Harmonic power validation of fiber ball imaging

Motivation: Fiber ball imaging (FBI) makes quantitative predictions beyond the previously reported diffusion MRI (dMRI) b-value scaling of the direction-averaged signal. Goal(s): Test the validity of the main FBI modeling assumptions. Approach: In vivo human dMRI data from white matter were acquired at b-values ranging from 1000 to 10,000 s/mm2, and the spherical harmonic expansion was calculated for degrees up to l = 6. Theoretical predictions from FBI for the harmonic power b-value dependence were then compared to the experimental results. Results: A close matching is observed between theory and experiment for b ≥ 4000 s/mm2. Impact: The observed harmonic power b-value dependence strongly support the two main FBI assumptions that 1) axons can be modeled as thin, impermeable cylinders, and 2) intra-axonal water dominates the dMRI signal for b ≥ 4000 s/mm2.