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 realigns hydrogen atoms in your body. Radio waves cause these aligned atoms 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 also be used 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 of the inside of the body that help diagnose a variety of problems.
MRI of the brain and spinal cord
MRI is the most frequently used imaging test of the brain and spinal cord. It's often performed to help diagnose:
- Aneurysms of cerebral vessels
- Disorders of the eye and inner ear
- Multiple sclerosis
- Spinal cord injuries
- Brain injury from trauma
A special type of MRI is the functional MRI of the brain (fMRI). It measures the metabolic changes that occur within the brain. It may be used to examine the brain's anatomy and determine which parts of the brain are handling critical functions. This helps identify important language and movement control areas in the brains of people being considered for brain surgery. Functional MRI may also be used to assess damage from a head injury or from disorders such as Alzheimer's disease.
MRI of the heart and blood vessels
MRI that focuses on the heart or blood vessels can assess:
- The size and function of the heart's chambers
- Thickness and movement of the walls of the heart
- The extent of damage caused by heart attack or heart disease
- Structural problems in the aorta, such as aneurysms or dissections
- Inflammation or blockages in the blood vessels
MRI of other internal organs
MRI may be used to check for tumors or other abnormalities of many organs in the body, including the:
- Liver and bile ducts
MRI of bones and joints
MRI may be used to help evaluate:
- Joint abnormalities caused by traumatic or repetitive injuries, such as torn cartilage or ligaments
- Disk abnormalities in the spine
- Bone infections
- Tumors of the bones and soft tissues
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.
Because MRI uses powerful magnets, the presence of metal in your body may be a safety hazard or affect a portion of the MRI image. Before having an MRI, 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
- Cochlear implants
- A bullet, shrapnel or any other type of metal fragment
If you have tattoos, ask your doctor whether they might affect your MRI. Some of the darker inks may contain metal.
Before you schedule an MRI, tell your doctor if you think you're pregnant. 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 doctor and the technologist, because problems with these organs may limit the use of injected contrast agents during your scan.
How you prepare
Before an MRI exam, eat normally and continue to take your usual medications, unless otherwise instructed. You will typically be asked to change into a gown and to remove things that might affect the magnetic imaging:
- Hearing aids
- Underwire bras
What you can expect
The MRI machine looks like a long narrow tube that has both ends open. You lie down on a movable table that slides into the opening of the tube. A technologist monitors you from another room. You can talk with the person by microphone. If you have a fear of enclosed spaces (claustrophobia), you may be given a medicine to help you feel sleepy and less anxious. Most people get through the exam without difficulty.
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 and other noises. Earplugs or music may be provided to help block the noise.
In some cases, a contrast material, typically gadolinium, may be injected through an intravenous (IV) line into a vein in your hand or arm. The contrast material enhances the appearance of certain details. The contrast material used for MRIs is less likely to cause an allergic reaction than the contrast material used for CT scans.
An MRI can last anywhere from 15 minutes to more than an hour. You must hold very still because movement can blur the resulting 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.
A doctor specially trained to interpret MRIs (radiologist) will analyze the images from your scan and report the findings to your doctor. Your doctor will discuss any important findings and next steps with you.