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

• Published in: NeuroImage (Orlando, Fla.) Vol. 215; p. 116835
• Main Authors: Palombo, Marco, Ianus, Andrada, Guerreri, Michele, Nunes, Daniel, Alexander, Daniel C., Shemesh, Noam, Zhang, Hui
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15.07.2020; Elsevier Limited; Academic Press; Elsevier
• Subjects: Adult; Animals; Axons; Brain; Brain - cytology; Brain - diagnostic imaging; Brain - physiology; Brain architecture; Brain mapping; Cell body; Cell Body - physiology; Diffusion Magnetic Resonance Imaging - methods; Female; Glia; Humans; Magnetic resonance imaging; Male; Mice; Mice, Inbred C57BL; Models, Neurological; Nervous system; Neurites - physiology; Neuroimaging; Simulation
• ISSN: 1053-8119; 1095-9572; 1095-9572
• DOI: 10.1016/j.neuroimage.2020.116835

This work introduces a compartment-based model for apparent cell body (namely soma) and neurite density imaging (SANDI) using non-invasive diffusion-weighted MRI (DW-MRI). The existing conjecture in brain microstructure imaging through DW-MRI presents water diffusion in white (WM) and gray (GM) matter as restricted diffusion in neurites, modelled by infinite cylinders of null radius embedded in the hindered extra-neurite water. The extra-neurite pool in WM corresponds to water in the extra-axonal space, but in GM it combines water in the extra-cellular space with water in soma. While several studies showed that this microstructure model successfully describe DW-MRI data in WM and GM at b ​≤ ​3,000 ​s/mm2 (or 3 ​ms/μm2), it has been also shown to fail in GM at high b values (b≫3,000 ​s/mm2 or 3 ​ms/μm2). Here we hypothesise that the unmodelled soma compartment (i.e. cell body of any brain cell type: from neuroglia to neurons) may be responsible for this failure and propose SANDI as a new model of brain microstructure where soma of any brain cell type is explicitly included. We assess the effects of size and density of soma on the direction-averaged DW-MRI signal at high b values and the regime of validity of the model using numerical simulations and comparison with experimental data from mouse (bmax ​= ​40,000 ​s/mm2, or 40 ​ms/μm2) and human (bmax ​= ​10,000 ​s/mm2, or 10 ​ms/μm2) brain. We show that SANDI defines new contrasts representing complementary information on the brain cyto- and myelo-architecture. Indeed, we show maps from 25 healthy human subjects of MR soma and neurite signal fractions, that remarkably mirror contrasts of histological images of brain cyto- and myelo-architecture. Although still under validation, SANDI might provide new insight into tissue architecture by introducing a new set of biomarkers of potential great value for biomedical applications and pure neuroscience.