Maturational changes in diffusion anisotropy in the rat corpus callosum: comparison with quantitative histological evaluation.

PURPOSE To determine the main histological components that affect fractional anisotropy (FA) in postnatal development of the rat corpus callosum and compare FA values with histological changes evaluated quantitatively. MATERIALS AND METHODS Diffusion tensor image (DTI) data of the rat (postnatal 1-10 weeks) corpus callosum were obtained with a 7.0 T MR scanner. Histological parameters were quantitatively assessed in toluidine blue-stained semithin sections. Simple and multiple linear regression analyses were performed to investigate relationships between FA values and histological variables. RESULTS The mean FA value (mFA) increased significantly in the early growth stages, whereas the change became smaller after postnatal week 4. Simple regression analysis showed a high correlation between the area of myelin sheath and mFA (r = 0.856; P < 0.01). The area of extracellular space correlated negatively with mFA (r = -0.813; P < 0.01). In a forward stepwise analysis, the area of myelin sheath had the strongest influence on mFA (P < 0.001), followed by the number of unmyelinated axons (P = 0.113). Multiple linear regression analysis revealed that both parameters predicted mFA with a highly significant adjusted correlation coefficient (r(2) adj. = 0.738, P < 0.001). CONCLUSION During the early development stage in the rat corpus callosum, the strongest contribution to FA value is the area of myelin sheath.