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 and PET 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
Doctors may use MRI instead of a CT scan when more detail is needed to find and see:
Doctors may choose MRI test instead of a CT scan if:
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.
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.
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.