In vivo DTI evaluation of white matter tracts in rat spinal cord

PURPOSE: To determine whether differences in specific spinal cord white matter (WM) tracts can be detected with in vivo DTI. MATERIALS AND METHODS: In vivo DTI was performed on six rats at the lower thoracic region using a 4.7T magnet. Axial diffusion images were obtained with diffusion gradients applied in six independent directions, with low and high b-values equal to 0 and 692 seconds/mm(2), respectively. Regions of interest (ROIs) were selected corresponding to the major spinal cord tracts, including the dorsal cortical spinal tract (dCST), fasciculus gracilis (FG), rubrospinal tract (RST), vestibulospinal tract (VST), and reticulospinal tract (ReST). RESULTS: ANOVA demonstrated overall differences between tracts for all of the DTI parameters, including fractional anisotropy (FA), trace diffusion (Tr), longitudinal diffusivity (EL = lambda(1)), and transverse diffusivity (ET = (lambda(2) + lambda(3))/2). Similarly to previous ex vivo analyses, the spinal cord tract with the largest and most widely spaced axons (VST) had the largest EL and ET. CONCLUSION: The principal diffusivities appear to reflect axon morphologic differences between the WM tracts that are not as well appreciated with FA and Tr.