Viable and fixed white matter: diffusion magnetic resonance comparisons and contrasts at physiological temperature

Fixed samples have been used extensively in diffusion MRI (dMRI) studies. However, fixation causes significant structural changes in tissue. The purpose of this study was to evaluate fixed white matter as a surrogate for viable white matter during development and validation of dMRI methods.

dMRI data was acquired from fixed and viable rat optic nerves maintained in identical conditions in a viable isolated tissue (VIT) chamber. The chamber preserves tissue integrity for 10 h at 37°C. Diffusion tensors (DT) and multi-compartment white matter signal models were fitted to the data.

When comparing VIT and fixed tissue, DT parameters demonstrated that fixation causes significant reductions in axial diffusivity and increases in radial diffusivity. However, both tissues exhibited similar responses to changes in diffusion times and gradient strengths. Multicompartment models demonstrated differences in parameter estimates (e.g., directional diffusivities) that were analogous to differences in DT parameters. Similarities in multi-compartment model rankings suggested that tissue water populations were broadly maintained postfixation.

The data demonstrate that fixed tissue, while maintaining the broad water environment of viable tissue, differs significantly in diffusion parameters. Results from dMRI experiments on fixed tissue may correlate with-but will not directly translate into-results from viable tissue.