Magnetic resonance imaging (MRI) is a medical imaging technique that uses a magnetic field and computer-generated radio waves to create detailed images of the organs and tissues in your body.
Most MRI machines are large, tube-shaped magnets. When you lie inside an MRI machine, the magnetic field inside works with radio waves and hydrogen atoms in your body to create cross-sectional images — like slices in a loaf of bread.
The MRI machine also can produce 3D images that can be viewed from different angles.
Why it's done
MRI is a noninvasive way for a medical professional 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 conditions.
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.
- Conditions of the eye and inner ear.
- Multiple sclerosis.
- Spinal cord conditions.
- Brain injury from trauma.
A special type of MRI is the functional MRI of the brain, also known as fMRI. It produces images of blood flow to certain areas of the brain. Functional MRI can be used to examine the brain's anatomy and show which parts of the brain are handling critical functions, language and movements. This information can help guide decisions when considering someone for brain surgery.
Functional MRI also can check for damage from a head injury or from conditions such as Alzheimer's disease.
MRI of the heart and blood vessels
MRI that focuses on the heart or blood vessels can check:
- Size and function of the heart's chambers.
- Thickness and movement of the walls of the heart.
- Extent of damage caused by heart attacks 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 can check for tumors or other irregularities in many organs in the body, including the following:
- Liver and bile ducts.
MRI of bones and joints
MRI can help look for:
- Joint issues caused by traumatic or repetitive injuries, such as torn cartilage or ligaments.
- Disk problems in the spine.
- Bone infections.
- Tumors of the bones and soft tissues.
MRI of the breasts
MRI can be used with mammography to detect breast cancer, particularly in people who have dense breast tissue or who might be at high risk of the disease.
Because MRI uses powerful magnets, the presence of metal in your body can be a safety hazard if attracted to the magnet. Even if not attracted to the magnet, metal objects can distort the MRI images. Before having an MRI exam, you'll likely complete a questionnaire that includes whether you have metal or electronic devices in your body.
Unless the device you have is certified as MRI safe, you might not be able to have an MRI. Devices include:
- Metallic joint prostheses.
- Artificial heart valves.
- An implantable heart defibrillator.
- Implanted drug infusion pumps.
- Implanted nerve stimulators.
- A pacemaker.
- Metal clips.
- Metal pins, screws, plates, stents or surgical staples.
- Cochlear implants.
- A bullet, shrapnel or any other type of metal fragment.
- Intrauterine device.
If you have tattoos or permanent makeup, ask whether it might affect your MRI. Some of the darker inks contain metal.
Before you schedule an MRI, tell your doctor if you think you're pregnant. The effects of magnetic fields on an unborn baby aren't well understood. An alternative exam may be recommended, or the MRI may be postponed. Also tell your doctor if you're breastfeeding, especially if you're to receive contrast material during the procedure.
It's also important to discuss kidney or liver problems with your doctor and the technologist, because problems with these organs might limit the use of injected contrast agents during your MRI scan.
How you prepare
Before an MRI exam, eat as you would normally and continue to take your usual medicines, unless you're told otherwise. You will typically be asked to change into a gown and to remove things that might affect the magnetic imaging, such as:
- Hearing aids.
- Underwire bras.
- Cosmetics that contain metal particles.
What you can expect
During the test
The MRI machine looks like a long narrow tube that is open on both ends. 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 technologist by microphone.
If you have a fear of enclosed spaces, called claustrophobia, you might receive a drug 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. Wearing earplugs or having music playing can help block the noise.
In some cases, a contrast material, typically gadolinium, will be injected through an intravenous (IV) line into a vein in a hand or arm. The contrast material helps make certain details clearer. Gadolinium rarely causes allergic reactions.
An MRI exam can last anywhere from 15 minutes to more than an hour. You must hold still because movement can blur the images.
During a functional MRI exam, you might be asked to perform a few 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 can resume your usual activities immediately after the scan.
A doctor specially trained to interpret MRI scans, called a radiologist, will look over the images from your scan and report the findings to your doctor. Your doctor will discuss important findings and next steps with you.