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 (d‖) is set to 1.7 µm2⋅ms−1, though the effects of this assumption have not been extensively explored. This work seeks to optimize d‖ by minimizing the model residuals.
Methods The model residuals were evaluated in function of d‖ 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 d‖ was also explored.
Results Results show the optimum d‖ is significantly lower for gray matter relative to 1.7 µm2⋅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.
Conclusion Future implementations of NODDI would benefit from d‖ optimization, particularly when investigating young populations and/or gray matter.