Late Effects of X Irradiation on Regulation of Cerebral Blood Flow after Whole-Brain Exposure in Rats

Hemodynamic parameters such as total cerebral blood volume (total CBV), cerebral parenchymal blood volume (CBV), cerebral blood flow (CBF) and cerebral blood velocity index were measured in rats 6, 12 and 18 months after single exposures of brain to 5, 10, 15 and 20 Gy X rays for total CBV, CBF and blood velocity index, and only 20 Gy for CBV. Total CBV and blood velocity index were determined by a noninvasive blood dilution method using $[{}^{99{\rm m}}{\rm Tc}]\text{pertechnetate}$ and CBF by $[{}^{131}{\rm I}]\text{iodoantipyrine}$ brain extraction. The CBV was obtained from both parenchymal plasma and erythrocyte volumes measured in isolated brain by 125 I-labeled serum albumin and 51 Cr-labeled erythrocytes, respectively. Neither the dose nor the time after irradiation influenced total CBV. Nevertheless, CBV decreased slightly while CBF decreased strongly at 12 and 18 months after 20 Gy. In contrast, the blood velocity index increased progressively at 12 and 18 months after 15 Gy and at all times after 20 Gy. According to the coexistence in irradiated brains of a remodeling with microvascular occlusions and dilated abnormal vessels, this lowered CBF can be explained by the smaller number of open capillaries and a "steal phenomenon" through low-resistance channels developed in the parenchymal and extra-parenchymal vasculatures. Such a "steal phenomenon" is also supported by the response of the blood velocity index, which appears to be the earliest sensitive index for the detection of hemodynamic changes with respect to time (6 months) and dose of radiation (15 Gy).