Magnetic Resonance Imaging Scan

Magnetic resonance imaging (MRI) is a type of scan that is often used to diagnose health conditions that affect organs, tissue and bone.

Having an MRI scan is a painless procedure. MRI scans use strong magnetic fields and radio waves to produce a detailed image of the inside of the body. The device that carries out MRI scans is known as an MRI scanner. The scanner consists of a large tube that contains a series of powerful magnets. The scanner is illuminated and remains open at both ends at all times. The patient lies inside the tube during the scan.

History

The scientific principles behind MRI were discovered in 1946, but it was not until the 1970s that the technology became available to use these principles.

At first, most MRI scanners were only available at large research centres. However, as the price of the scanners decreased, they started to become more widely available and now many hospitals offer MRI facilities.

Safety

One of the main advantages of MRI is that, unlike X-rays, it does not involve exposing the body to radiation.

Extensive research has also been conducted into whether the magnetic and radio waves that are used in MRI could pose a risk to the human body. No evidence that there is a risk has ever been found. This means that MRI is one of the safest medical procedures currently available.

Glossary

MRI

MRI stands for magnetic resonance imaging. It is the use of magnets and radio waves to take detailed pictures of the inside of the body.

Joints

Joints are the connection point between two bones that allow movement.

Bone marrow

Bone marrow is the soft, spongy tissue in the centre of bones that produces blood cells.

Blood vessels

Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.

Structures

Structure is the way something is put together and how its different parts relate to each other.

Brain

The brain controls thought, memory and emotion. It also sends messages to the body controlling movement, speech and senses.

Spine

The spine supports the skeleton and surrounds and protects the delicate spinal cord and nerves. It is made up of 33 bones called the vertebrae.

Abdomen

The abdomen is the part of the body between the chest and the hips.

Magnetic resonance imaging (MRI) can be used to look at almost any part of the body. It is most often used to study:

  • the brain and spinal cord,
  • the heart and blood vessels,
  • other internal organs, such as the lungs or liver,
  • bones and joints, and
  • breasts

The brain and spinal cord

Due to the way MRI works, an MRI scanner can provide very detailed images of your brain and spinal cord (the long bundle of nerves that runs from your brain down your spine).

MRI scans are often used to diagnose conditions that can affect the brain and nervous system, such as:

The heart and blood vessels

MRI can be used to produce a detailed image of your heart. The image can often help detect specific problems with the heart, such as defects with the valves or chambers.

MRI can also be used to assess whether a person's heart has been significantly damaged after having a heart attack.

A type of MRI known as magnetic resonance angiography (MRA) can be used to study your blood vessels. MRA can help 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),
  • atherosclerosis (hardening of the arteries), and
  • narrowing of the veins (stenosis) that supply blood to important organs, such as the kidneys and lungs.

Internal organs

MRI is widely used to diagnose cancers that affect specific organs, such as lung cancer and prostate cancer. An MRI scan is also useful for assessing whether cancer has spread beyond a specific organ into nearby muscle or tissue.

MRI scans can also be used to diagnose other conditions that can affect the organs, such kidney disease.

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.

An MRI scan can be used to 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 provide an effective way of checking how far the cancer has spread through the breast.

Functional MRI

Functional MRI is a new technique used to study the workings of the brain. Rather than taking a single scan, functional MRI takes repeated scans, usually one a second. These are used to track the movement of blood through the brain.

By studying the movement of blood, it is possible to tell which sections of the brain are particularly active in real time and to see 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 will then be used to determine which parts of the brain are active during these tasks.

Functional MRI is a relatively new technique, but it has been used by a number of specialists to help plan complex brain surgery.

Glossary

Joints
Joints are the connection point between two bones that allow movement.
Brain
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
Heart attack
A heart attack happens when there is a blockage in one of the arteries in the heart.
Bone marrow
Bone marrow is the soft, spongy tissue in the centre of bones that produces blood cells.
Tissues
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.  
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Heart
The heart is a muscular organ that pumps blood around the body.
Oxygen
Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.

