The goal of this observational study is to explore the ability of intravascular pressure gradients to identify hemodynamic disturbance in patients with intracranial atherosclerotic diseases (ICAS). The main questions that aim to answer are: The correlation between intravascular pressure gradient and cerebral blood flow (CBF) The threshold for intravascular pressure gradients to predict hemodynamic disturbance in ICAS Patients will undergo intravascular pressure measurement and arterial spin labeling (ASL) for CBF during pre- and post-operation respectively.