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

By Jose M. Guerrero, Nagesh Adluru, Barbara B. Bendlin, H. Hill Goldsmith, Stacey M. Schaefer, Richard J. Davidson, Steven R. Kecskemeti, Hui Zhang, Andrew L. Alexander

Posted 08 May 2019
bioRxiv DOI: 10.1101/630541

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 should consider the optimal values of d‖ found in this report or conduct their own optimization, particularly when investigating young populations and/or gray matter.

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