Magnetic Resonance Imaging (MRI) is a method of examining body organs through the use of magnetic fields rather than X rays, and using a computer to construct a series of body scans. A MRI scan is particularly useful for producing pictures of the brain and spine, as well as the soft tissues of joints and the interior structure of bones. For some MRI scans you will be given an injection of a special dye called a contrast agent. This makes certain tissues or blood vessels show up more clearly and with greater detail on a scan.
MRI scans can provide detailed images of almost any part of the body, but are most commonly used to look at the brain, spinal cord, abdomen and pelvis, and to investigate injuries to bones, joints and soft tissue.
MRI scans are particularly effective in looking for tumours. They can show how deeply a tumour has grown into body tissue and whether it has spread. After surgery, MRI scans can be used to check whether there is any tumour tissue left. Repeated MRI scans could show whether a tumour is shrinking.
MRI can reveal detailed information about the brain and how it works. It can measure changes in blood flow within the brain, can identify the abnormal tissue formed when someone has multiple sclerosis (MS) and can be used to investigate the brain following a stroke, to see if brain tissues have been damaged by a lack of oxygen.
MRI can show the heart and the large blood vessels surrounding the heart. This is particularly useful for diagnosing heart defects, as well as changes in the thickness of the muscles around the heart following a heart attack.
MRI scans are often used to examine joints, particularly for common sports injuries affecting the knee. Scans can identify injured tendons, ligaments, muscles, cartilage and bone marrow and can help your doctor decide whether an injury needs surgery.
Not everyone can have an MRI scan. The magnetic field from the scan affects some metals. It's important to tell your radiographer if you have a medical device implanted in your body. These include:
- heart pacemaker
- heart defibrillator (a device to establish a regular heart rhythm)
- heart valve
- medicine infusion pump (such as an insulin pump)
- inner ear implant (a hearing aid)
- neurostimulator (a device that stimulates nerves)
- aneurysm clip (a metal clip on an artery)
- shunts (tubes) in the brain
- joint replacements/large metal implants
- stents (tubes) in the heart or arteries
- eye, penis, or breast implants
- an intra-uterine contraceptive device or coil
You will also need to tell your radiographer if you have:
- shrapnel or gunshot wounds
- metal fragments anywhere in your body
- tattoos or transdermal patches
Not all of these mean that you won't be able to have an MRI scan. Your radiographer will discuss with you whether it's safe for you to go ahead with the scan.
Jewellery and other accessories should be left at home if possible, or removed prior to the MRI scan. Because they can interfere with the magnetic field of the MRI unit, metal and electronic objects are not allowed in the exam room. These items include:
- jewellery, watches, credit cards and hearing aids, all of which can be damaged.
- pins, hairpins, metal zippers and similar metallic items, which can distort MRI images.
- removable dental work.
- pens, pocketknives and eyeglasses.
- body piercings
Usually there is no preparation for MRI. You can eat and drink normally before and after the scan. Some patients who are having an abdominal scan will be required not to eat or drink for four hours before the scan. If you are required to fast, full instructions will be included in your appointment letter.
Some MRI examinations may require the patient to swallow contrast material or receive an injection of contrast into the bloodstream. The radiologist may ask if you have allergies of any kind, such as allergy to iodine or x-ray contrast material, drugs, food, the environment, or asthma. However, the contrast material used for an MRI exam does not contain iodine and is less likely to cause side effects or an allergic reaction.
During the Procedure
You will be positioned on a moveable examination table. Straps and bolsters may be used to help you stay still and maintain the correct position during imaging.
Small devices that contain coils capable of sending and receiving radio waves may be placed around or adjacent to the area of the body being studied.
If a contrast material will be used in the MRI exam, a radiologist or nurse will insert an intravenous (IV) line into a vein in your hand or arm. A saline solution may be used. The solution will drip through the IV to prevent blockage of the IV line until the contrast material is injected.
You will be moved into the magnet of the MRI unit and the radiologist and radiographer will leave the room while the MRI examination is performed.
If a contrast material is used during the examination, it will be injected into the intravenous line (IV) after an initial series of scans. Additional series of images will be taken during or following the injection.
When the examination is completed, you may be asked to wait until the radiographer or radiologist checks the images in case additional images are needed.
Your intravenous line will be removed.
MRI exams generally include multiple runs (sequences), some of which may last several minutes.
Depending on the type of exam and the equipment used, the entire exam is usually completed in 15 to 45 minutes.
What will I experience during and after the procedure?
Most MRI exams are painless.
Some patients, however, find it uncomfortable to remain still during MR imaging. Others experience a sense of being closed-in (claustrophobia). Therefore, sedation can be arranged for those patients who anticipate anxiety, but few require it.
It is normal for the area of your body being imaged to feel slightly warm, but if it bothers you, notify the radiologist or radiographer. It is important that you remain perfectly still while the images are being recorded, which is typically only a few seconds to a few minutes at a time. For some types of exams, you may be asked to hold your breath. You will know when images are being recorded because you will hear tapping or thumping sounds when the coils that generate the radiofrequency pulses are activated. You will be able to relax between imaging sequences, but will be asked to maintain your position as much as possible.
You will usually be alone in the exam room during the MRI procedure. However, the radiologist and radiographer will be able to see, hear and speak with you at all times using a two-way intercom. You may be offered or you may request earplugs to reduce the noise of the MRI scanner, which produces loud thumping and humming noises during imaging. MRI scanners are air-conditioned and well-lit. Some scanners have music to help you pass the time.
When the contrast material is injected, it is normal to feel coolness and a flushing sensation for a minute or two. The intravenous needle may cause you some discomfort when it is inserted and once it is removed, you may experience some bruising. There is also a very small chance of irritation of your skin at the site of the IV tube insertion.
If you have not been sedated, no recovery period is necessary. You may resume your usual activities and normal diet immediately after the exam. A few patients experience side effects from the contrast material, including nausea and local pain. Very rarely, patients are allergic to the contrast material and experience hives, itchy eyes or other reactions. If you experience allergic symptoms, a radiologist or other physician will be available for immediate assistance.
It is recommended that nursing mothers not breastfeed for 36 to 48 hours after an MRI in which a contrast material was given.