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 almost any part of the body, including the:
- brain and spinal cord
- bones and joints
- heart and blood vessels
- internal organs, such as the liver, womb or prostate gland
The results of an MRI scan can be used to help diagnose conditions, plan treatments and assess how effective previous treatment has been.
Blackrock Clinic use the referral criteria, ‘iRefer Guidelines, Making the Best Use of Clinical Radiology’. These guidelines are considered when requesting a diagnostic radiology/nuclear medicine examination.
What happens during an MRI scan?
During an MRI scan, you lie on a flat bed that’s moved into the scanner.
Depending on the part of your body being scanned, you’ll be moved into the scanner either head first or feet first.
The MRI scanner is operated by a radiographer, who is trained in carrying out imaging investigations.
They control the scanner using a computer, which 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 and they’ll be able to see you on a television monitor throughout 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.
You’ll be given earplugs or headphones to wear.
It’s very important to keep as still as possible during your MRI scan.
The scan lasts 15 to 90 minutes, depending on the size of the area being scanned and how many images are taken.
How does an MRI scan work?
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 like tiny magnets and are very sensitive to magnetic fields.
When you lie under the powerful scanner magnets, the protons in your body line up in the same direction, in the same way that a magnet can pull the needle of a compass.
Short bursts of radio waves are then sent to certain areas of 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 distinguish between the various types of tissue in the body, because the protons in different types of tissue realign at different speeds and produce distinct signals.
In the same way that millions of pixels on a computer screen can create complex pictures, the signals from the millions of protons in the body are combined to create a detailed image of the inside of the body.
An MRI scan is a painless and safe procedure. You may find it uncomfortable if you have claustrophobia, but most people are able to manage it with support from the radiographer.
Going into the scanner feet first may be easier, although this isn’t always possible.
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 has been found to suggest there’s a risk, which means MRI scans are one of the safest medical procedures available.
But MRI scans may not be recommended in certain situations. For example, if you have a metal implant fitted, such as a pacemaker or artificial joint, you may not be able to have an MRI scan.
They’re also not usually recommended during pregnancy.
Metal implants or fragments
Having something metallic in your body doesn’t necessarily mean you can’t have an MRI scan, but it’s important for medical staff carrying out the scan to be aware of it.
They can decide on a case-by-case basis if there are any risks, or if further measures need to be taken to ensure the scan is as safe as possible.
For example, it may be possible to make a pacemaker or defibrillator MRI-safe, or to monitor your heart rhythm during the procedure.
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 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 (common in people who do welding or metalwork for a living)
- prosthetic (artificial) metal heart valves
- penile implants – used to treat erectile dysfunction (impotence)
- eye implants – such as small metal clips used to hold the retina in place
- an intrauterine device (IUD) – a contraceptive device made of plastic and copper that fits inside the womb
- artificial joints – such as 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
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 in your tattoo during the scan.
Please watch this video for a guided tour through MRI and CT scanning: