What is an MRI? (Overview)
An MRI (magnetic resonance imaging) is a test that makes use of a magnetic field, as well as pulses of radio wave energy to produce pictures of organs, tissues and other structures inside the body.
This testing method can also give different information about the structures in the body that can also be picked up with an ultrasound, X-ray or CT (computed tomography) scan. An MRI can pick up other abnormalities or problems that cannot be seen with other types of imaging methods.
An MRI test allows for an area of the body to be studied inside a special machine that contains a strong magnet. MRI machines are large, tube-shaped magnets which allow for a person to lie inside. A magnetic field temporarily realigns hydrogen atoms in the body. Radio waves then cause the aligned atoms to produce very faint signals. These are used to create cross-sectional digital images (sometimes in 3-D or 3 dimensional visuals which can be viewed from many different angles), much like slices in a loaf of bread.
If available, an open MRI machine that doesn’t enclose the entire body may be used (these are not available everywhere and the image quality may not be as good as those from a standard MRI machine). Contrast material (or fluid) containing gadolinium can also be used during the scan to show up certain structures or parts of the body more clearly.
Digital images are created from an MRI scan and saved or stored on a computer to study later on or to be reviewed remotely.
An MRI is a non-invasive way for a medical professional to examine the goings on inside the body (organs, tissues, skeletal system and other structures). The high-resolution images that an MRI scan produces can help a doctor or specialist to accurately diagnose variety of problems or conditions.
Why would an MRI be recommended?
A medical professional may need to assess various parts of the body to determine a symptomatic cause and diagnosis of a specific condition.
An MRI may be recommended to look at:
- The brain and spinal cord: Images are often performed to diagnose aneurysms of cerebral vessels, disorders of the eye and inner ear (damage to optic or auditory nerves), multiple sclerosis, spinal cord injuries, spinal stenosis, disc bulges, spinal tumours, stroke, tumours, bleeding in the brain, and brain or nerve injury due to trauma. A fMRI (functional MRI) is a type of MRI that measures metabolic changes in the brain. It is useful for examining the anatomy of the brain and determining which parts are handling critical functions (or not). Movement control areas in the brain can also be easily identified using an fMRI. It can be used to assess damage from a head injury if a person is being considered for brain surgery or may be suffering from disorders such as Alzheimer’s disease.
- The heart and blood vessels: An MRI can focus on the size and function of the heart’s chambers, the thickness and movement of the walls of the heart, the extent of damage caused by heart disease or a heart attack, structural problems in the aorta (aneurysms or dissections) or inflammation or blockages in the blood vessels.
- Other internal organs: An MRI is often used to check for tumours or other abnormalities affecting areas around the abdomen and pelvis. These can include areas in or around the liver, bile ducts, kidneys, bladder, gallbladder, spleen, pancreas, uterus, ovaries or prostate.
- The bones and joints: An MRI can help identify joint abnormalities (caused by repetitive or traumatic injury) such as torn cartilage or ligaments, spinal disc abnormalities, bone infections or tumours of the bones and soft tissues, and arthritis. An MRI can also be used to tell if a bone is broken in the body if an X-ray result is not clear.
- The breasts: In addition to a mammogram used to detect and diagnose breast cancer, an MRI may be recommended for women who have dense breast tissue. Women who are at particularly high risk for the disease may also be asked to have an MRI.
Preparing for an MRI
In preparation for an MRI scan, your doctor and an MRI technologist will need to know the following:
- If you have an allergy to any medications: You may have an allergic reaction to the contrast material used. It is best to mention any known allergies to your doctor and technologist as a precaution.
- If you have a known health condition: Diabetes, sickle cell anaemia, kidney disease or other types of conditions may prevent you from being able to have an MRI using contrast material due to the adverse effects it can have on the body.
- If you are pregnant
- If you have metal implants in your body: Your doctor will need to assess whether an MRI scan is safe for you. If you have heart or blood vessel devices (coronary artery stent, a pacemaker, an ICD / implantable cardioverter-defibrillator), metal pins, clips, metal heart valve or other metal parts in the body (artificial limbs, dental work or braces), a medicine infusion pump, cochlear implant, or cosmetic metal implants (including tattooed eyeliner or in your ears).
- If you have recently had surgery on a blood vessel
- If you have an intrauterine (IUD) device in place: This is a type of birth control method.
- If you are typically nervous or suffer anxiety when in confined spaces (claustrophobic)
- If you wear any medicine patches: An MRI scan can burn a patch site on the body.
You may be advised to arrange for someone to drive you home safely following an MRI, especially if you have been given a sedative to help you relax because you are particularly nervous about being in a confined space for a lengthy period.
If your abdomen or pelvis is the area your doctor wishes to concentrate on, you may be asked not to eat or drink anything several hours before the scan is done. For any other area being scanned, it may be absolutely fine for you to eat normally or continue to take your usual medications on schedule. If necessary, your doctor will instruct otherwise.
As with any test or procedure that involves some degree of risk, you may need to sign a consent form once potential complications have been clearly discussed with you.
There is no need to feel in the dark about what the scan will involve or what you can expect to go through. Your doctor can help you to understand what this scan will involve, how it will work, what you are likely to experience and how to interpret the results.
