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Recent observations of several preferred orientations of diffusion in deep white matter may indicate either (a) that axons in different directions are independently bundled in thick sheets and function noninteractively, or more interestingly, (b) that the axons are closely interwoven and would exhibit branching and sharp turns. This study aims to investigate whether the dependence of dMRI Q‐ball signal on the interpulse timecan decode the smaller‐than‐voxel‐size brain structure, in particular, to distinguish scenarios (a) and (b).

High‐resolution Q‐ball images of a healthy brain taken with s/mm²for 3 different values ofwere analyzed. The exchange of water molecules between crossing fibers was characterized by the fourth Fourier coefficientof the signal profile in the plane of crossing. To interpret the empirical results, a model consisting of differently oriented parallel sheets of cylinders was developed. Diffusion of water molecules inside and outside cylinders was simulated by the Monte Carlo method.

Simulations predict that, agreeing with the empirical results, must increase withfor largeb‐values, but may peak at a typicalthat depends on the thickness of the cylinder sheets for intermediateb‐values. Thus, the thickness of axon layers in voxels with 2 predominant orientations can be detected from empiricaltaken at smallerb‐values.

Based on the simulation results, recommendations are made on how to design a dMRI experiment with optimalb‐value and range ofin order to measure the thickness of axon sheets in the white matter, hence to distinguish (a) and (b).