MRI scan
Introduction
Magnetic resonance imaging (MRI) is a type of scan used to diagnose health conditions that affect organs, tissue and bone.
MRI scanners use 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 a series of powerful magnets. You lie inside the tube during the scan.
An MRI scan can be used to investigate almost any part of the body including the:
- brain and spinal cord
- bones and joints
- breasts
- heart and blood vessels
- internal organs, such as the lungs and liver
Read more about why MRI scans are used.
What happens during an MRI scan?
During an MRI scan, you lie on a flat bed which is moved into the scanning tube. Depending on the part of your body being scanned, you will be moved into the scanner either head first or feet first.
The MRI scanner is controlled by a computer which is in a different room from the scanner. This is to keep it away from the magnetic field generated by the scanner.
A radiographer will operate the computer. A radiographer is someone who is trained in carrying out X-rays and similar procedures. You will be able to talk to them through an intercom and they will be able to see you on a television monitor throughout the scan.
At certain times during the MRI scan, the scanner will make a loud clicking sound. This is the magnets being turned on and off. You will be given earplugs or headphones to wear.
It is very important that you keep still during your MRI scan. The scan will last between 15 and 90 minutes, depending on the size of the area being scanned and how many images are taken.
Read more about how an MRI scan is performed.
Safety
An MRI scan is a painless and harmless procedure. You may experience a slight sense of claustrophobia (fear of enclosed spaces) when you are inside the scanner.
Unlike X-rays, MRI scans do not involve exposing the body to radiation. This means that people who may be vulnerable to the effects of radiation, such as pregnant women and babies, can safely use them.
However, not everyone can have an MRI scan. For example, they are not recommended for people who have certain types of implants fitted, such as a pacemaker (a battery operated device that helps control an irregular heartbeat).
Read more about when MRI scans are not recommended.
Extensive research has been carried out into whether the magnetic fields and radio waves used during MRI scans could pose a risk to the human body.
No evidence to suggest that there is a risk has been found, which means that MRI is one of the safest medical procedures currently available.
^^ Back to top
What is it used for?
Magnetic resonance imaging (MRI) can be used to produce detailed images of almost any part of the body.
MRI scans are often used to examine the:
- brain and spinal cord
- bones and joints
- breasts
- heart and blood vessels
It is also possible for an MRI scan to be used to examine other internal organs, such as the lungs or liver.
Brain and spinal cord
An MRI scan can produce very detailed images of the brain and spinal cord (the bundle of nerves that runs from your brain down your spine).
This makes MRI ideal for helping to diagnose conditions that affect the brain and nervous system. For example:
Bones and joints
MRI scans are a very effective way of checking for damage or abnormalities to the soft tissue found in bones and joints, such as cartilage, tendons, muscles and ligaments.
This means that an MRI scan can be used to help diagnose conditions that affect the bones and joints, such as arthritis and osteomyelitis (infection of the bones).
An MRI scan can also be used to assess damage to the cartilage, tendons, muscles and ligaments in sports injuries, as well as other types of injury that can damage a joint.
Breasts
MRI scans are now often used to help diagnose breast cancer. They can provide a way of checking how much of the breast is affected by cancer, although they are not needed in all cases. MRI is also sometimes used to assess breast implants.
Heart and blood vessels
MRI scans can be used to produce detailed images of your heart. The images can often help detect specific problems with the heart, such as defects with the valves or chambers.
MRI scans can also help assess whether a person’s heart has been significantly damaged after having a heart attack.
A type of MRI that is known as magnetic resonance angiography (MRA) can be used to study your blood vessels. It can help to diagnose conditions that affect the blood vessels such as:
- aneurysms – a bulge in a blood vessel that is caused by a weakness in the vessel wall
- narrowing of the arteries that supply blood to organs, such as the kidneys, or the legs
Internal organs
MRI is widely used to study how far cancers that affect specific organs have spread – for example, in cases of prostate cancer and cancer of the womb (uterus).
An MRI scan is also useful for assessing whether cancer has spread beyond a specific organ into nearby tissues or to distant sites such as lymph nodes.
Lymph nodes are small glands that are found in several areas of the body, including in the armpits and groin. They act as filters, preventing foreign particles from entering the bloodstream.
Functional MRI
Functional MRI is a fairly new technique that is used to study the workings of the brain. A functional MRI takes repeated scans, usually one a second, rather than a single scan.
