The prevalence of SBI varies from 5% to 62% in healthy population. To date, few studies investigate the association between SBI and ethnicity. The effectiveness of antithrombotics including aspirin against future symptomatic stroke in SBI patients remains to be established. Due to the high prevalence of ICAS among Chinese, and its nature of artery-to-artery microembolisms, investigators hypothesize that the prevalence of SBI among Chinese might be significantly higher than other races such as Caucasians and African-Americans.