P163. Fiber Density vs. Dispersion in 16p11.2 Deletion: A Multi-Site Study of Advanced Diffusion MRI Measures

Abstract

16p11.2 Deletion Syndrome (16pDel) is associated with neurodevelopmental disorders, such as autism spectrum disorder. Previous 16pDel diffusion MRI (dMRI) studies report increased white matter (WM) fractional anisotropy (FA) from DTI. The underlying microstructural sources of increased FA in 16pDel carriers are unknown. We used advanced dMRI metrics in a large multi-site sample of 16p11Del carriers to disentangle two sources of FA variability: axonal density and fiber dispersion. Mean Signal Kurtosis (MSK) and Return-to-Origin Probability (RTOP) model axonal density with less bias from fiber incoherence. Conversely, NODDI Orientation Dispersion Index (ODI) is sensitive to fiber dispersion.