Optimizing the intrinsic parallel diffusivity in NODDI: an extensive empirical evaluation
PurposeNODDI is widely used in parameterizing microstructural brain properties. The model includes three signal compartments: intracellular, extracellular, and free water. The neurite compartment intrinsic parallel diffusivity (d||) is set to 1.7 {micro}m2*ms-1, though the effects of this assumption have not been extensively explored. This work seeks to optimize d|| by minimizing the model residuals.\n\nMethodsThe model residuals were evaluated in function of d|| over the range from 0.5 to 3.0 {micro}m2*ms-1. This was done with respect to tissue type (i.e., white matter versus gray matter), sex, age (infancy to late adulthood), and diffusion-weighting protocol (maximum b-value). Variation in the estimated parameters with respect to d|| was also explored.\n\nResultsResults show the optimum d|| is significantly lower for gray matter relative to 1.7 {micro}m2*ms-1 and to white matter. Infants showed significantly decreased optimum d|| in gray and white matter. Minor optimum d|| differences were observed versus diffusion protocol. No significant sex effects were observed. Additionally, changes in d|| resulted in significant changes to the estimated NODDI parameters.\n\nConclusionFuture implementations of NODDI would benefit from d|| optimization, particularly when investigating young populations and/or gray matter.