In vivo mapping of striatal neurodegeneration in Huntington's disease with Soma and Neurite Density Imaging.

Huntington's disease (HD) is an inherited neurodegenerative disorder characterised by progressive cognitive and motor decline driven by basal ganglia (BG) atrophy. Clinical trials of novel disease-modifying therapies are ongoing, creating a need for sensitive non-invasive imaging biomarkers. Soma and Neurite Density Imaging (SANDI) is a multi-shell diffusion MRI model that estimates intracellular signal fractions from sphere-shaped soma and shows promise as a marker of neurodegeneration. The objectives of this study were to characterise HD-related microstructural abnormalities in the BG using SANDI and to examine relationships between SANDI and volumetric measurements and motor performance. T1- and diffusion-weighted images (b-values 200-6000 s/mm²) were acquired on a 3T Siemens Connectom scanner (300 mT/m) in 56 individuals with HD and 57 age- and sex-matched controls. HD participants completed Quantitative Motor (Q-Motor) tasks, summarised using principal component analysis. SANDI estimated apparent soma and neurite density, apparent soma size, and extracellular signal fraction. Microstructural and volumetric indices were extracted from bilateral caudate, putamen, pallidum and thalamus regions, compared between groups, and correlated with Q-Motor performance. HD was associated with reduced apparent soma density and increased apparent soma size and extracellular signal fraction in the BG but not the thalami. No group differences were present for apparent neurite density. SANDI metrics correlated with Q-Motor performance and explained up to 63% of striatal atrophy in HD. SANDI indices detected HD-related striatal neurodegeneration, explained atrophy, and correlated with motor impairments, demonstrating its potential as an in vivo biomarker and surrogate clinical outcome measure for HD and other neurodegenerative diseases.