Finsler fractional anisotropy (FFA): a directionally sensitive descriptor for multi-fiber white matter characterization in HARDI.

Diffusion tensor imaging has significantly advanced the study of brain microstructure, but it faces challenges in regions with complex fiber architectures. High angular resolution diffusion imaging provides improved angular resolution, but still lacks straightforward scalar measures for complex diffusion patterns. We propose Finsler fractional anisotropy (FFA), a directionally sensitive biomarker derived from the Finsler geometry framework to capture local features in multi-fiber white matter. The descriptor was evaluated using both synthetic and in vivo data. Results suggest that FFA offers improved sensitivity in differentiating fiber crossings from parallel bundles, showing enhanced contrast and directional sensitivity, particularly within multi-fiber voxels where traditional scalar measures tend to be less informative. Statistical evaluations (t-test and Wilcoxon signed-rank test) indicated significant differences, with p-values [Formula: see text]. We also discuss the strengths and limitations of FFA in analyzing complex white matter. Overall, these preliminary findings suggest that FFA may serve as a valuable complementary tool for characterizing complex brain microstructure. While results show enhanced directional sensitivity in regions with crossing fibers, further validation in larger clinical cohorts is required to establish its utility for investigating conditions such as multiple sclerosis and Alzheimer’s disease.