Mapping measures of microscopic diffusion anisotropy in human brain white matter in vivo with double-wave-vector diffusion-weighted imaging

Abstract

Purpose: To demonstrate that rotationally invariant measures of the diffusion anisotropy on a microscopic scale can be mapped in human brain white matter in vivo. Methods: Echo-planar imaging experiments (resolution 3.0 x 3.0 x 3.0 mm3) involving two diffusion-weighting periods (δ = 22 ms, Δ = 25 ms) in the same acquisition, so-called double-wave-vector or double-pulsed-field-gradient diffusion-weighting experiments, were performed on a 3 T whole-body magnetic resonance system with a long mixing time (τm = 45 ms) between the two diffusion weightings. Results: The disturbing influences of background gradient fields, eddy currents, and the finite mixing time can be minimized using 84 direction combinations based on nine directions and their antipodes. In healthy volunteers, measures of the microscopic diffusion anisotropy (IMA and MA indexes) could be mapped in white matter across the human brain. The measures were independent (i) of the absolute orientation of the head and of the diffusion directions and (ii) of the predominant fiber orientation. Compared to the fractional anisotropy derived from the conventional diffusion tensor, the double-wave-vector indexes exhibit a narrower distribution, which could reflect their independence of the fiber orientation distribution. Conclusions: Mapping measures of the microscopic diffusion anisotropy in human brain white matter is feasible in vivo and could help to characterize tissue microstructure in the healthy and pathological brain.