Magnetic Resonance Imaging (MRI)
MRI is a method of producing extremely detailed pictures of body tissues and organs without the need for X-rays. The electromagnetic energy that is released when exposing a patient to radio frequency waves in a strong magnetic field is measured and analyzed to produce both two- and three-dimensional images that may be viewed on a computer monitor. This advanced imaging technique allows for the evaluation of some body structures that may not be as visible with other imaging methods.
Areas of Application
Benefits vs. Risks
- Images of the brain, spine, joints and soft tissue structures are clearer and more detailed than with other imaging methods.
- On the occasion that IV contrast is needed, it is less likely to produce an allergic reaction than the iodine-based materials used for conventional X-rays and CT scans.
- There is no exposure to radiation.
- Ability to detect abnormalities that might be obscured by bone tissue.
- New MRI systems can depict brain function, and in this way detect a stroke at a very early stage.
- The examination poses virtually no risk when appropriate guidelines are followed.
- Metal implants may be affected by the magnetic field. The procedure should be avoided in any patient with a pacemaker, implanted neurostimulator, certain types of metallic ear implants (cochlear implants) or metallic object within the eye socket. It should also be avoided if the patient has a port for delivering insulin or chemotherapy (however, many ports are detachable so patients can safely undergo MRI examinations). If you have an implant, please bring the implant identification card or letter from the physician who placed the implant. Please note, most orthopedic appliances such as artificial hips, knees, hearts stents and valves are all imaged safely.
- Pregnant patients should be informed that, to date, there has been no evidence that the use of clinical MR imaging during pregnancy has produced any deleterious effects. For more information, please see our Patient Safety Guidelines section and click the link to Pregnancy and MRI.
- Women who are breast-feeding should inform the radiologist and ask how to proceed. MRI poses no risk to milk or breast tissue.
About MRI Examinations
How does the procedure work?
MRI is a unique imaging method because, unlike the usual radiographs (x-rays), radioisotope studies and even CT scanning, it does not rely on ionizing radiation. Instead, radio waves are directed at protons, the nuclei of hydrogen atoms, in a strong magnetic field. The protons are first "excited" and then "relaxed," emitting radio signals, which can be computer-processed to form an image. In the body, protons are most abundant in the hydrogen atoms of water - the "H" of H2O - so that an MRI image depicts differences in the content and distribution of water in various tissues. With MRI, different types of tissue within the same body structure are clearly displayed in fine anatomic detail. In the spine, for instance, fatty tissue, cerebrospinal fluid and the intervertebral discs contain considerable water, more than is found in bone, cartilage and nerve tissue. MRI is well suited to detecting conditions that increase the amount of fluid, such as tumors, inflammation and infection. Even different types of tissue within the same organ, such as the gray and white matter of the brain, can easily be distinguished. Special imaging techniques are used to image various structures, such as arteries in the case of MR Angiography, or MRA.
How is the procedure performed?
The patient is placed on a special table and positioned inside the opening of the MRI unit. A typical exam consists of two to six imaging sequences, each taking two to 15 minutes. Each sequence provides a specific image orientation and a specified degree of image clarity or contrast. Depending on the type of exam being done, the total time needed can range from 20 to 60 minutes, not counting the time needed to change clothing, and answer questions. When contrast material is needed, a substance called gadolinium is given by IV injection during one of the imaging sequences. It highlights blood vessels and pathology, making them stand out from surrounding tissues. If you are having the shoulder, knee, or other specific joint imaged, direct injection of contrast into the joint may be required. The injection is performed either prior to, or about half-way through the exam (please see section on “MRI Arthrography” for details). The technologist leaves the room during the actual imaging process, but the patient can communicate with them at any time using an intercom. A friend/relative or, if a child is being examined, a parent may stay in the room. When the exam is completed you may be asked to wait to make sure that more images are not needed.