Magnetic resonance imaging (MRI) is a technique that uses a magnetic field and radio waves to create detailed images of the organs and tissues within your body.
Most MRI machines are large, tube-shaped magnets. When you lie inside an MRI machine, the magnetic field temporarily aligns all the water molecules in your body. Radio waves cause these aligned particles to produce very faint signals, which are used to create cross-sectional MRI images — like slices in a loaf of bread.
The MRI machine can combine these slices to produce 3-D images that may be viewed from many different angles.
Why it's done
MRI is a noninvasive way for your doctor to examine your organs, tissues and skeletal system. It produces high-resolution images that help diagnose a variety of problems.
MRI of the brain and spinal cord
MRI is the most sensitive imaging test of the brain and spinal cord. It's often performed to help diagnose:
- Developmental abnormalities
- Pituitary gland diseases
- Multiple sclerosis
- Dementia progression
- Spinal cord injuries
Functional MRI of the brain (fMRI) can be used to identify important language and movement control areas in the brain in people who are being considered for brain surgery.
MRI of the heart and blood vessels
An MRI that focuses on the heart or blood vessels can assess:
- The size and thickness of walls in the heart's chambers
- The extent of damage caused by heart attack or heart disease
- The buildup of plaques and blockages in the blood vessels
- Structural problems in the aorta, such as aneurysms or dissections
MRI of other internal organs
An MRI may be used to check for tumors or other abnormalities of the:
MRI of bones and joints
MRI may be used to help evaluate:
- Joint disorders, such as arthritis
- Joint abnormalities caused by traumatic or repetitive injuries
- Disk abnormalities in the spine
- Bone infections
MRI of the breasts
MRI may be used in addition to mammography to detect breast cancer, particularly in women who have dense breast tissue or who may be at high risk of the disease.
For most individuals, there are no known harmful effects from exposure to the magnetic field or radio waves used in making MRI images.
How you prepare
Before an MRI exam, eat normally and continue to take your usual medications, unless otherwise instructed. You will be asked to change into a gown and to remove:
- Hearing aids
- Underwire bras
The presence of metal in your body may be a safety hazard or affect a portion of the MRI image. Tell the technologist if you have any metal or electronic devices in your body, such as:
- Metallic joint prostheses
- Artificial heart valves
- An implantable heart defibrillator
- A pacemaker
- Metal clips to prevent aneurysms from leaking
- Cochlear implants
- A bullet, shrapnel or any other type of metal fragment
Also tell the technologist if you think you're pregnant, because the effects of magnetic fields on fetuses aren't well understood. Your doctor may recommend choosing an alternative exam or postponing the MRI.
It's also important to discuss any kidney or liver problems with your physician and the technologist, because problems with these organs may impose limitations on the use of injected contrast agents during your scan.
What you can expect
During the test
The MRI machine looks like a tunnel that has both ends open. You lie down on a movable table that slides into the opening of the tunnel. A technologist monitors you from another room. You can talk with him or her by microphone.
The MRI machine creates a strong magnetic field around you, and radio waves are directed at your body. The procedure is painless. You don't feel the magnetic field or radio waves, and there are no moving parts around you.
During the MRI scan, the internal part of the magnet produces repetitive tapping, thumping sounds and other noises. Earplugs or music may be provided to help block the noise. If you are worried about feeling claustrophobic inside the MRI machine, talk to your doctor beforehand. He or she may make arrangements for you to receive a sedative before the scan.
An MRI typically lasts about an hour. You must hold very still because movement can blur the resulting images. In some cases, contrast agents are injected into your veins to enhance the appearance of certain tissues or blood vessels in the images.
During a functional MRI, you may be asked to perform a number of small tasks — such as tapping your thumb against your fingers, rubbing a block of sandpaper or answering simple questions. This helps pinpoint the portions of your brain that control these actions.
After the test
If you haven't been sedated, you may resume your usual activities immediately after the scan. Nursing mothers shouldn't breast-feed for 36 to 48 hours after an MRI if a contrast material was used. Very rarely, the contrast material can cause hives and itching.
A radiologist — a doctor specially trained to interpret MRIs — will analyze the images from your scan and report the findings to your doctor. Your doctor will then discuss any important findings and next steps with you.