THEORETICAL-MODEL FOR WATER DIFFUSION IN TISSUES

Water diffusion in a tissue model is studied both analytically and num erically. Tissue is regarded as a periodic array of boxes surrounded b y partially permeable membranes (cells), embedded in an extracellular medium, intracellular and extracellular diffusion coefficients may dif fer. Expressions for the apparent diffusion coefficients (ADC) in isot ropic and nonisotropic tissues are derived and compared with Monte Car te simulations. Calculated ADCs disagree with values obtained from the widely used ''fast exchange'' formula. Effects of differences between intracellular and extracellular T-2 relaxation times on measured valu es of ADC and T-2 are discussed. The general analysis is specifically applied to the changes occurring in ADC following ischemic insults to brain tissue. It is found that although membranes affect ADC significa ntly, the observed changes in diffusion cannot be due to reduced membr ane permeabilities. They may result from the combined effect of change s in cellular volume fraction, extracellular and intracellular diffusi on.