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 creates 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 (computed tomography) scans and PET (positron emission tomography) 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 scans, 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
Image provided by Jon A. Jacobson, MD.
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 jewelry, 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, people who are very large 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.
