Water diffusion compartmentation and anisotropy at high b values in the human brain

Biexponential diffusion decay is demonstrated in the human brain in vivo us ing b factors up to 4000 sec mm(-2). Fitting of the signal decay data yield s values for the slow and fast diffusion components and volume fractions in agreement with previous studies in rat and human brain. In addition, diffe rences in the fitted parameters are demonstrated in the white and gray matt er and diffusion anisotropy is demonstrated in both the slow and fast diffu sing components. Apparent anisotropy in the component fractions is discusse d in terms of directionally dependent exchange rates between the compartmen ts. The lack of a relationship between the estimated contribution to the si gnal of the fast and slow components and echo time appears to rule out T-2 differences in the observed water compartments. Values obtained for the fas t diffusion coefficient, including differences between white and gray matte r and the degree of anisotropy are compatible with the predictions of extra cellular diffusion of water based on tortuosity models and the diffusion of tetramethylammonium ions in rat brain. (C) 2000 Wiley-Liss, Inc.