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 is an even smaller particle called a proton. Protons are very sensitive to magnetic fields.

MRI scanners use powerful magnets

When the powerful magnets that are used in magnetic resonance imaging (MRI) 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.

The MRI scanner sends radio signals to certain areas of the body which 'snap' the protons out of position. When 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, just as millions of pixels can create images, so the radio signals of millions of protons can be collected together and combined to create a detailed image of the inside of the body.

Glossary

Tissue
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.  

Magnetic resonance imaging (MRI) can disrupt the working of certain medical devices that are implanted in the body, such as: 

  • Pacemakers: an electrical device used to control an irregular heartbeat.
  • Implantable cardioverter-defibrillator (ICD): a similar device to a pacemaker that uses electrical shocks to regulate heartbeats.
  • Nerve stimulators: electrical implants that are used to treat chronic nerve pain.
  • Cochlea implants: similar to hearing aids but surgically implanted inside the ear.
  • Drug pumps: implantable pumps that are used to treat chronic pain by delivering painkilling medication directly to an area of the body, such as the lower back.

An MRI scan is not usually recommended if you have any of the devices above. They may also be unsuitable for people who have:

  • 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.
  • Prosthetic (artificial) metal heart valves.
  • Penile implants, which are used to treat erectile dysfunction (impotence).
  • Eye implants, such as small metal clips that are used to hold the retina in place.
  • An artificial joint that has been fitted within the last six weeks.
  • Surgery to stop bleeding in the brain.

If you have a contraceptive coil (IUD) fitted, you must tell the radiographer (a specialist in using medical imaging techniques such as MRI) because some have copper wire in them.

If you think you may have metal fragments in your body, for example if you do metalwork or welding for a living, you will have an X-ray to find out if this is the case.

MRI scans are usually suitable for people with:

  • Artificial joints, such as hip or knee replacements.
  • Dental fillings and bridges.
  • Tubal ligation clips, which are used in female sterilisation.
  • Surgical clips or staples.

Some people with tattoos have reported a burning sensation during an MRI scan. This is because some tattoo ink contains traces of metal. If, during the scan, you experience any pain related to your tattoo, tell the radiographer immediately. Covering the tattoo with an ice pack during the scan may help to prevent further burning.

While there is no evidence that MRI scans pose a risk during pregnancy, as a precaution, scanning is not usually recommended during the first three months of pregnancy.

Glossary

Aneurysm
An aneurysm is a blood-filled sac that forms in a weakened part of a blood vessel.
Brain
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
X-ray
An X-ray is a painless way of producing pictures of inside the body using radiation.
Joint
Joints are the connection point between two bones that allow movement.
Heart
The heart is a muscular organ that pumps blood around the body.
Heart valves
Heart valves are four sets of flaps that control the direction that blood pumps around the heart.

Preparing for the scan

On the day of your magnetic resonance imaging (MRI) scan, you should be able to eat and drink and take any medication as usual.

One exception is if you are having an MRI scan of your bile ducts, known as magnetic resonance cholangiopancreatography or MRCP. In this case, you will usually be asked not to eat or drink anything for two to three hours before the scan.

Due to the strong magnetic fields that are used by the MRI scanner, it is important to remove any metal objects from your body, including:

  • watches,
  • jewellery, such as rings,
  • piercings, such as ear, nipple and nose rings,
  • dentures (false teeth),
  • hearing aids, and
  • wigs (as 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. Otherwise, wear clothing without metal zips, fasteners, buttons, belts or buckles.

For some MRI scans, you will be given an injection of a special dye, known as a contrast agent. This makes certain tissues and blood vessels show up more clearly and with greater detail on the scan.

As MRI scans are painless, anaesthetic is not usually required. If you are claustrophobic, you may want to request a mild sedative during the scan in order to help you relax. If this is the case, you should inform your GP or consultant well in advance of having your scan so that it can be arranged. If you decide to be sedated, you will need to arrange for a friend or a family member to drive you home after the scan because you will be unable to drive for the first 24 hours (see After the scan, below).