The scans are used to track the movement of blood through the brain. This allows the sections of the brain that are particularly active to be identified. It also shows how brain activity responds to outside events and activities.
For example, a volunteer may be asked to solve a problem or to remember a short phrase. Functional MRI can then be used to determine which parts of the brain are active during these tasks.
Functional MRI is sometimes used to help surgeons when planning complex brain surgery.
^^ Back to top
How does it work?
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 very sensitive to magnetic fields.
When the powerful magnets that are used in MRI scans are switched on, all the protons in your body are pulled towards the same direction, in the same way that a magnet can pull the needle of a compass.
Short bursts of radio waves are sent to certain areas of the body and pull the protons out of position. As this happens, each proton transmits a radio signal that provides information about its exact location in the body.
On its own, a single proton will not provide much useful information, in the same way that a single pixel on a computer screen is essentially just a coloured dot.
However, in the same way that millions of pixels can create images, the radio signals of millions of protons can be collected together and combined to create a detailed image of the inside of the body.
^^ Back to top
Who can use it?
Magnetic resonance imaging (MRI) is very safe and most people can have the procedure, including pregnant women and babies.
Medical devices
However, the strong magnets that are used during MRI can interfere with certain medical devices that are implanted in the body. Therefore, an MRI scan may not be recommended if you have:
- a pacemaker: an electrical device that is used to control an irregular heartbeat
- an implantable cardioverter-defibrillator (ICD): a similar device to a pacemaker that uses electrical shocks to regulate heartbeats
- a nerve stimulator: an electrical implant that is used to treat long-term nerve pain
- a cochlea implant: a device that is similar to a hearing aid but is surgically implanted inside the ear
- a drug pump: used to treat long-term pain by delivering painkilling medication directly to an area of the body such as the lower back
In some cases, it is safe for people with pacemakers and implanted defibrillators to have MRI scans if certain procedures are followed.
Before the scan, a cardiologist (heart specialist) or another trained healthcare professional will need to make your pacemaker or defibrillator MRI-safe.
They will also need to carefully monitor your heart rhythm during the procedure. However, in many situations this is not possible and alternatives to MRI may be necessary.
MRI may also be unsuitable for people who have:
- brain aneurysm clips: small metal clips that are 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
- prosthetic (artificial) metal heart valves
- penile implants: used to treat erectile dysfunction (impotence)
- eye implants: such as small metal clips that are used to hold the retina in place
It may be possible for you to have a scan even if you have brain aneurysm clips in place. This will need to be considered on an individual basis.
You must tell the radiographer if you have an intrauterine device (IUD) fitted, although it is not likely to prevent you having an MRI scan. A IUD is a contraceptive device made of plastic or copper that fits inside the womb.
If you're unsure whether or not you have metal fragments in your body, for example, from metalwork or welding, you will need to have an X-ray to confirm whether this is the case.
MRI is usually suitable for people with:
- artificial joints, such as those who have had a hip replacement or knee replacement
- dental fillings and bridges
- tubal ligation clips, which are used in female sterilisation (an operation that permanently prevents a woman from being able to get pregnant)
- surgical clips or staples
Tattoos
Some tattoo ink contains traces of metal, but most tattoos are safe in an MRI scanner. Tell the radiographer immediately if you feel any discomfort or heat to your tattoo.
Pregnancy
There is no evidence to suggest that MRI scans pose a risk during pregnancy. However, as a precaution, scanning is not usually recommended during the first three months of pregnancy.
^^ Back to top
How is it 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 and drink and take any medication as usual.
However, if you are having an MRI scan of your bile ducts, you will usually be asked not to eat or drink anything for up to four hours before the scan. This type of MRI scan is known as magnetic resonance cholangiopancreatography (MRCP).
Due to the strong magnetic fields that are produced by the MRI scanner, it's important to remove any metal objects from your body, including:
- watches
- 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)
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, 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 with greater detail.
It's possible for contrast dye to cause tissue and organ damage in people with severe kidney disease. Therefore, if you have a history of kidney disease you 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. If you are claustrophobic, you could ask for a mild sedative to help you relax. 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 because you will be unable to drive for the first 24 hours (see below).
A 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 tunnel 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.
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.
At certain times during the procedure, the MRI scanner will make loud tapping noises. This is the electric current in the scanner coils being turned on and off. You will be given earplugs or headphones to wear.
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.
^^ Back to top