Magnetic Resonance Imaging (MRI)

Authors: Mr Richard McIntyre*
                            Prof Stacy Goergen *

What is magnetic resonance imaging (MRI)?

Magnetic resonance imaging (MRI) is a scanning procedure that uses strong magnets and radiofrequency pulses to generate signals from the body. These signals are detected by a radio antenna and processed by a computer to create images (or pictures) of the inside of your body.

The MRI scanner is generally shaped like a large, covered box with a tunnel passing through it. A table, on which you lie, slides into the tunnel. Both ends of the scanner are open and will not close. The tunnel has lights in it and sometimes a mirror. Some of the MRI scanners have wider tunnels or are partially open (more like a ‘C’, rather than an ‘O’).

Why would my doctor refer me to have an MRI?

There are many imaging procedures that help doctors look for and assess medical conditions. Different procedures provide different specialised information. Sometimes more than one type of imaging might be used to show information about a problem.

MRI might be the best way of showing certain problems, such as for a knee injury, the brain or spine. MRI is also often used to provide additional information to other tests, such as X-ray or ultrasound.

There are many different techniques or types of MRI scans that can be carried out. Each one offers the radiologist (specialist doctor) specific information, to best show what the problem might be. The information gained using MRI should help to diagnose the medical condition you might have.

How do I prepare for a MRI?

Safety in the MRI scanner is vital. The strong magnetic fields can attract and interfere with metal objects that you might have in or on you (including electronic and magnetic devices). Some of these interactions can cause harm or death (see the section on Risks of MRI below).

To ensure it is safe for you to have an MRI, you will be required to complete a safety questionnaire. Sometimes a questionnaire is mailed to you that you will need to complete and take with you to the appointment. If a friend or relative will be in the scanning room with you, they will also need to complete a safety questionnaire.

If you have a pacemaker or other implants, it is important to tell the radiology practice before having the scan. An alternative test might need to be arranged.

Objects in your body that can cause particular harm or be damaged include: pacemakers, aneurysm clips, heart valve replacements, neurostimulators, cochlear implants, metal fragments in the eye, metal foreign bodies, magnetic dental implants and drug infusion pumps. Some of these implants, particularly more recent devices, might be safe to go into the MRI scanner, but have to be accurately identified for the scan to proceed.

You should take any documents about your implants to the appointment. These can help to correctly identify the type of implant to assess if it is safe for you to have the MRI.

It is important that you do not wear any makeup or hairspray, as many of these products have tiny metal particles that could interfere with the scan and reduce the quality of the images. They might cause the area to heat up and, on the rare occasion, burn your skin.

You will not be able to take anything with you into the scan room, and there are usually lockers available. It is easier if you leave objects such as watches, jewellery, mobile phones, belts, safety pins, hairpins and credit cards at home.

If you are pregnant, please discuss this with your doctor and tell the radiology practice before having the scan.

If you are claustrophobic (a fear of small or enclosed spaces) and think you might not be able to proceed with the scan, advise your doctor or the MRI facility when making your appointment. Sedative (calming) medication can be given. If this happens, you will not be able to leave the facility until you are fully awake and someone else will need to drive you home.

Some MRI facilities have stereos or CD and DVD players attached to the MRI scanner. You can take CDs or DVDs to listen to or watch while you are having the scan.

Fasting (going without food) for a MRI procedure might be required in some cases. When you make your MRI appointment, you will be advised of any fasting requirements.

Continue to take all your normal medications, unless you are otherwise advised when you make the booking for your MRI scan.

Please bring any previous X-ray, computed tomography or ultrasound films. The radiologist might like to review the older studies or see if your condition has changed since your last scan.

What happens during a MRI?

The MRI procedure will be thoroughly explained to you, and your safety questionnaire reviewed and discussed before you enter the scan room. If you have any questions, please ask the radiographer (medical imaging technologist), who will be operating the MRI scanner, as it is important that you are comfortable and know what will be happening. The radiographer will be able to see you from the control room throughout the scan.

You will usually be asked to change into a gown. This increases safety, with items in your pockets not being accidentally taken into the scan room.

