The acronym for magnetic resonance imaging, a non-invasive method of imaging the body and its organs. It may also be used to study tissue metabolism. The area to be imaged is placed in a magnetic field which causes atomic nuclei to align in the direction of the field. Pulses of radio-frequency radiation are then applied; interpretation of the frequencies absorbed and re-emitted allows an image in any body plane to be built up. Different tissues – for example, fat and water – can be separately identified and, if the resonance signal for the fat is suppressed, only the signal from any abnormalities in the fat can be identified. Many diseases result in a rise in the water content of tissues, so MRI is a valuable test for identifying disease, and the operating radiologist is skilled in interpreting the meaning of altered signals.