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Magnetic resonance imaging (MRI) uses computer-generated radio waves and a powerful magnetic field to produce detailed images of body structures including tissues, organs, bones, and nerves. Neurological uses include the diagnosis of brain and spinal cord tumors, eye disease, inflammation, infection, and vascular irregularities that may lead to stroke. MRI can also detect and monitor degenerative disorders such as multiple sclerosis and can document brain injury from trauma.
The equipment houses a hollow tube that is surrounded by a very large cylindrical magnet. The patient, who must remain still during the test, lies on a special table that is slid into the tube. The patient will be asked to remove jewelry, eyeglasses, removable dental work, or other items that might interfere with the magnetic imaging. The patient should wear a sweat shirt and sweat pants or other clothing free of metal eyelets or buckles. MRI scanning equipment creates a magnetic field around the body strong enough to temporarily realign water molecules in the tissues. Radio waves are then passed through the body to detect the “relaxation” of the molecules back to a random alignment and trigger a resonance signal at different angles within the body. A computer processes this resonance into either a three-dimensional picture or a two-dimensional “slice” of the tissue being scanned, and differentiates between bone, soft tissues and fluid-filled spaces by their water content and structural properties. A contrast dye may be used to enhance visibility of certain areas or tissues. The patient may hear grating or knocking noises when the magnetic field is turned on and off. (Patients may wear special earphones to block out the sounds.) Unlike CT scanning, MRI does not use ionizing radiation to produce images. Depending on the part(s) of the body to be scanned, MRI can take up to an hour to complete. The test is painless and risk-free, although persons who are obese or claustrophobic may find it somewhat uncomfortable. (Some centers also use open MRI machines that do not completely surround the person being tested and are less confining. However, open MRI does not currently provide the same picture quality as standard MRI and some tests may not be available using this equipment). Due to the incredibly strong magnetic field generated by an MRI, patients with implanted medical devices such as a pacemaker should avoid the test.
Functional MRI (fMRI) uses the blood’s magnetic properties to produce real-time images of blood flow to particular areas of the brain. An fMRI can pinpoint areas of the brain that become active and note how long they stay active. It can also tell if brain activity within a region occurs simultaneously or sequentially. This imaging process is used to assess brain damage from head injury or degenerative disorders such as Alzheimer’s disease and to identify and monitor other neurological disorders, including multiple sclerosis, stroke, and brain tumors.