TIME-DEPENDENT DIFFUSION OF WATER IN A BIOLOGICAL MODEL SYSTEM

Packed erythrocytes are ideally suited as a model system for the study of water diffusion in biological tissue, because cell size, membrane permeability and extracellular volume fraction can be varied independe ntly. We used a pulsed-field-gradient spin echo NMR technique to measu re the time-dependent diffusion coefficient D(t) in packed erythrocyte s. The long-time diffusion constant, D-eff, depends sensitively on the extracellular volume fraction. This may explain the drop in D-eff dur ing the early stages of brain ischemia, where just minutes after an is chemic insult the extra-cellular volume in the affected region of the brain is significantly reduced. Using an effective medium formula, we estimate the erythrocyte membrane permeability, in good agreement with measurements on isolated cells. From the short-time behavior of D(t), we determine the. surface-to-volume ratio of the cells, approximate t o(0.72 mu m)(-1).