Optimizing the intrinsic parallel diffusivity in NODDI: An extensive empirical evaluation

Purpose NODDI 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(parallel to)) is set to 1.7 mu m(2).ms(-1), though the effects of this assumption have not been extensively explored. This work investigates the optimality of d(parallel to) = 1.7 mu m(2).ms(-1) under varying imaging protocol, age groups, sex, and tissue type in comparison to other biologically plausible values of d(parallel to). Methods Model residuals were used as the optimality criterion. The model residuals were evaluated in function of d(parallel to) over the range from 0.5 to 3.0 mu m(2).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(parallel to) was also explored. Results Results show d(parallel to) = 1.7 mu m(2).ms(-1) is appropriate for adult brain white matter but it is suboptimal for gray matter with optimal values being significantly lower. d(parallel to) = 1.7 mu m(2).ms(-1) was also suboptimal in the infant brain for both white and gray matter with optimal values being significantly lower. Minor optimum d(parallel to) differences were observed versus diffusion protocol. No significant sex effects were observed. Additionally, changes in d(parallel to) resulted in significant changes to the estimated NODDI parameters. Conclusion The default (d(parallel to)) of 1.7 mu m(2).ms(-1) is suboptimal in gray matter and infant brains.