n. (Physics) A phenomenon resulting from quantization in the magnetic field of electrons or atoms or molecules or nuclei exposed to an external magnetic field, by which a narrow frequency band of radio waves may be absorbed and re-emitted by the object. [ WordNet 1.5 +PJC ]

n. (Medicine) a medical diagnostic procedure utilizing the phenomenon of nuclear magnetic resonance to generate images of internal parts of the body. It depends on the differential absorption of electromagnetic radiation by different types of living tissue in a magnetic field. It is complementary to X-ray imaging in that the softer tissue show more prominently in magnetic resonance images, rather than bone, as with X-rays. It is a non-invasive procedure, allowing such images to be obtained without penetration of the tissue by objects. It is abbreviated MRI. As with computerized tomography, the results are usually presented as images of sequential planar sections of that part of the body of concern to the physician. [ PJC ]

n. (Physics) The specific absorption and re-emission of electromagnetic radiation at characteristic wavelengths by atomic nuclei in a magnetic field. It is abbreviated NMR. The wavelength of the radiation absorbed depends on the type of nucleus, the intensity of the magnetic field, and the local chemical environment in which the nucleus resides. It is the latter effect (called the chemical shift), by which atoms of specific elements in different chemical compounds show a different resonance frequency, which gives rise to the greatest utility of this phenomenon in analyzing the chemical structure of substances. Similar effects of the chemical environment permit the discrimination of different types of living tissue by virtue of their different chemical composition, thus permitting utilization of the phenomenon in medical diagnostic instruments, especially for magnetic resonance imaging. [ PJC ]