What Is an MRI Scan?
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Medically reviewed by Anju Goel, MD, MPH
A magnetic resonance imaging (MRI) scan is a painless medical imaging procedure that uses a strong magnetic field and radio waves to generate images of the body. Unlike imaging tests such as positron emission testing (PET) scans and computed tomography (CT) scans, an MRI does not involve radiation.
A lab technician will usually conduct the scan. A radiologist (a doctor who specializes in medical imaging) will usually read the MRI results and send a report to the referring healthcare provider (the provider who ordered your scan).
MRI can produce detailed images of soft tissues, including the brain, muscles, and ligaments. It's also better than other types of scans at distinguishing between diseased and normal tissue. For this reason, healthcare providers use MRI scans to diagnose and monitor a variety of medical conditions, as well as to monitor fetuses in people who are pregnant.
Purpose
A healthcare provider may order an MRI to evaluate the appearance of an organ, blood vessel, or lymph nodes. They may also recommend an MRI scan to diagnose or monitor different medical conditions, such as:
Abdominal or pelvic tumors
Bowel inflammation caused by conditions like appendicitis (inflammation of the appendix) or Crohn's disease (a type of inflammatory bowel disease [IBD] that causes inflammation in the gastrointestinal tract)
Liver disease, such as cirrhosis (a condition that causes extreme scarring and damage to the liver) or abnormal bile ducts (tubes that connect digestive organs)
Vasculitis, a condition in which a person's immune system attacks their blood vessels
MRI is a valuable tool for examining the brain and spinal cord. For example, it can distinguish between white matter (WM) and gray matter (GM) in the brain. GM is made of cells that help with functioning like cognition and movement. WM consists of nerve fibers that help areas of GM communicate. Some conditions affect one type of matter more than another. For example, people with Parkinson's disease (PD) and Alzheimer's disease (AD) have less gray matter. MRI can also detect aneurysms and tumors.
A healthcare provider may also order an MRI scan for someone who is pregnant and has symptoms or test results that require further examination before delivery. This helps protect the pregnant person and, in turn, the health of the fetus. An MRI does not involve exposure to ionizing radiation, which can be harmful to the developing fetus.
Types of MRI
Most MRI scanning approaches examine organ or blood vessel functioning. The type of MRI you receive depends on what area of the body needs to be scanned. Common MRI scans include:
Cranial (head)
Cervical (the part of your spine that's in your neck)
Chest
Heart
Abdominal
Lumbar (the lower part of your spine)
Pelvic (the area between the hip bones)
There is also a type of MRI called magnetic resonance angiography (MRA), which examines blood vessels. Another type of MRI, magnetic resonance venography (MRV), examines veins.
Meanwhile, functional MRI (fMRI) evaluates how much blood flows to the brain during certain tasks. An fMRI scan allows a healthcare provider to determine which areas of the brain are most active and if brain damage has occurred.
Related: Should You Get a Full-Body MRI?
How Does It Work?
An MRI machine looks very similar to a CT or PET scan machine: a large machine with a round hole in the middle, in which you lie during the procedure. An open MRI machine is open on the sides.
In most MRI machines, electric currents pass through wire coils in the machine. Coils might also be placed around the body part examined. The coils create a magnetic field. They produce signals via radio waves that the coils send and receive. These signals create images.
MRI works by using a high-powered magnet. The magnet's magnetic field works to temporarily change the orientation of hydrogen atoms in your body. The hydrogen atoms generate energy when they return to their original place, or realign. This energy is transported back to the scanner to generate an image of your body.
Different types of body tissues generate different amounts of energy. The time it takes for the hydrogen atoms to realign also varies depending on the type of tissue. This helps healthcare providers tell the difference between different types of tissues, such as bones and muscles.
Some MRI tests use a special dye called a contrast. This contrast material makes the images clearer. It's usually given to you through an intravenous (IV) injection into your vein.
Before the Test
Your healthcare provider will need to confirm that MRI is safe for you based on any health devices you may use. If you have an implanted device like a pacemaker (a device that controls your heartbeat), you'll need to tell the person who helps you schedule the MRI scan. They'll probably ask you to bring the pamphlet for the device with you on the day of the scan.
You'll complete a questionnaire before the MRI scan. The pre-questionnaire helps identify any potential risks that would prevent you from having the procedure. For example, the technician will ask if you're pregnant or could be pregnant. Contrast material is typically not used during pregnancy because the effects of fetal exposure are not known.
Let your healthcare provider know if you're claustrophobic, or extremely fearful of confined spaces. You might be able to use an open MRI machine or take a mild sedative before the test. A sedative is a medication that causes sleepiness and helps you relax.
You'll also need to tell the technician if you have any of the following:
Foreign bodies, such as retained bullets or shrapnel
Jewelry, hearing aids, or removable dental work
Body piercings
When you arrive at the test location, you'll be asked to remove any metal or electronic items, including:
Mobile phones
Electronic watches
Eyeglasses
Hairpins
The MRI magnet is very powerful and could potentially attract these items, causing damage to you or your MRI technician. Some types of metal can also blur the images.
An MRI's duration depends on what is being scanned. Most MRI scans last between 20 and 90 minutes.