You will be asked to lie on the scan table and given a buzzer to hold. When you squeeze it, an alarm sounds in the control room and you will be able to talk to the radiographer.

The MRI scanner is very noisy during the scans. It is at a noise level that can damage your hearing. You will be given earplugs or headphones to reduce the noise to safe levels.

Depending on the type of MRI you are having and your particular situation, at this point you might have:

• leads placed on your chest to monitor your heartbeat if having a heart scan;
• a small plastic tube (pulsoximeter) taped on your finger to check your breathing and heart rate if having sedative (calming) medication; and/or
• a needle inserted into a vein in your arm if any medication is required during the scan.

If you are claustrophobic and find you are unable to proceed with the scan, a sedative can be injected. The radiology facility has special procedures for people with claustrophobia and will advise you of what to do if this applies to you.

The most common medication injected is a contrast agent or ‘dye’ called gadolinium contrast medium. This highlights the part of the body being scanned, which can give more information to the radiologist who is assessing your problem.

Other medication might be injected; for example, to slow down your intestinal movement if having an MRI of the rectum.

The part of your body to be scanned will be carefully positioned and gently secured, so you are comfortable and more likely to remain still. This part will then have special antennae (coils) positioned around it to pick up signals from your body for the computer to create images. The coils are usually encased in a plastic pad or frame. Depending on the part of the body being scanned, they might be wrapped around your shoulder or lie on top of your stomach. A frame containing the coils can be used; for example, around your knee or wrist, and also for your head and upper neck. Some coils are in the mattress of the scan bed, used when your back is being scanned.

The scan table will then move into the centre of the machine. Your head might be inside or outside the scanner, depending on the part of the body being scanned.

When the scan begins, you will hear a knocking noise that continues during each scan. Scanning is not continuous, and each scan varies in length from about 1 to several minutes, with a break in between. You will be able to talk to the radiographer between each scan and can press the buzzer if you are not comfortable or want to come out of the machine at any time.

The scanning process is painless. You might feel warm during scanning. If you do feel anything at all, it is important you tell the radiographer carrying out the scan.

You need to lie still and hold your position during the scan. In general, you can breathe normally. Occasionally, during some types of MRI, you will need to hold your breath. Breathing and movement can make the images blurry and assessment of your problem more difficult.

Are there any after effects of an MRI?

There are no after effects of the MRI itself. You will be able to carry on your day as planned once the test has been completed.

If you require sedation for the scan, the MRI facility will give clear instructions on what to do before and after the scan. You will not be able to leave until you are fully awake and you will need someone to drive you home.

If you are given any medication during the scan, you might experience after effects. These are detailed in individual items on this website (such as MRI of the Rectum). The staff looking after you will discuss any after affects with you, and will help you if they do occur.

How long does an MRI take?

The scan can take between 10 minutes to over an hour to complete. This depends on the part of the body being imaged and what type of MRI is required to show the information. Before the scan begins, the radiographer will tell you how long the scan takes, so you know what to expect. Occasionally, you might need to return for delayed scans, usually after 1 or 2 hours, mostly with scanning of the liver.

If you received sedative medication, this will increase the time you will need to stay at the MRI facility.

What are the risks of an MRI?

There are no known side-effects of an MRI, providing you do not have any implants or objects that must not go in the scanner.

The danger from the MRI is due to interactions of objects with magnetic fields. Metal objects can move, as well as get hot, and electrical currents can be produced and lead to malfunction of a device. A strong magnet can alter or wipe information from other magnetic devices. Some of these interactions can cause harm or death.

Some implants, such as pacemakers, defibrillators, hearing devices and drug pumps, make it unsafe for a person to have a scan. It is important you complete the safety questionnaire fully and contact the facility where you are having the MRI if you have any questions about the implants you have. The radiographers have literature about most implants. They might ask you or your doctor for more information on any implants you have, and will find out whether it is safe for you to go in the scanner.

Metal objects that are attracted to magnets (called ferromagnetic objects), such as buckets, chairs or objects in your pocket, can be pulled rapidly, like a missile, into the MRI machine. These can damage the machine, as well as injure anyone in the way. People have died due to injuries cause by this.

