Multi-echo NODDI with released intrinsic diffusivity: Initial insights for rat brain tissue

Motivation: The applicability of NODDI and its spinoffs, e.g. multi-echo (MTE) NODDI, is limited to those tissue conditions where the intrinsic diffusivity is known a priori. Goal(s): We propose an estimation approach for MTE-NODDI parameters in which the intrinsic diffusivity is released whilst ensuring fitting stability by adding an l2-norm regularisation term to the cost function. Approach: The regularisation parameter was optimised via the generalised cross-validation approach. The MTE-NODDI with released intrinsic diffusivity was tested in a healthy rat at 3 T. Results: The estimation of MTE-NODDI with released intrinsic diffusivity is well-conditioned if an l2-norm regularisation term is used. Impact: The proposed estimation approach for MTE-NODDI parameters with released intrinsic diffusivity was shown to be stable. Thus, a new range of tissue conditions in which the intrinsic diffusivity is known to be affected, e.g. stroke, can be accurately characterised.