Fiber orientation dependence of diffusion MRI in whole-brain white matter

Motivation: In addition to the restriction of water movement by the microstructure of myelin, the spin coherence in diffusion MRI could be affected by the orientation dependence of magnetic field perturbation caused by the myelin sheath. Consequently, diffusion parameters are expected to vary with the orientation. Goal(s): To explore how and to what extent the quantitative diffusion parameters will vary with orientation. Approach: Diffusion parameters derived from DTI and NODDI models were analyzed. Results: Substantial orientation dependence of quantitative diffusion parameters was demonstrated. This suggests that current quantitative diffusion MRI techniques, which do not account for this effect, might be biased. Impact: The substantial orientation-dependent variations in diffusion parameters suggest that current techniques, which do not account for this orientation effect, might be biased. Further work to fully explore and correct for the orientation effect is vital for accurate quantitative analysis.