Knowledge Bank

A radiologist’s guide to getting an MRI

Illustration of torso x-ray

Dr. Ania Kielar, vice-president of the Canadian Association of Radiologists and vice-chair of the University of Toronto’s department of medical imaging, explains the ins and outs of one of the most powerful tools at a radiologist’s disposal

What is an MRI?
Magnetic resonance imaging (better known as an MRI) uses a strong magnetic field and radio waves to create detailed images of the body’s organs and soft tissues. Patients are required to lie face up on a bed that is threaded into a doughnut-shaped machine that is similar in appearance to a CT scanner but longer and able to accommodate a larger portion of the body. The magnetic field forces the body’s protons to align in the same direction and the radio waves, when turned on, send these protons spinning back out of alignment. Once the radio waves are turned off, a computer uses the amount of time it takes the protons to realign with the field, and the energy released, to create an image of the area of interest. “Basically, you’re sitting inside a big magnet,” Dr. Kielar says. “It very temporarily changes how your atoms orient inside your body but it doesn’t change your DNA, it doesn’t change anything in your body.”

What is the difference between a CT scan vs. MRI?
While both of these scans provide clear images of the inner workings of the body, a CT scan achieves its results by using X-rays instead of a magnetic field and radio waves. CT scans are more commonly used because they are faster, cheaper and more readily available. MRI scans, on the other hand, are often ordered when doctors need to see more detail in hard-to-reach areas, such as those inside the brain.

What is a contrast agent?
MRI scans often require a contrast agent, usually gadolinium, to alter the magnetic properties of water molecules inside the body and improve image quality. “This is excreted through the kidneys,” Dr. Kielar says. “So, we need your kidneys to work reasonably, in the same fashion as the CT scanner needs your kidneys to work OK for the contrast agent. They’re different agents, though, so if you’re allergic to one, it doesn’t mean you’re allergic to the other. The chance of being allergic to gadolinium is very low.”

What does an MRI show?
“The images we get are exquisite,” Dr. Kielar says. “They’re very useful for looking at the brain and very useful for looking at pelvic structures, such as the reproductive organs. They’re also very good for looking at soft tissue in general for, say, tumours in the bones and they’re excellent for looking at the bones themselves.”

How long does an MRI take?
Patients will typically spend anywhere between 15 and 45 minutes inside an MRI machine and should be prepared to hear a lot of loud knocking and clicking during the procedure. “Each sequence has its own sound so you may feel like you’re in a dance club at times,” Dr. Kielar says. “They do put stuff over your ears and they talk to you through earphones so if you’re uncomfortable you just have to tell the tech. We give you a button you can press and we’ll pull you out immediately if you’re having any issues.”

How can patients prepare for an MRI?
Clothing/jewelry — Patients will be asked to change into a gown before the procedure and must remove any metal or jewelry because they will be drawn to the powerful magnet used for the scan. There is a list of other items patients will be asked about prior to the procedure. “If you have a pacemaker or if you have a cochlear implant or if you had brain surgery with clips, the technologist will ask you about that even before you arrive,” Dr. Kielar says. “When you’re making your appointment, they’ll ask your doctor to fill out a form to make sure you don’t have anything that’s dangerous inside a magnet.”
Dietary requirements — Patients can eat or drink as usual prior to a scan unless otherwise instructed.
Bloodwork — Certain patients, particularly those who are elderly or have kidney issues, may be required to have a blood test before an MRI if a contrast agent is required.
Medications — While regular medications can generally be taken, Dr. Kielar encourages people to come prepared anytime they enter a healthcare facility. “Whenever you go to the hospital, I strongly recommend that you have a list of your past medical history and your medications.”

Is there anything patients should do after an MRI?
If a contrast agent is required for the procedure, patients should try to drink plenty of water to help clear it from their system.

How long does it take for results to come back?
A radiologist’s report is typically ready for a doctor within a couple of days.

Are there any concerns or risks that accompany an MRI?
Claustrophobia — While modern MRI machines are roomier than older models, patients who have a fear of enclosed spaces should share any concerns with their doctor or radiologist beforehand. “There is one to two per cent of the population that will be claustrophobic and feel a little bit uncomfortable in there, so they can get Adavin or other medications to kind of make them a little bit calm or sleepy,” Dr. Kielar says. “If you do have to take those medications then you cannot drive so you will need someone to drive you home.”
Muscle twitching — Most MRI machines operate at a strength of 1.5 Tesla but some operate at double the strength, or 3 Tesla. “Sometimes, this can cause your muscles to twitch a tiny bit,” Dr. Kielar says. “It’s normal but good to know in advance.”
Tattoos — Body art can be a problem because some tattoo ink contains magnetic particles that may heat up inside the powerful magnetic field used for the scan. “We’ll warn you that if you do start feeling hot on your skin where your tattoos are you have to tell us and we’ll take you out immediately.”
Pregnancy concerns — Similar to a CT scan, radiologists exercise extreme caution when it comes to expecting mothers. “If you’re pregnant we need to know because we cannot give intravenous contrasts,” Dr. Kielar says. “If you’re pregnant, we also need to know because we’re not 100 per cent sure what the long term effects are of magnets and all that stuff are on developing babies. If we need to do it, we will do it but we will avoid any sort of imaging on pregnant women until after the baby is born.”

How has the pandemic changed the way MRI scans are performed?
Like all scans performed by a radiologist, wait times for an MRI have generally gone from bad to worse, Dr. Kielar says. Radiologists have decreased the use of contrast agents, where possible, to limit the amount of time patients must spend in the waiting room. Any equipment used is cleaned even more frequently than it used to be. “We use various filters to try to keep the room clean, everyone has to wear a mask all the time — the technologist as well as the patient — and the technologist also wears a shield over their eyes.”

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