BIEXPONENTIAL DIFFUSION ATTENUATION IN VARIOUS STATES OF BRAIN-TISSUE- IMPLICATIONS FOR DIFFUSION-WEIGHTED IMAGING

Diffusion-weighted single voxel experiments conducted at b-values up t o 1 x 10(4) smm(-2) yielded biexponential signal attenuation curves fo r both normal and ischemic brain. The relative fractions of the rapidl y and slowly decaying components (f(1), f(2)) are f(1) = 0.80 +/- 0.02 , f(2) = 0.17 +/- 0.02 in healthy adult rat brain and f(1) = 0.90 +/- 0.02, f(2) = 0.11 +/- 0.01 in normal neonatal rat brain, whereas the c orresponding values for the postmortem situation are f(1) = 0.69 +/- 0 .02, f(2) = 0.33 +/- 0.02. It is demonstrated that the changes in f(1) and f(2) occur simultaneously to those in the extracellular and intra cellular space fractions (f(ex), f(in)) during: (i) cell swelling afte r total circulatory arrest, and (ii) the recovery from N-methyl-D-aspa rtate induced excitotoxic brain edema evoked by MK-801, as measured by changes in the electrical impedance. Possible reasons for the discrep ancy between the estimated magnitude components and the physiological values are presented and evaluated. Implications of the biexponential signal attenuation curves for diffusion-weighted imaging experiments a re discussed.