Viable and Fixed White Matter: DTI and Microstructural Comparisons at Physiological Temperature.

We compare fixed and viable isolated tissue (VIT) in identical conditions at physiological temperature. We acquired DTI data sets with a range of acquisition parameters and a rich multi-b-value diffusion weighted MR (DW-MR) dataset for microstructural tissue model fitting. DTI data demonstrated a significant increase in radial diffusivity (RD) in fixed samples in comparison to VIT. Model fitting demonstrated that similar models best explain data from both samples and differences in parameter estimates reflect DTI measured RD and MD differences. The stability of the model ranking allows us to conclude that fixed tissue is a reasonable model for in-vivo, although significant differences in the fitted model parameters (and DTI measured FA and RD) suggest that water in individual compartments within the tissue behaves quite differently.