Diffusion time dependence of magnetic resonance diffusion signal decays: an investigation of water exchange in human brain in vivo.

OBJECT: To understand the behavior of diffusion signal decays of water in white matter of human brain in vivo and to estimate tissue microstructure parameters such as exchange time of diffusing water molecules in human brain.
MATERIALS AND METHODS: Diffusion decays were measured over an extended range of diffusion weightings (b-values) up to a maximum of 12,500 s/mm(2) and diffusion times between 19.9 and 53.8 ms in eight healthy human subjects using MRI scans. The diffusion signal decays were all Rician noise corrected and then analyzed using multi-component non-negative least squares (NNLS) data analysis.
RESULTS: Three diffusion coefficients including one at (0.930 ± 0.003) × 10(-3) (80 ± 1%) mm(2)/s, another at (0.067 ± 0.002) × 10(-3) (19 ± 1%) mm(2)/s and a small contribution at (1.20 ± 0.02) × 10(-2) (1.00 ± 0.01%) mm(2)/s were observed in the diffusion decay using the highest b-value. The diffusion decays show diffusion time dependence for the slow diffusion coefficient which has not previously been reported.
CONCLUSION: This study presents the accurate diffusion parameters by the use of very large b-values along with Rician noise correction and multi-component data analysis. The experimental results are consistent with the theoretical predictions used to estimate the exchange time of diffusing water molecules for a model of human brain tissue.