Multi-echo NODDI with released intrinsic diffusivity in the healthy human brain tissue

Motivation: A well-known limitation of NODDI is that the intrinsic diffusivity parameter must be fixed to a brain-wide value to stabilise estimation, which restricts its applicability. Goal(s): An estimation approach for multi-echo NODDI (MTE-NODDI) with released intrinsic diffusivity recently proposed for the rat brain is applied here to investigate human brain tissue microstructure. Approach: Parameter estimation was performed via non-linear least-squares with an additional l2-norm regularisation term to enforce fitting stability. Results: Estimation of MTE-NODDI parameters with released intrinsic diffusivity was stable over several tissue types. Significant changes in tissue values were observed compared to the conventional approach. Impact: Incorporating an l2-norm regularisation term into the estimation of MTE-NODDI parameters allows the intrinsic diffusivity to be released whilst ensuring fitting stability. This opens the possibility of using MTE-NODDI to investigate a great variety of tissue pathologies.