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 MRIHow the Test is Performed:The MRI scanner is in an area that has been shielded from outside magnetic fields. You are asked to lie on a narrow table that can slide inside a large tunnel-like tube within the scanner. The scanner creates a magnetic field around you, then pulses radio waves that are directed at the tissues in question. Several sets of images are usually required, each taking from 2 to 15 minutes. The complete scan takes about 1 hour; some scans take more than 90 minutes. How to Prepare: No preparatory tests, diets, or medications are usually needed. Occasionally, the person may be asked to fast for 4 to 6 hours prior to the scan. You must sign a consent form. Because of the strong magnets, no metallic objects are allowed into the room. Items such as jewelry, watches, credit cards, and hearing aids can be damaged. Pins, hairpins, metal zippers, and similar metallic items can distort the images. Frequent exposure to ferrous metals (such as a sheet metal worker has) may make MRI difficult. A hospital gown may be recommended, or you may be allowed to wear "sweats" or similar clothing without metal fasteners. Removable dental work should be taken out just prior to the scan. How it Feels: There is no pain. The magnetic field and radio waves are not felt. The primary discomfort is the claustrophobic feeling that some people experience from being inside the scanner. The table may be hard or cold, but you can request a blanket or pillow. The machine produces loud thumping and humming noises. Ear plugs are usually given to reduce the noise. A technologist observes you during the entire procedure and may enter the room to speak to you or may speak with you through an intercom in the scanner. Excessive movement can blur MRI images. If you have difficulty lying still or are very anxious, you may be given a sedative. There is no recovery required (unless you have been sedated). After an MRI scan, you can resume normal diet, activity, and medications. Risks: There is no ionizing radiation and usually no contrast dye is needed. The procedure is safe in most cases, and there are no documented side effects. However, because the effects of strong magnetic fields on a fetus are not well documented at this time, pregnant women are usually advised to avoid MRI scans. Because the strong magnets can displace or disrupt the action of implanted metallic objects, people with cardiac pacemakers cannot be scanned and should not enter the MRI scan area. MRI also should not be used for people with metallic objects in their bodies, such as artificial joints, inner ear (cochlear) implants, brain aneurysm clips, some artificial heart valves, and metallic foreign bodies in the eye or other body areas. MRI is usually not recommended for acute trauma situations, because traction and life-support equipment cannot enter the scanner area and because of the long scan times. For specific information about why the test is performed and normal and abnormal results, please see the specific MRI topics: |
