What Is an MRI?

Magnetic resonance imaging (MRI) is a test that uses powerful magnets, radio waves, and a computer to make detailed pictures inside 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 CT scans, an MRI doesn't use radiation.

Why Would You Get an MRI?

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.

An MRI of the brain and spinal cord looks for:

An MRI of the heart and blood vessels looks for:

An MRI of the bones and joints looks for:

MRI 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 for brain mapping if you need brain surgery for epilepsy or tumors. Your doctor can use this test to plan your treatment.

How Do I Prepare for an MRI?

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

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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.

What Does the Equipment Look Like?

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.

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What Happens During the Test?

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 between 20 and 90 minutes.

Who Shouldn't Get an MRI?

Pregnant women should not 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.

Don't get contrast dye if you've had an allergic reaction to it in the past or you have severe kidney disease.

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

Your Results

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.

WebMD Medical Reference Reviewed by Sabrina Felson, MD on January 11, 2017

Sources

SOURCES:

American Academy of Orthopaedic Surgeons: "X-Rays, CT Scans, and MRIs."

FDA: "MRI (Magnetic Resonance Imaging)."

Mayo Clinic: "MRI: How you prepare," "MRI: Risks." "MRI: What you can expect," "MRI: Why it's done."

National Institute of Biomedical Imaging and Bioengineering: "Magnetic Resonance Imaging (MRI)."

Radiological Society of North America: "Magnetic Resonance Imaging (MRI) -- Body." "Magnetic Resonance, Functional (fMRI) -- Brain."

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