Validation of double diffusion schemes of microscopic fractional anisotropy

Diffusion sequences with two diffusion encoding blocks have recently been proposed to allow model free characterization of microscopic diffusion anisotropy, i.e. diffusion anisotropy of the individual water compartments insensitive to their directional dispersion. For example, the d-PFG 5-design was shown to provide metrics of e.g. microscopic fractional anisotropy, μFA, using 72 pairs of gradient directions. In contrast to FA, μFA in theory remains unaffected by the presence of fiber dispersion or crossing, and theoretically equals FA for single straight fiber bundles. The purpose of this work was to test the ability to distinguish μFA from macro FA in controllable phantoms, and then to evaluate the relative performance of different sampling schemes.