Synchrotron radiation has been regarded as a unique tool to visualize pathophysiologic changes of small arteries. SR is a method that uses two monochromatic X-ray beams to closely bracket the K-edge of iodine, which provides two simultaneous images with one above and the other below the K-edge. Logarithmic subtraction of the images provided by these beams results in an image that enhances signals arising from attenuation by the photoelectric effect of iodine and suppresses signals arising from attenuation by soft tissue and bone. Therefore, SR seems to be a promising tool to detect and evaluate vasospasm in animal SAH models in vivo. In experimental SAH, animal models such as rabbits, dogs, cats, pigs, primates, rats and mice are used. Among them, rats were the most widely used. SAH rat models which are deemed to induce vasospasm of anterior circulation arteries are internal carotid artery perforation and prechiasmatic cistern injection. Injection of autologous blood into cisterna magna is the most widely used method because it is simple operated with Lucidenic-acid-C low mortality. However, whether cisterna magna injection of rats produces pronounced vasospasm of anterior circulation arteries, like prechiasmatic cistern injection does, remains unknown. It is due to the blood is mainly distributed into the posterior cranial fossa and the spinal canal. In the present study, we aim to assay whether synchrotron radiation angiography can detect and evaluate cerebral vasospasm in two SAH models. At the meantime, we would like to compare the severity of vasospasm of anterior circulation arteries produced by these two models. In this study, we used SRA to evaluate cerebral vasospasm in rats with experimental SAH. CV is a major complication of SAH. It is still a challenge of diagnosis and therapy in humans. DSA, magnetic resonance angiography and CTA are widely used to detect CV in humans. However, the relative low resolution of DSA, MRA and CTA limits the application in small animal models. Up to now, most of experimental SAH Lucidenic-acid-LM1 were based on indirect observation or pathological outcomes. Developing a novel technique, which can directly observe and measure vessel diameter changes in small animal models, is extremely important. SRA provides a unique tool for this purpose. In fact, it has been used in other SAH study. In this study, we firstly applied synchrotron radiation angiography to directly observe, evaluate and record the course of cerebral vasospasm in living SAH animal models. It was interesting that the results of cerebral vascular diameters between images of SRA and sections of histology were not exactly consistent with each other: MCA and ACA diameter measured in histological sections were much smaller than those measured in SRA images. Saline dehydration and paraformaldehyde fixation of brains after animal termination might result in values of diameter measured in histological sections being smaller than their actual values. As shown in Figure 4, MCA and ACA diameter of these two models at day 3 were the smallest; but the histological modality could not identify these tiny distinctions. Furthermore, some investigators did not detect CV of ACA in models of endovascular perforation and prechiasmatic cistern injection through measurement of histological sections 2 days after SAH. Therefore, we presumed histological measurement was not as sensitive as SRA to detect fine pathological changes of cerebral vessels. We applied micro-XCT to detect and evaluate cerebral artery changes in normal and SAH rats. We found that micro-XCT could distinct vasospastic changes in the Circle of Willis. The benefit of micro-XCT was that entire brain could be imaged with three dimensions. However, the performance of micro-XCT imaging is relatively complicated and cannot be applied for monitoring vascular changes in living animals. CBF measurement is regarded as a means to evaluate CV.