Related: What I Wish I Knew About Choosing Sedation
During the Test
During an MRI scan, you will lie on a moveable table. The technician might place small coils around the area of your body being examined. This helps create clearer images. If your test includes a contrast material, a provider will attach the IV line to your hand or arm.
The technician will push a button to move the table inside the opening of the machine. Once the test begins, you will hear thumping and clicking noises. An MRI scanner can be very loud, so the technician will usually give you earplugs or earphones to wear during the scan.
If the test includes a contrast material, you'll receive it via the IV line after a few initial scans. Feeling some warmth or a strange taste in your mouth is normal. The technician might ask you to hold your breath for a few seconds afterward to get clear images.
An MRI is not a painful procedure, but you will need to lie very still during the exam. You might feel anxious, especially if you don't like loud noises or small spaces. You also might feel warmth in the area being imaged, which is normal.
The technician will probably sit in an adjacent room with windows so they can see you. Most MRIs are equipped with an intercom so you and the technician can communicate throughout the procedure. You also might have a "squeeze ball" in case you need immediate attention.
Your MRI technician will remove your IV (if you had one) after the test.
After the Test
An MRI scan is usually performed on an outpatient basis. If you underwent the MRI with no sedation, you can typically leave once your scan is complete and resume all usual activities immediately.
If you receive sedation medication, you'll need someone to drive you home. And you may be instructed about some things you shouldn't do for a brief period of time following the MRI because of the sedation, like operate heavy machinery.
Risks and Precautions
MRI is safe for most people. However, there are some potential risks. For example, some people can have an allergic reaction to contrast material. Symptoms are generally mild and resolve on their own.
Nephrogenic systemic fibrosis is a very rare but serious complication that might be related to contrast materials that contain a chemical called gadolinium. Your healthcare provider will evaluate your kidney function before the MRI test, as this is most often a complication for people with severe renal (kidney) failure who require dialysis. More research is needed to determine whether there's a causal link.
People with certain implants should avoid MRI. The magnetic field might affect some implanted medical devices or pose a potential risk. These implants include:
Capsules from capsule endoscopy: Pills that are ingested before an endoscopy, a procedure that involves using a tiny camera to take images of your digestive tract
Cochlear implants: Devices that improve hearing
Insulin pumps: Devices that deliver the hormone insulin via a catheter (small tube placed in the body)
Loop recorders: Devices that record heart activity
Older pacemakers that contain iron: Devices that help control the heartbeat
Vagus nerve stimulators: Devices that stimulate the vagus nerve, which helps regulate heart rate, digestion, and blood pressure
While MRI is safe for children, very young children often have difficulty staying still or holding their breath to complete the test. As a result, they may require sedation or anesthesia.
People who are pregnant can also undergo an MRI. However, your healthcare provider may recommend waiting to get an MRI until after your first trimester to be especially cautious.
MRI is typically quite loud, and the space may feel constricting. Talk to your healthcare provider to make sure you fully understand the process and take precautionary measures as needed, including ear protection, sedation or anesthesia, and music to listen to during the scan.
How to Prepare for an MRI
Your MRI will likely take place at an imaging center or inside a hospital. You do not usually need to adjust how you eat or drink prior to undergoing your scan. You can also typically take your medications as directed, unless your healthcare provider says otherwise.
Things you may wish to bring to an MRI include:
Comfortable clothing: Wear clothing you can easily remove and put back on. You may be asked to change into a gown for the exam.
Identification and medical cards: Bring any medical cards, like an insurance card, so you can present them before the procedure. If you have metal implants, you may need to bring cards that say whether or not they're MRI-compatible.
Order form: Your healthcare provider may give you a form called an order that describes the imaging tests being performed. They may also send an order in advance.
Support person: You might be able to bring someone with you. Ask if this is allowed when you schedule the appointment. You will need someone to drive you if you receive IV sedation.
You may have questions about how much the MRI test costs and whether your insurance will cover your procedure. Your insurance company will likely cover a portion of the costs if the MRI scan is medically necessary. Contact your insurance company before the procedure to see what they cover and if there are any payment plan options.
Related: What Should a Medical Scan Cost?
Results
After your MRI scan, a radiologist will review the images for abnormalities or changes from previous scans (if you've had them). The radiologist will then send a report to the referring healthcare provider.
A report will generally include the following information:
Exam type (MRI)
Reason for the exam
Comparison to previous imaging tests, if applicable
Technique, including whether the exam included contrast
Findings, including normal or abnormal results
Impressions, including the most important findings and possible causes
The time it takes to receive this report depends on whether there are any immediate findings that could be life-threatening or require rapid decisions regarding your care. You'll likely be able to see the report in an electronic health records system. You also might receive your MRI results at your next visit to your healthcare provider.
Your healthcare provider might recommend additional testing. For example, other imaging or blood tests might help them make a diagnosis if the results of the MRI didn't lead to a firm conclusion.
A Quick Review
A magnetic resonance imaging (MRI) scan is a painless medical imaging procedure that uses a strong magnetic field and radio waves to generate images of inside the body. It can take longer than some other scan types, such as X-rays or computed tomography scans, but it doesn't involve radiation.
MRI scans can be more helpful than other imaging scans in identifying injuries to soft tissue as well as conditions like aneurysms and tumors. If your healthcare provider prescribes an MRI, talk to them about the procedure and when you can expect results.
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