NODDI with dispersion anisotropy

PURPOSE This work presents a technique for estimating the dispersion anisotropy in neurite orientations, using Neurite Orientation Dispersion and Density Imaging (NODDI) [1]. NODDI is a diffusion MRI technique, recently developed to directly quantify the microstructural features (density and orientation dispersion) of neurites in vivo, in the human brain. The parameters it provides offer higher specificity and sensitivity than standard indices from Diffusion Tensor Imaging (DTI), as shown in the preliminary application to epilepsy [2], dementia [3] and brain development [4]. One limitation of the current implementation is that it models neurite orientation dispersion with the Watson distribution, which constrains the dispersion about the dominant orientation to be isotropic (Fig. 1). This can neither accurately represent nor fully characterise the anisotropic orientation dispersion associated with complex fibre configurations, such as fanning and bending fibres. The present work develops a new NODDI model that can quantify the degree of this anisotropy and estimate the primary dispersion orientation. These features are not only important for mapping brain connectivity [5] but are also potential markers for disease diagnosis and monitoring. Using simulated and in vivo brain data, we assess the accuracy and precision of estimating the dispersion anisotropy using the NODDI protocol [1] and quantify the potential bias of the current NODDI model in estimating tissue microstructure.