Echo-time dependence of microscopic fractional anisotropy using single-shot spiral encoding and free water elimination

Motivation: Understand echo time (TE) dependence of microscopic fractional anisotropy (µFA) in white matter (WM), cortical grey matter (GM), and deep GM. Goal(s): Measure µFA at varying TEs and observe the trends in WM, cortical GM, and deep GM. Approach: Healthy volunteers were scanned with dMRI at 3T using a spiral imaging sequence with linear tensor encoding and spherical tensor encoding. µFA was calculated with and without free water elimination (FWE). Results: Linear regression fitting showed a downward trend in µFA in cortical GM and deep GM with increasing TE, both with and without FWE. Impact: We observed reducing µFA with increasing TE in grey matter for the first time, which was enabled by a spiral readout that greatly reduced the minimum TE (~20 ms). This TE-dependence can potentially be exploited for improved microstructural modelling.