NMR imaging

Nuclear magnetic resonance (NMR) has, in recent years, been developed as a new imaging technique in medicine. The NMR phenomenon is discussed with emphasis upon the nuclei of hydrogen atoms. The use of radio frequency pulses to obtain an NMR signal is described. The period over which this signal may be observed is limited by the relaxation properties of the tissues of the body. Sequences of pulses are used to emphasize these relaxation parameters, and the resultant images provide more information than is readily available from the simplest, proton density pictures. One method of imaging a single slice is described, which has two variations. The image is a map of the transverse nuclear magnetization at a given time during the pulse sequence. A field gradient together with a radio frequency field is used to rotate the magnetization selectively. It is found that the receiver signal corresponds to a point in the spatial frequency spectrum of the image; the position of this point is determined by an additional field gradient. Two ways of mapping the spatial frequency spectrum of the image are described. Finally two developments of this imaging method are briefly given. (author)