[The measurement of fractional anisotropy values at the corpus callosum in an irradiated-rat model by 7T-MRI: comparison with quantitative histological evaluation].

Radiation-induced white matter injury has recently been evaluated by fractional anisotropy (FA) values using diffusion tensor image (DTI) data on magnetic resonance (MR) imaging. However, controversy remains as to which histological components affect anisotropy. FA values were compared with histological findings in rat irradiated brains. Forty Gy whole brain radiation was performed on 9 rats, with another 9 rats not subjected to irradiation prepared as the control group. DTI data for the corpus callosum were obtained using a 7.0 Tesla MR scanner at post radiation 18+2 to 48 +/- 2 weeks (p18w-p48w), and the mean FA value (mFA) was calculated. Histological parameters were assessed at p24w, p36w, and p48w in hematoxylin-eosin stained and immunohistochemically processed sections. Numbers of dilated vessels, hypertrophic glial fibrillary acidic protein (GFAP)-positive cells and Olig2-positive cells were quantitatively evaluated. Qualitative assessment of change in neurofilament (NF)-and myelin basic protein (MBP)-positive structures at each time point was also performed. The mFA decreased from p30w to p42w, significantly decreasing at p42w compared to the control group. However, It recovered to control levels at p48w. Histological examination showed that hypertrophic GFAP-positive cells and dilated vessels had increased at p48w. Olig2-positive cells decreased significantly at p48w compared to p24w and p36w (p<0.05). Although NF-positive structures continued to decrease over time, MBP-positive structures recovered at p48w in agreement with the change in mFA. In the subacute/chronic stage, rat irradiated brain FA value in the corpus callosum appears to mainly reflect the change in myelin structure.