast expanders
  • insulin pumps
  • glucose monitoring devices

Most implants can be scanned safely using MRI. However, it's important that the Radiology department are aware of them. This means they can check whether any adjustments need to be made to ensure the scan is safe for you.

Last updated:
14 September 2022