Inclusion Criteria:  Patients aged ≥ 40 years ≥ 50% stenosis in unilateral intracranial / carotid artery Hypoperfusion in the territory of the culprit artery as determined on imaging Written informed consent available  Exclusion Criteria:  Previous history of major head trauma and any intracranial surgery Intracranial abnormalities, such as intracerebral hemorrhage, subarachnoid hemorrhage, cerebral infarction (except lacunar lesions) and other space occupying lesions Extrapyramidal symptoms or mental illness which may affect neuropsychological measurement severe loss of vision, hearing, or communicative ability plan to be treated with surgery