Investigating Diffusion-MRI based neurite density estimation model dependency: an in-vivo study on the HCP dataset

Neurite density is one of the most promising microstructural features that can be estimated from Diffusion MRI multi-shell data. This index is generally calculated from diffusion MRI compartmental models as the "stick" compartment volume fraction. In this work we provide evidence that the distribution of stick volume fraction is characteristic of the brain tissue (white matter or gray matter) and is highly reproducible between subjects, but strongly depends on the underlying multi-compartment model. In particular, in-vivo results on 10 subjects of the Human Connectome Project show that the neurite density distribution hardly depends on both the stick parallel diffusivity of the extra-axonal compartment model. This witnesses against the exploitability of such a parameter as a biomarker due to the lack of accuracy and precision across models.