magnetic resonance imaging

magnetic resonance imaging (MRI), noninvasive diagnostic technique that uses nuclear magnetic resonance to produce cross-sectional images of organs and other internal body structures. The patient lies inside a large, hollow cylinder containing a strong electromagnet, which causes the nuclei of certain atoms in the body (especially those of hydrogen) to align magnetically. The patient is then subjected to radio waves, which cause the aligned nuclei to "flip" ; when the radio waves are withdrawn the nuclei return to their original positions, emitting radio waves that are then detected by a receiver and translated into a two-dimensional picture by computer. Unhampered by bone and capable of producing images in a variety of planes, MRI is used in the diagnosis of brain tumors and disorders, spinal disorders, multiple sclerosis, and cardiovascular disease. The procedure is considered to be without risk, but the scanner may interfere with pacemakers, hearing aids, or other mechanical devices. Although the images are similar in many ways to those of CAT scans , they are obtained without X rays or other radiation, and generally provide more contrast between normal and abnormal tissue.

magnetic resonance imaging (MRI) Diagnostic scanning system, based on the use of powerful magnets, which produces images of soft tissues in the body. Magnetic resonance imaging (MRI) is invaluable for producing images of the brain and spinal cord in particular. The scanner's magnet causes the nuclei within the atoms of the patient's body to line themselves up in one direction. A brief radio pulse is then beamed at the nuclei, causing them to spin. As they realign themselves to the magnet, they give off weak radio signals that can be recorded and converted electronically into images.

magnetic resonance imaging (MRI) (mag-net-ik) n. a diagnostic imaging technique based on the emission of electromagnetic waves from the body when the patient is placed in a strong magnetic field and exposed to radiofrequency radiation (see nuclear magnetic resonance). Most images rely on the signal from hydrogen in water, which is particularly strong, although other elements can be used. A major advantage over computerized tomography is the lack of X-rays, which reduces exposure to ionizing radiation.