Other metal in your body might move if not well fixed. These would include metal fragments in your eyes, which can interfere with vision if they move in the MRI. Most implants (hip replacements for example) are well fixed, and are usually made of non-magnetic or only weakly magnetic materials and are not a problem. Clips in the brain, used on an aneurysm (dilated blood vessel or out-pouching of a vessel), must be non-magnetic or they cannot be scanned.

Heating of metal can cause burns, such as from necklaces, and these will be removed before the scan. Some catheters (fine tubes usually in your blood vessels) can melt if they contain a wire.
Electronic devices can be damaged and not work – by moving in the body, heating and also having abnormal electrical currents. A pacemaker is a common device that can exclude you from having an MRI scan.

Magnetic dental implants will no longer remain attached if placed in the strong magnetic fields of the MRI scanner. Magnetic strips, such as on credit cards, can also be damaged.

If you are pregnant, please discuss the scan with your doctor and tell the MRI facility before your procedure. This will not necessarily stop you from having the scan. There are no reported effects of an MRI on the unborn child, but caution is always used in pregnancy.

If you are required to have an injection of gadolinium contrast medium, there is a very small risk of an allergic reaction. Gadolinium contrast medium is generally very safe, but as with all medications, allergic reactions can occur. Minor reactions (such as hives or itchy eyes) can occur in approximately 1 in 1000 people. More significant reactions (difficulty in breathing or collapse) might occur in 1 in 10,000 people. The hospital radiology department or radiology practice where you are having the scan will treat you if you have an allergic reaction. There is a small risk of an allergic reaction to any of the medications that might be given during an MRI scan.

If you have a history of kidney disease, you should have a blood test before the scan to ensure that the contrast medium can be given safely. Nephrogenic systemic fibrosis is a rare, but serious complication following a gadolinium chelate injection (see Gadolinium Contrast Medium (MRI Contrast agents)) in people with very poor kidney function.

The chance of an allergic reaction to the contrast medium is very small, but please ask the MRI radiographer about the injection to obtain more information.

What are the benefits of MRI?

MRI has no known long-term harmful effects, provided the safety precautions are followed. MRI does not use ionising radiation. Avoiding the need for exposure to ionising radiation (X-rays) is of significant benefit to younger people and children, and MRI can also be used safely in pregnancy, if required.
MRI is capable of providing your doctor with a wide range of information about your body and particular diseases or conditions you might have. It can show certain conditions that other tests can’t show. The benefits of MRI depend on the part of the body being imaged, and specific benefits are covered in individual items on this website (see Breast MRI or MRI of the Rectum, for example).
MRI can image most parts of the body in any direction to obtain maximum information and provides this information in high-quality images. These images give accurate details about certain processes or structures within the body and can also provide information as data or graphs.

Who does MRI?

A radiographer (medical imaging technologist), who is specially trained in MRI, is responsible for looking after you in the hospital department or private radiology practice, and for taking the scans. The scans are then passed to a radiologist (a specialist doctor), who interprets these and provides a written report to the doctor who referred you for the scan.

Where is a MRI done?

Most private and public hospitals, and private radiology practices have MRI scanners. You can contact your local hospital, radiology practice or your referring doctor for a list of places that have an MRI scanner.

When can I expect the results of my MRI?

The time that it takes your doctor to receive a written report on the test or procedure you have had will vary, depending on:

• the urgency with which the result is needed;
• the complexity of the examination;
• whether more information is needed from your doctor before the examination can be interpreted by the radiologist;
• whether you have had previous X-rays or other medical imaging that needs to be compared with this new test or procedure (this is commonly the case if you have a disease or condition that is being followed to assess your progress);
• how the report is conveyed from the practice or hospital to your doctor (i.e. phone, email, fax or mail).

Please feel free to ask the private practice, clinic or hospital where you are having your test or procedure when your doctor is likely to have the written report.

It is important that you discuss the results with the doctor who referred you, either in person or on the telephone, so that they can explain what the results mean for you.

*The author has no conflict of interest with this topic.

Page last modified on 31/10/2018.