What Is an MRI?
Magnetic resonance imaging (MRI) is a test that uses powerful magnets, radio waves, and a computer to make detailed pictures of the inside of your body.
Your doctor can use this test to diagnose you or to see how well you've responded to treatment. Unlike X-rays and computed tomography (CT) scans, MRIs don’t use the damaging ionizing radiation of X-rays.
An MRI helps a doctor diagnose a disease or injury, and it can monitor how well you’re doing with a treatment. MRIs can be done on different parts of your body. It's especially useful for looking at soft tissues and the nervous system.
An MRI of the brain and spinal cord can help find many things, including:
- Blood vessel damage
- Brain injury
- Multiple sclerosis (MS)
- Spinal cord injuries
- Eye problems
- Inner ear problems
An MRI of the heart and blood vessels looks for:
- Blocked blood vessels
- Damage caused by a heart attack
- Heart disease
- Problems with the structure of the heart
An MRI of the bones and joints looks for:
- Bone infections
- Damage to joints
- Disk problems in the spine
- Neck or low back pain with nerve signs
MRIs can also be done to check the health of these organs:
A special kind of MRI called a functional MRI (fMRI) maps brain activity.
This test looks at blood flow in your brain to see which areas become active when you do certain tasks. An fMRI can detect brain problems, such as the effects of a stroke, or it can be used for brain mapping if you need brain surgery for epilepsy or tumors. Your doctor can use this test to plan your treatment.
Risks of MRI
Pregnant women shouldn’t get an MRI during their first trimester unless they absolutely need the test. The first trimester is when the baby's organs develop. You also shouldn't get contrast dye when you’re pregnant.
Certain people with metal inside their body can't get this test, including those with:
- Some clips used to treat brain aneurysms
- Pacemakers and cardiac defibrillators
- Cochlear implants
- Certain metal coils placed in blood vessels
Before your MRI, let your doctor know if you:
- Have any health problems, such as kidney or liver disease
- Recently had surgery
- Have any allergies to food or medicine, or if you have asthma
- Are pregnant, or might be pregnant
No metal is allowed in the MRI room, because the magnetic field in the machine can attract metal. Tell your doctor whether you have any metal-based devices that might cause problems during the test. These can include:
- Artificial heart valves
- Body piercings
- Cochlear implants
- Drug pumps
- Fillings and other dental work
- Implanted nerve stimulator
- Insulin pump
- Metal fragments, such as a bullet or shrapnel
- Metal joints or limbs
- Pacemaker or implantable cardioverter-defibrillator (ICD)
- Pins or screws
If you have tattoos, talk with your doctor. Some inks contain metal.
On the day of the test, wear loose, comfortable clothing that doesn't have snaps or other metal fasteners. You might need to take off your own clothes and wear a gown during the test.
Remove all of these before you go into the MRI room:
If you don't like enclosed spaces or you're nervous about the test, tell your doctor. You may be able to have an open MRI or get medicine to relax you before the test.
A typical MRI machine is a large tube with a hole at both ends. A magnet surrounds the tube. You lie on a table that slides all the way into the tube.
In a short-bore system, you are not totally inside the MRI machine. Only the part of your body that's being scanned is inside. The rest of your body is outside the machine.
An open MRI is open on all sides. This type of machine may be best if you have claustrophobia -- a fear of tight spaces -- or you're very overweight. The quality of images from some open MRI machines isn't as good as it is with a closed MRI.
During an MRI
Before some MRIs, you'll get contrast dye into a vein in your arm or hand. This dye helps the doctor more clearly see structures inside your body. The dye often used in MRIs is called gadolinium. It can leave a metal taste in your mouth.
You will lie on a table that slides into the MRI machine. Straps might be used to hold you still during the test. Your body might be completely inside the machine. Or, part of your body may stay outside the machine.
The MRI machine creates a strong magnetic field inside your body. A computer takes the signals from the MRI and uses them to make a series of pictures. Each picture shows a thin slice of your body.
You might hear a loud thumping or tapping sound during the test. This is the machine creating energy to take pictures inside your body. You can ask for earplugs or headphones to muffle the sound.
You might feel a twitching sensation during the test. This happens as the MRI stimulates nerves in your body. It's normal, and nothing to worry about.
The MRI scan should take 20-90 minutes.
After an MRI
You can usually go home after an MRI and get back to your normal routine. If you had medicine to help you relax, you’ll stay in the imaging center until you’re fully awake. You’ll also need someone to drive you home.
MRI Side Effects
Contrast dye helps your doctor better see what’s going on, and for most people it doesn’t cause any problems. But it can cause an allergic reaction in some people. There are steps doctors take to treat them.
Your doctor likely won’t use it on you if you're pregnant, even though there's no evidence it could harm a fetus. Your doctor may check your kidney function before the test. People with severe kidney disease are at risk of a rare disease called nephrogenic sclerosing fibrosis.
There’s a chance some of the dye may stay in your body and build up in the brain and other organs. It’s not clear yet if this buildup is harmful. The FDA hasn’t restricted its use.
A specially trained doctor called a radiologist will read the results of your MRI and send the report to your doctor.
Your doctor will explain the meaning of your test results and what to do next.