Tensor-valued and frequency-dependent diffusion MRI and magnetization transfer saturation MRI evolution during adult mouse brain maturation

Although rodent models are a predominant study model in neuroscience research, research investigating healthy rodent brain maturation remains limited. This motivates further study of normal brain maturation in rodents to exclude confounds of developmental changes from interpretations of disease mechanisms. 11 C57Bl/6 mice (6 males) were scanned longitudinally at 3, 4, 5, and 8 months of age using frequency-dependent and tensor-valued diffusion MRI (dMRI), and Magnetization Transfer saturation (MTsat) MRI. Total kurtosis showed significant increases over time in all regions, which was driven by increases in isotropic kurtosis while anisotropic kurtosis remained stable. Increases in total and isotropic kurtosis with age were matched with increases in MTsat. Quadratic fits revealed that most metrics show a maximum/minimum around 5-6 months of age. Most dMRI metrics revealed significantly different trajectories between males and females, while the MT metrics did not. Linear fits between kurtosis and MT metrics highlighted that changes in total kurtosis found throughout normal brain development are driven by isotropic kurtosis, while differences in total kurtosis between brain regions are driven by anisotropic kurtosis. Overall, the trends observed in conventional dMRI and MT metrics are comparable to previous studies on normal brain development, while the trajectories of our more advanced dMRI metrics provide novel insight. Based on the developmental trajectories of tensor-valued dMRI and MT metrics, our results suggest myelination during brain maturation is not a main contributor to microscopic diffusion anisotropy and anisotropic kurtosis in axons. For studies that only calculate total kurtosis, we suggest caution in attributing neurobiological changes to changes in total kurtosis as we show here constant anisotropic kurtosis in the presence of increasing myelin content.