Computed tomography perfusion (CTP) can be used to evaluate vasoreactivity and cerebrovascular reserve in patients with AICAS and predict future stroke. The parameters of CTP include ①cerebral blood flow(CBF),② cerebral blood volume(CBV) and ③mean transit time(MTT). Relative regional cerebral blood flow (rCBF) was evaluated as the percentage radioisotope counts in the region of interest (ROI) of the affected side against the corresponding ROI in the unaffected contralateral side.