Review of relaxation times of tissues

NMR proton imaging techniques are based on fundamental properties of the water molecules of the body tissues, in particular the varying ability of different populations of water protons to lose spin orientation energy to the immediate environment (spin-lattice relaxation with a time constant of T1) or to randomise their spin orientation (spin-spin relaxation which contributes largely to T2). In general it has been shown that changes in the T1 and T2 relaxation times of water protons in pathological tissue are non-specific. For this reason it is most unlikely that examination of the relaxation times of excised tissues samples will ever be a primary diagnostic tool in clinical medicine. NMR imaging, however, offers this information in a different way, since spatial information is added to the relaxation time, and the pathological tissue is shown within its host tissue so that differences in relaxation time appear as contrast in a visual display. The imaging technique is also of use when a relaxation time is seen which is sufficiently different to identify the tissue as abnormal, but when there are several different conditions which could cause such an abnormal relaxation time. In this case the spatial information, showing a precise position or, perhaps, a mottling throughout a large region of the organ, can provide the missing factor which, along with previous clinical knowledge, enables a certain diagnosis to be made. As one of the battery of hospital diagnostic tests it is believed that knowledge of the extent and location of changes in the state of the body water will prove to be of great assistance