Correlation of Diffusion Tensor Imaging Indices with Histological Parameters in Rat Cervical Spinal Cord Gray Matter Following Distal Contusion Spinal Cord Injury

The purpose of this study was to delineate the diffusion tensor imaging (DTI) parameters across the cervical spinal cord gray matter (GM) in a distal (T8) rat contusion spinal cord injury (SCI) model. DTI data were obtained from ex vivo rat spinal cords and registered to corresponding histological slices in samples from the acute through chronic stages of SCI including uninjured control, 2 weeks post injury, 15 weeks post injury and 25 weeks post injury groups (n = 5 in all groups). After imaging, samples were dehydrated, blocked in paraffin, sliced axially and stained with eriochrome cyanine R stain and H&E counter-stain. A corresponding sample was post fixed with osmium tetroxide and stained with toluidine blue. Histology images of the eriochrome cyanine R stained and H&E counter-stained slices were captured at 4x and then segmented into white matter (WM) and GM and dorsal and ventral GM using a custom cluster analysis. Using whole cord templates, DTI images for each animal were then registered to the corresponding histology images. The WM and the GM regions of interest (ROI) histological templates were then used to map DTI indices, including fractional anisotropy (FA), longitudinal apparent diffusion coefficient (lADC) and transverse apparent diffusion coefficient (tADC) across the GM. The average values for each index were also calculated in predefined gray matter ROIs. Histology images of the above mentioned ROIs were captured at 40x resolution using the toluidine blue stained slices for the control and post injury groups (n=4). Motoneuron size in the ventral GM was calculated for each of the control and post injury groups. It was observed that the FA and lADC values in the dorsal GM ROI were significantly higher than that in the ventral GM ROI in controls, fifteen weeks post injury and twenty five weeks post injury groups (P \u3c 0.05). The overall GM FA value at twenty five weeks was significantly higher than the FA value at two weeks post injury (P \u3c 0.05) and the FA value in controls (P \u3c 0.05). Group analysis of the size of the motor neurons showed a 9% increase in the motoneuron size at two weeks (P \u3c 0.01) and 42% increase at twenty five weeks (P \u3c 0.01) post injury as compared to controls. The motor neurons also showed a significant increase in size at twenty five weeks post injury (P \u3c 0.01) as compared to the motor neuron size at two weeks post injury. These results indicate changes in gray matter structure rostral to a contusion injury that can be detected and monitored using DTI