Participants with significant MCA stenosis（≥50%）on TCD, MRA, and/or CTA together with one or more risk factors of atherosclerosis will be prospectively recruited. Symtomatic patients will be included if they had suffered from a recent transient ischemic attack or ischemic stroke(NHISS≤6) in the territory of the stenotic MCA. Asymptomatic patients will be included if they had no history of cerebrovascular events or a history of an ischemic event in a territory outside that the affected MCA.