An Accelerated, Alternative Approach for Estimating Zero-Displacement Probability in Hybrid Diffusion Imaging

Inroduction: Hybrid diffusion imaging (HYDI) is a diffusion-weighted (DW) technique whose encoding scheme is composed of multiple concentric (spherical) shells of constant diffusion weighting in q-space, which may be used to characterize the signal behavior with low, moderate, and high diffusion weighting [1]. The zero-displacement probability (Po) describes the probability density of water molecules that do not diffuse within the diffusion time [2], and is a measure of restricted diffusion, which may be modulated by axons and myelin in white matter diseases like multiple sclerosis (MS). In HYDI, the Po is conventionally estimated by using the signal measurements in all the shells (Poall), which requires long scan time. However, the highest diffusion-weighting measurements are likely to contribute most heavily to the restricted diffusion signal. Thus, an alternative and faster approach for characterizing restricted diffusion would be to use the signal measurements only in the outermost shell, or highest diffusion weighting (Poouter).