The abbreviation for magnetic resonance imaging. MRI is a diagnostic technique that produces cross-sectional or 3-dimensional images of organs and other body structures. The patient lies inside a scanner surrounded by a large, powerful magnet. A receiving magnet is then placed around the part of the body to be investigated. If large areas, such as the abdomen, are to be imaged, the receiving magnet is fitted inside the scanner; for a smaller area, such as a joint, a magnet may be placed around the part to be scanned. The scanner generates a strong magnetic field, which causes the atoms in the body to line up parallel to each other. Short pulses of radio waves from a radiofrequency source briefly knock the atoms out of alignment. As the atoms realign they emit tiny signals, which are detected by the receiving magnet. Information about these signals is passed to a computer, which builds up an image based on the signals’ strength and location. MRI images can be enhanced by use of a contrast medium to highlight particular body structures, such as tumours and blood vessels. Images from MRI are similar to those produced by CT scanning but give greater contrast between normal and abnormal tissues. MRI is useful in studying the brain and spinal cord, the internal structure of the eye and ear, the internal organs, and blood flow. There are no known risks or side effects. The technique does not use ionizing radiation and can be performed repeatedly, but the scanner may interfere with the functioning of pacemakers, hearing aids, and other electrical devices.


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