Analysis of corpus callosum diffusion tensor imaging parameters in infants

Correlation of diffusion tensor imaging parameters in the canine brain

The goal of this study was to determine the degree to which ex vivo diffusion tensor imaging (DTI) parameters correlate to one another in white matter regions on very high resolution MR scans. Specifically, we hypothesized that radial diffusivity (RD) and apparent diffusion coefficient (ADC) would correlate more closely than either would correlate with fractional anisotropy (FA). We performed post mortem DTI imaging on three canine brains on a 7 T MR scanner (TR = 100 ms, NEX = 1, gradient amplitude = 600 mT/m, b = 1492-1,565 s/mm²) and generated maps of FA, RD, and ADC. We measured RD, FA and ADC within 14 regions of interest representative of various portions of white matter. We compared the three combinations of values, i.e., FA vs ADC, FA vs RD and ADC vs RD, using linear regression models. Linear regression demonstrated that RD was significantly correlated with FA (p << 0.01; R² = 0.3053) and also with ADC (p << 0.01; R² = 0.6755), but to a much greater degree. However, ADC was not significantly correlated with FA (p = 0.526; R² = 0.101). Our findings suggest that both RD and ADC reflect similar cytoarchitectural features, most likely that of myelination, whereas FA values likely reflect both myelination and additional microstructural features that constrain the diffusion of water in white matter.

Diffusion tensor imaging of neural tissue organization: correlations between radiologic and histologic parameters

This study examined the relationship between histological variables and diffusion tensor imaging (DTI) values in a normal canine brain. We hypothesized that radial diffusivity (RD) would correlate with myelin density and fractional anisotropy (FA) would correlate with white matter fiber coherence. We acquired DTI maps of a normal canine brain post mortem on a 7T MR scanner (TR = 100 ms, TE = 18.1 ms, NEX = 1, width [d] = 4 ms, separation [D] = 8.9 ms, gradient amplitude = 600 mT/m, b=1,565 s/mm(2)) and generated maps of FA, RD, and axial diffusivity. The brain was subsequently sectioned and stained for myelin with gold chloride, which also allowed for measurement of fiber coherence. DTI metrics were then directly compared with the optical density of the myelin stain and the coherence of stained fibers. Multivariate linear regression demonstrated that RD, but not FA, significantly correlated with both myelin stain intensity (p = 0.031) and fiber coherence (p = 0.035). The Pearson correlation coefficient between these two histological variables and FA was 0.122; and was 0.607 for the histological variables and RD. We found that RD significantly correlated with both optical density of myelinated fibers and fiber coherence, but FA correlated with neither histological finding. Factors other than degree of myelination and fiber coherence should be considered to fully account for regional variation in FA.