NMR relaxation behavior in living and ischemically damaged tissue.

A study by pulsed NMR techniques in living liver tissue has led to the discovery that the observed longitudinal relaxation decay behavior is strongly multicomponent. After death of the experimental animal, the relaxation decay curves evolve toward a single-component behavior. These changes can also be observed within a few minutes after the liver is excised and placed in a test tube, and they involve a high degree of quantitative and qualitative regularity and reproducibility. An excellent description of all observed NMR behavior is obtained from a dynamic two-compartment model. Rapidly relaxing volumes exchange water molecules with slowly relaxing volumes; associating only an increasing water molecule exchange rate with increasing ischemia accounts in quantitative detail for all observed changes. The exchange-rate values and their variation with tissue deterioration are in good agreement with that estimated for intra- to extracellular water exchange as limited by cell-membrane osmotic permeabilities. Possible applications of these results in different biomedical areas are discussed.