Disentangling microscopic anisotropy and diffusion time dependence in the brain using oscillating gradients with spherical b-tensor encoding

Motivation: Tensor-valued diffusion MRI allows estimation of microscopic fractional anisotropy (μFA) independent of orientation dispersion. However, diffusion time/frequency-dependence of μFA in the brain is not well understood. Goal(s): To investigate the frequency dependence of diffusional variances and μFA in the rat brain using oscillating-gradient spherical (STE) and linear (LTE) b-tensor encoding waveforms. Approach: Oscillating-gradient STE and LTE waveforms were implemented at frequencies of 65, 117, and 169 Hz. Tensor-valued dMRI data were acquired from fixed rat brains using the spectrally-matched STE and LTE gradients. Results: Spectrally-modulated STE and LTE gradients revealed significant time/frequency-dependence of isotropic diffusivity, diffusional variances, and μFA in the rat brain. Impact: We demonstrate that oscillating spherical and linear b-tensor encoding waveforms allow mapping of frequency-dependent isotropic diffusivity, diffusional variances, and μFA in the brain. The results are important to understand the effects of spectral content of the waveforms in tensor-valued dMRI.