Diffusion MRI in the cortex of the brain: Reducing partial volume effects from CSF and white matter in the mean diffusivity using high b‐values and spherical b‐tensor encoding

Purpose The mean diffusivity (MD) is sensitive to the microstructure of the cortex. However, partial volume effects with CSF and white matter (WM) may obscure pathology‐related alterations. This work investigates both existing approaches and a novel approach for reducing partial volume effects. Theory and Methods A bias in MD arises due to partial volume effects, higher‐order terms, and the noise floor in magnitude data. We propose to reduce this bias by using high b ‐value encoding to limit partial volume effects with CSF, spherical b‐tensor encoding to reduce the influence of higher‐order terms, and super‐resolution acquisition and reconstruction to suppress the noise floor. This approach was investigated, along with established approaches (inversion recovery and free water elimination) for reducing partial volume effects, using simulations and in vivo data. Results High b ‐value diffusion MRI with spherical b‐tensor encoding reduced partial volume effects with CSF relative to conventional diffusion MRI. Maximum errors decreased from 0.51 to 0.01 μm 2 /ms in simulations. In vivo, the median absolute deviation of cortical MD decreased from 0.17 to 0.06 μm 2 /ms, whereas the median decreased slightly from 0.77 to 0.73 μm 2 /ms. The other methods yielded bias from either CSF, WM, or model assumptions. Conclusion The mean diffusivity of the cortex can be mapped in high precision with reduced influence of partial volume effects with CSF and WM matter using high b ‐values and spherical b‐tensor encoding and super‐resolution reconstruction.