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Background and purpose: Standard superficial temporal artery-middle cerebral artery (STA-MCA) bypass surgery is an effective treatment for ischemic cerebrovascular diseases, including moyamoya disease and occlusive cerebrovascular disease. Our purpose in this study was to evaluate the ischemic status based on the imaging modality of computed tomographic perfusion (CTP) before and after STA-MCA bypass in patients with moyamoya disease and occlusive cerebrovascular disease. Methods: CTP was performed on 19 patients, comprising 10 patients with moyamoya disease and nine patients with occlusive cerebrovascular disease, preoperatively and on the third postoperative day. According to the regional cerebral microcirculatory change and modality of CTP, the regional cerebral ischemic status was graded into two stages with four substages (I1, I2, II1, and II2) to indicate the regional cerebral perfusion and ischemic status of the patients. The ischemic status was analyzed in all 19 patients according to those stages preoperatively and postoperatively. Results: Among the 19 patients, nine (47.4%) showed improvement in the regional cerebral ischemic status, six (31.6%) showed no change, and four (21.1%) showed deterioration. Both improvement and no change in the regional cerebral ischemic status were regarded as effective, while deterioration was regarded as ineffective, meaning that 15 (78.9%) patients were effective and four (21.1%) patients were ineffective. The effective rate for moyamoya disease (one of ten, 90.0%) was significantly higher than that for occlusive cerebrovascular disease (three of nine, 66.7%). Postoperatively, only one patient (case N11) exhibited deterioration of symptoms, which presented as right hemiplegia and aphasia, and no obvious changes in symptoms were observed for the other patients in the transient period. Conclusions: This study has shown that STA-MCA bypass is an effective and safe way to improve the regional cerebral perfusion and ischemic status in ischemic cerebrovascular diseases. The stage based on the regional cerebral microcirculatory change and presentation of CTP can directly reflect the pathological mechanism underlying the regional cerebral ischemic status. [ABSTRACT FROM AUTHOR]