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

Abstract

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 (dk) is set to 1.7 μm2•ms−1, though the effects of this assumption have not been extensively explored. This work investigates the optimality of dk = 1.7 μm2•ms−1 under varying imaging protocol, age groups, sex, and tissue type in comparison to other biologically plausible values of dk. Methods Model residuals were used as the optimality criterion. The model residuals were evaluated in function of dk over the range from 0.5 to 3.0 μm2•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 dk was also explored. Results Results show dk = 1.7 μm2•ms−1 is appropriate for adult brain white matter but it is suboptimal for gray matter with optimal values being significantly lower. dk = 1.7 μm2•ms−1 was also suboptimal in the infant brain for both white and gray matter with optimal values being significantly lower. Minor optimum dk differences were observed versus diffusion protocol. No significant sex effects were observed. Additionally, changes in dk resulted in significant changes to the estimated NODDI parameters. Conclusion The default (dk) of 1.7 μm2•ms−1 is suboptimal in gray matter and infant brains.