A general anaesthetic is often used when babies and young children are given an MRI scan. This is because it is important to stay very still during the scan, which babies and young children are often unable to do if they are awake.

During the scan

An MRI scanner is a short tunnel which is open at both ends. During the procedure, you lie on a motorised bed, which is moved inside the scanner.

A small receiving device is placed behind or around the part of your body being scanned. You are moved into the scanning tube, either head- or feet-first, depending on which part of your body is being scanned.

A computer is used to operate the MRI scanner. The computer is located in a different room to the scanner to keep it away from the magnetic field generated by the scanner.

As the radiographer operates the computer, they will also be in a separate room to you. You will hold a bell in your hand during the scan and if you need to talk to the radiographer you can ring the bell and talk to them via the intercom. The radiographer will be able to see you at all times on a television monitor.

During 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 in the scanner room with you will be asked the same questions as you about pacemakers and metal objects in their body, and will have to follow the same guidelines about 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 procedure. Depending on the size of the area being scanned and how many pictures are taken, a typical scan lasts between 15 and 90 minutes.

At certain times during the procedure, the MRI scanner will make a loud knocking noise. You may be given earplugs or headphones to wear. The noise is caused by the magnets in the machine being turned on and off.

After your scan has been completed, you will be moved back out of the scanner.

After the scan

MRI scans are usually performed as an outpatient procedure, so there is no need for an overnight stay in hospital. Once the scan is over, most people can resume their normal activities immediately.

However, if you have been given a sedative, you will need to be taken home by a friend or relative, and someone should stay with you for the first 24 hours. It is not safe to drive, operate machinery or drink alcohol for 24 hours after you have had a sedative.

If you have been given an injection of a contrast agent, it is a good idea to drink a lot of water for the following 24 hours to help flush the dye out of your body.

As your MRI scan will need to be studied by the radiologist and possibly a number of other related specialists, it is unlikely that the results of your scan will be known immediately.

The radiologist will report their findings to the doctor who arranged the scan, who will discuss the results with you. Unless they are needed urgently, it usually takes a couple of weeks for the results of an MRI scan to come through.

Glossary

Tissues
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.  
Joint
Joints are the connection point between two bones that allow movement.
Brain
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
Anaesthetic
Anaesthetic is a drug used to either numb a part of the body (local) or to put a patient to sleep (general) during surgery.
Heart valve
Heart valves are four sets of flaps that control the direction that blood pumps around the heart.

Advantages and disadvantages

Advantages

The main advantages of magnetic resonance imaging (MRI) scans are:

  • They do not involve exposure to radiation, so they can be safely used in people who may be vulnerable to the effects of radiation, such as pregnant women and babies.
  • They are particularly useful for showing soft tissue structures, such as ligaments and cartilage, and organs such as the brain, heart and eyes.
  • They can provide information about how the blood moves through certain organs and blood vessels, allowing problems with blood circulation, such as blockages, to be identified.

Disadvantages

The main disadvantages of MRI scans are:

  • MRI scanners are very expensive. A single scanner can cost over £1 million. This means that the number of scanners the HSE can afford to fund is limited. Therefore, if your condition is non-urgent, you may have to wait several months to have an MRI scan.
  • The combination of being put in an enclosed space and the loud noises that are made by the magnets can make some people feel claustrophobic while they are having a MRI scan.
  • MRI scanners can be affected by movement, making them unsuitable for investigating problems such as mouth tumours because coughing or swallowing can make the images that are produced less clear.

Since magnetic resonance imaging (MRI) scans were first introduced in the 1970s, millions of scans have been carried out. During that time, there has been no evidence to suggest that the magnetic waves used during MRI scans pose any health risks.

There is also no evidence that MRI scans pose a risk during pregnancy. However, as a precaution, the use of MRI scans is not recommended during the first trimester of pregnancy.

Projectile accidents

There have been a number of accidents where unsecured metal objects, such as mops or oxygen cylinders, were pulled towards the MRI scanner when the magnetic field was turned on, resulting in the person in the scanner being injured. These types of accidents are known as projectile accidents.

Glossary

Allergic
An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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