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Quick Facts

Magnetic Resonance Imaging (MRI)

By The Manual's Editorial Staff,

What is MRI?

MRI is a test that uses a machine with a powerful magnet to make pictures of the inside of your body. A computer records changes in the magnetic field around your body. The computer then uses the changes to create a series of detailed pictures. Each picture looks like a slice taken through your body. The computer can also create a 3-D image of the inside of your body. Unlike CT scans, an MRI doesn't use x-rays (radiation).

  • An MRI test doesn't use x-rays or radiation and is usually very safe

  • MRI usually provide more detail than CT, but MRI takes a lot longer and is more uncomfortable

  • Because it uses powerful magnets, you can't have an MRI if you have certain kinds of metal objects in your body

  • Most MRI machines put you in a narrow tunnel, so some people get very anxious (claustrophobic) and can't do the test

  • Some MRI machines have a bigger opening ("open MRI") that doesn't bother people as much

Why would I need an MRI?

Doctors may use MRI instead of a CT scan when more detail is needed to find and see:

  • Problems in your brain, spinal cord, muscles, and liver

  • Problems in female reproductive organs

  • Breaks in hip bones and pelvis

  • Problems in your joints, such as tears or sprains

  • Bleeding or infection

Doctors may choose MRI test instead of a CT scan if:

  • You’ve had an allergic reaction (such as sneezing, a rash, or trouble breathing) to the type of contrast agent used in CT scans

  • You’re pregnant (because CT scans give off radiation and MRI does not)

What happens during an MRI test?

Before the test

You'll empty your pockets and remove your jewelery, belts, and any other metal objects. Often you can leave your clothes on.

Sometimes, doctors inject a liquid (called a paramagnetic contrast agent) into a vein or joint. The MRI contrast agent makes certain parts of your body show up on the pictures more clearly.

If you're anxious about being enclosed in the MRI machine, doctors may give you a medicine to help you relax.

During the test

  • You’ll lie still on a table as it moves into a large scanner shaped like a tube

  • Doctors may have you wear headphones or earplugs to block out the loud banging noises made by the scanner

  • Doctors may ask you to hold your breath at certain times

A scan usually lasts for 20 to 60 minutes.

After the test

You can go back to your usual activities.

What are the problems with MRI tests?

Slower than CT scans

MRI tests take longer than CT scans. They aren’t often used in emergencies when quick results are needed, such as if you have a serious injury or stroke.

Small, enclosed space

Most MRI scanners are small and enclosed. You may feel claustrophobic (afraid of being in a confined space) during the scan, even if you aren’t usually afraid of confined spaces. Also, very large people may not fit into the scanner.

Some MRI scanners are made with a larger tube that’s open on one side (open MRI). But the pictures aren't as clear as those from regular scanners.

Problems with the magnetic field

If you have certain kinds of metal objects inside your body, the MRI’s magnetic field may be a problem. The MRI technician will ask you about all metal objects that are in your body. Some metal is safe and some is not. The technicians have a detailed list of what's safe for a given MRI machine. But in general, MRI is a problem for:

  • Medical devices controlled by magnets, such as a heart pacemaker, defibrillator, or cochlear implant—the MRI can make the device malfunction

  • Medical devices with wires or other metals that conduct electricity—the MRI can make the device heat up and burn you

  • Metal, such as iron, that can be pulled by a magnet—the MRI can make this metal move inside you

Some medical devices are safe for MRI tests, including common dental implants, artificial hips, and rods used to straighten the spine.

Reactions to the contrast agent, if used

An allergic reaction to the dye used during an MRI test is less likely than during a CT scan. However, you may get a headache, dizziness, upset stomach, pain, or an unusual taste in your mouth. Rarely, you may get kidney damage. Kidney damage is more likely if you already have kidney problems.

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