magnetic resonance (MR), a phenomenon in which the atomic nuclei of certain materials placed in a strong, static magnetic field absorb radio waves supplied by a transmitter at particular frequencies. The energy of the radio waves promotes the nuclei from a low-energy state, in which the nuclear spin is aligned parallel to the magnetic field, to a higher-energy state, in which the nuclear spin has a component transverse or opposed to the field. These nuclei occasionally revert to the lower-energy state by emitting photons at characteristic (resonance) frequencies, providing information about the local magnetic field at the nuclei. The rate at which the nuclei revert, or relax, to the lower-energy state when the source of radio waves is turned off is another important factor. See also relaxation time.
