Thinning of fibrous cap in atherosclerotic plaques is associated with plaque vulnerability. The high resolution of optical coherence tomography (OCT) provides an accurate measurement of fibrous cap thickness. Endothelial dysfunction is a key component of vulnerable plaque and digital reactive hyperemia-peripheral arterial tonometry (RH-PAT) is a non-invasive automatic and quantitative method to evaluate endothelial function. The investigators will investigate the association between endothelial function assessed by RH-PAT and plaque vulnerability determined by OCT-derived thin-cap fibroatheroma (TCFA).