What happens during the procedure?
On the day, you will be asked to change into a gown (generally supplied to you) and to remove all things on your body that may affect the magnetic imaging (objects that will interfere with the powerful magnet). This includes jewellery, hairpins, eyeglasses, watches, wigs, dentures, hearing aids and underwire bras.
Depending on which area of the body is being examined, you will need to take off most or all your clothes and change into the supplied gown. If you are allowed to keep some clothing on, all pockets must be emptied (Fun fact: An MRI magnet can erase all information stored on the scanner strips of ATM bank cards).
The scan will usually be done by an MRI technologist and the images supplied to a radiologist for interpretation, your doctor will then explain the results to you once he/she has reviewed them. A technologist will monitor you from another room during the entire scan process (meaning you will be alone in the room during the scanning process).
The MRI machine will direct radio waves at your body and generate a strong magnetic field around you. The entire scanning process is painless and you cannot physically feel the radio waves or magnetic field. No parts of the scanning machine move around you.
During the scan, you will be requested to lie on your back on a table that is built in as part of the MRI scanner. You will be required to lie completely still for a period during the scan, with little or no movement. Some have straps which can be used on the head, arms and chest to assist you with remaining still.
The table will then slide into the space in the scanner that accommodates the magnet. A coil device will be placed over the area your MRI technologist would like to examine. A belt strap may also be wrapped around the area to ‘sense’ your breathing or heartbeat and trigger the machine to take a scan at an appropriate time.
If you have trouble keeping still or experience some degree of claustrophobia once inside the MRI magnet, you can be given a sedative to aid relaxation or you can request that mirror mounts be attached over your head so that you can see outside the MRI machine – this will significantly alleviate the feeling of claustrophobia. Alternatively, an open MRI machine may be an option if you have claustrophobia.
Once inside the scanner, you may feel some movement of air (or hear a fan) and slight vibrations. You may also hear a tapping noise as the MRI scan images are taken. Before sliding into the scanner that contains the magnet you may be given headphones with music or earplugs to counteract or reduce this noise.
You will be able to hear your MRI technologist during the test and may be asked to hold your breath for brief periods while a scan is taken. You can also talk to the technologist by microphone or intercom. It is very important to stay as still as possible throughout the scan process as movement can blur the images taken.
If you need contrast material, your technologist will administer it via an intravenous line (IV line) through a vein in your arm. You may feel some coolness as the material is administered. It can happen, although rarely, that some will feel a tingling sensation in the mouth (especially if you have dental fillings), warmth in the examined area of the body, nausea, vomiting, headache, pain, burning or trouble breathing, as well as dizziness. The material will be given over 1 to 2 minutes, followed by more scans.
During a fMRI (functional MRI) a few small tasks may be requested of you. You may be asked to tap your thumb against your fingers, answer a series of simple questions or rub a block of sandpaper. This may be done to pinpoint sections of your brain that typically control these types of functions.
An MRI can last anywhere from 15 minutes to more than an hour (sometimes 2 hours) depending on the purpose and areas being examined. If you haven’t been sedated, you are usually able to resume normal activities immediately after your scan.
The magnet is very powerful and may affect artificial limbs, artificial heart valves, implantable heart defibrillators, pacemakers and other medical devices what contain iron. There are no known serious effects as a result of the strong magnetic field used.
Any loose metal objects (such as a watch) can cause damage or injury of it is pulled toward the magnet during a scan. Any metal parts in or near the eyes can harm the retina. Irritation in the skin or the eye can occur due to iron pigments in tattoos or tattooed eyeliner.
There is a slight risk of an adverse reaction to the contrast material used (or an allergic reaction). This is typically mild and can easily be treated, so if you feel particulary strange after this has been administered, inform the person conducting the MRI immediately. Some may experience a slight risk of infection at the IV site.
If you are breastfeeding at the time of your MRI scan and need contrast material it is best to speak with your doctor or technologist beforehand to understand how it may affect your baby. A little dye may pass into breast milk and even less is believed to then be passed on to the baby while breast feeding. If you are nervous about this, you can store some of your breast milk ahead of your scan for use in the days thereafter.
Results and patient follow-up
A radiologist may discuss initial results of your MRI scan with you directly after the test. More thorough results are typically ready within a day or two after your scan and will be provided to your doctor or other specialist.
Sometimes your MRI results will be different from those of a CT scan, ultrasound or X-ray because it shows tissue differently. Your radiologist and specialist will carefully analyse any, and all tests conducted before making a diagnosis or drawing a conclusion.
From there he or he will discuss the findings in more detail with you, as well as any potential next steps which may be necessary.
How does an MRI scanner work?
The human body is made up of mostly water (H2O). Water molecules contain hydrogen nuclei (protons), which align in a magnetic field. An MRI scanner applies a very powerful magnetic field during the scanning process (roughly a thousand times the strength of a fridge magnet) and aligns these protons in the body.
What shows up on an MRI?
An MRI is a highly technical and informative way of creating images of the human body. The created images are very useful for medical professionals to detect and subsequently diagnose and treat, structural abnormalities of the body’s organs, soft tissues, muscles, ligaments and other structures inside the body.