Magnetic resonance imaging (MRI) is an alternative to computed tomography (CT) scans for obtaining three-dimensional images of the body. Unlike X-rays and CT scans, which use radiation, MRI uses powerful magnets and radio waves.
The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals.
Single MRI images are called slices and the slices can be stored on a computer or printed on film for viewing. One MRI exam produces dozens, or sometimes hundreds, of images.
How is an MRI performed?
You may be asked to wear a hospital gown or clothing without metal fasteners (such as sweatpants and a t-shirt). Certain types of metal can cause inaccurate images.
During the exam, you will lie on a narrow table, which slides into the middle of the MRI machine. If you fear confined spaces, tell your doctor before the exam. You may be prescribed a mild sedative, or your doctor may recommend an “open” MRI, in which the machine is not as close to the body.
Small devices, called coils, may be placed around your head, arm, leg, or other areas to be studied. These devices help send and receive the radio waves, and improve the quality of the images.
Some exams require a special dye (contrast) to help the radiologist see certain areas more clearly. The colorless dye is usually given at the time of the test through a vein (IV) in your hand or forearm.
Several sets of images are often obtained during the examination, each taking 2-15 minutes (somewhat longer than a CT examination). Depending on the areas being studied and the type of equipment, the exam may take one hour or longer. In many cases, this longer time allows for a clearer or higher resolution picture than a typical CT.
Why is the test performed?
Magnetic resonance imaging (MRI) is often used to:
- Evaluate tumors of the chest, abdomen or pelvis
- Diagnose coronary artery disease and heart problems
- Check tumors and other abnormalities of the reproductive organs
- Evaluate functional and anatomical abnormalities of the heart
- Discover causes of pelvic pain in women, such as endometriosis
- Check for diseases of the liver
- Diagnose congenital arterial and venous vascular anomalies and diseases of the chest, abdomen and pelvis
- Evaluate the bile duct, gallbladder and pancreatic ducts
- Check for breast cancer and monitor breast implants
- Evaluate the pancreas and spleen
- Evaluate pediatric diseases where radiation exposure carries a greater risk