The relationships between extent and microstructural properties of the midsagittal corpus callosum in human brain

Recent quantitative analyses of the corpus callosum (CC) have tried to assess the interhemispheric connectivity. Based on histological results showing an expansion of callosal extent at the midsagittal plane, without fiber density alterations, callosal extent was interpreted as an index of interhemispheric connectivity. The microstructural properties of the CC have also been investigated extensively using diffusion tensor imaging, to assess interhemispheric connectivity. The relationships between axonal density and callosal extent need to be investigated to understand how these parameters reflect interhemispheric connectivity. We used a semi-automated CC segmentation scheme in T1-weighted magnetic resonance image and fractional anisotropy (FA) image, respectively. The parameterization method of the segmented CC was applied to 47 right-handed healthy adult subjects. The callosal extent and microstructural properties were measured using the callosal thickness and diffusion indices (FA, mean diffusivity, and axial and radial diffusivity), respectively. Our results revealed a correlation between callosal thickness and FA on the posterior body and isthmus of the CC, which suggests that these regions are more sensitive to fiber alterations than other regions. Based on this result, we suggest that both the extent of the CC and its microstructural properties should be considered together in the estimation of interhemispheric connectivity in healthy adult populations.