Test-retest reproducibility of in vivo oscillating gradient and microscopic anisotropy diffusion MRI in mice at 9.4 Tesla

Background and Purpose: Microstructure imaging with advanced diffusion MRI (dMRI) techniques have shown increased sensitivity and specificity to microstructural changes in various disease and injury models. Oscillating gradient spin echo (OGSE) dMRI, implemented by varying the oscillating gradient frequency, and microscopic anisotropy ({micro}A) dMRI, implemented via tensor valued diffusion encoding, may provide additional insight by increasing sensitivity to smaller spatial scales and disentangling fiber orientation dispersion from true microstructural changes, respectively. The aims of this study were to characterize the test-retest reproducibility of in vivo OGSE and {micro}A dMRI metrics in the mouse brain at 9.4 Tesla and provide estimates of required sample sizes for future investigations. Methods: Eight adult C57Bl/6 mice were scanned twice (5 days apart). Each imaging session consisted of multifrequency OGSE and {micro}A dMRI protocols. Metrics investigated included {micro}A, isotropic and anisotropic kurtosis, and the diffusion dispersion rate ({Lambda}), which explores the power-law frequency dependence of mean diffusivity. The dMRI metric maps were analyzed with mean region-of-interest (ROI) and whole brain voxel-wise analysis. Bland-Altman plots and coefficients of variation (CV) were used to assess the reproducibility of OGSE and {micro}A metrics. Furthermore, we estimated sample sizes required to detect a variety of effect sizes. Results: Bland-Altman plots showed negligible biases between test and retest sessions. ROI-based CVs revealed high reproducibility for both {micro}A (CVs < 8 %) and {Lambda} (CVs < 15 %). Voxel-wise CV maps revealed high reproducibility for {micro}A (CVs ~ 10 %), but low reproducibility for OGSE metrics (CVs ~ 50 %). Conclusion: Most of the {micro}A dMRI metrics are reproducible in both ROI-based and voxel-wise analysis, while the OGSE dMRI metrics are only reproducible in ROI-based analysis. {micro}A and {Lambda} may provide sensitivity to subtle microstructural changes (4 - 8 %) with feasible sample sizes (10 - 15).