In vivo diffusion tensor imaging in acute and subacute phases of mild traumatic brain injury in rats.

Mild traumatic brain injury (mTBI) is the most common form of TBI with 10-25% of the patients experiencing long-lasting symptoms. The potential of diffusion tensor imaging (DTI) for evaluating microstructural damage after TBI is widely recognized, but the interpretation of DTI changes and their relationship with the underlying tissue damage is unclear. We studied how both axonal damage and gliosis contribute to DTI alterations after mTBI. We induced mTBI using the lateral fluid percussion injury model in adult male Sprague-Dawley rats, and scanned them at 3 and 28 days post-mTBI. To characterize the DTI findings in the tissue, we assessed the histology by performing structure tensor-based analysis and cell counting on myelin- and Nissl-stained sections, respectively. In particular, we studied the contribution of two tissue components, myelinated axons and cellularity, to the DTI changes. Fractional anisotropy (FA), mean diffusivity (MD), and axial diffusivity (AD) were decreased in both white and grey matter areas in the acute phase post-mTBI, mainly at the primary lesion site. In the subacute phase, FA and AD were decreased in the white matter, external capsule, corpus callosum, and internal capsule. Our quantitative histologic assessment revealed axonal damage and gliosis throughout the brain in both white and grey matter, consistent with the FA and AD changes. Our findings suggest that the usefulness of DTI is limited in its detection of secondary damage distal to the primary lesion, while at the lesion site, DTI detected progressive microstructural damage in the white and grey matter after mTBI. Mild traumatic brain injury (mTBI) is a major health problem worldwide with an unclear diagnosis. Using the lateral fluid percussion injury model in rats, we induced mTBI to assess the potential of diffusion tensor imaging (DTI) for non-invasively detecting progressive microstructural tissue damage. To interpret the changes observed in DTI, we performed extensive quantitative histologic assessment of the tissue microstructure. From the acute to subacute phases after mTBI, DTI detected progressive microstructural tissue alterations in the white and grey matter associated with axonal damage and gliosis. Although DTI failed to detect secondary tissue damage far from the primary lesion, these findings provide new insights for detecting mild tissue damage using DTI.