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This work introduces a compartment-based model for apparent soma and neuritedensity imaging (SANDI) using non-invasive diffusion-weighted MRI (DW-MRI). Theexisting conjecture in brain microstructure imaging trough DW-MRI presentswater diffusion in white (WM) and grey (GM) matter as restricted diffusion inneurites, modelled by infinite cylinders of null radius embedded in thehindered extra-neurite water. The extra-neurite pool in WM corresponds to waterin the extra-axonal space, but in GM it combines water in the extra-cellularspace with water in soma. While several studies showed that this microstructuremodel successfully describe DW-MRI data in WM and GM at b<3 ms/{\mum^2}, it hasbeen also shown to fail in GM at high b values (b>>3 ms/{\mum^2}). Here wehypothesize that the unmodelled soma compartment may be responsible for thisfailure and propose SANDI as a new model of brain microstructure where soma(i.e. cell body of any brain cell type: from neuroglia to neurons) isexplicitly included. We assess the effects of size and density of soma on thedirection-averaged DW-MRI signal at high b values and the regime of validity ofthe model using numerical simulations and comparison with experimental datafrom mouse (bmax = 40 ms/{/mum^2}) and human (bmax = 10 ms/{\mum^2}) brain. Weshow that SANDI defines new contrasts representing new complementaryinformation on the brain cyto- and myelo-architecture. Indeed, we show for thefirst-time maps from 25 healthy human subjects of MR soma and neurite signalfractions, that remarkably mirror contrasts of histological images of braincyto- and myelo-architecture. Although still under validation, SANDI mightprovide new insight into tissue architecture by introducing a new set ofbiomarkers of potential great value for biomedical applications and pureneuroscience.

SANDI: a compartment-based model for non-invasive apparent soma and neurite imaging by diffusion MRI