Review of Two Models for Water Proton Relaxation in Tissue

Pulsed NMR spectroscopy has been used extensively to study the motion of water in tissue by proton and deuteron resonance methods. While some studies (1-4) of this type had been done in the 1960's, activity in this area received a great stimulus from the report by Damadian (5) in 1971 that the proton spin-lattice relaxation time in the laboratory frame (7", ) was substantially longer for rat tumors than for apparently healthy tissue. In the many studies which followed, generally one of two types of measurements was made. In most studies, relaxation times were measured—mainly 7", and the spinspin relaxation time T2. The results (6) were markedly different than for comparable measurements on pure water. T, generally was in the 300 900-ms range and 7"2 was of the order of 50 ms, while for pure water at 20°C, F, and T2 are equal and have values of 2.5 to 3.3 s (7), depending on purity. In other studies, the self-diffusion constant (D) of the water molecules was measured via proton resonance. For tissue, the value of Dwas found to be about half that observed for pure water (8). At the simplest level, the fact that the diffusion constant for water in tissue was lower than that of pure water by only about a factor of two was usually interpreted to indicate that the mobility of most of the water in tissue was comparable to that of pure water (8). On the other hand, the relaxation times (both 7", and 7"2 are shorter in tissue than in pure water, and 7~2 is about an order of magnitude shorter than 7",) were more like those in solids than in liquids. Interpretations (1,9) of these results have tended to center around models in which water may be in one of several phases or situations. Two examples of models used to interpret such relaxation time and diffusion constant results are reviewed below.