Assessment of Distal Protection Device in Patients at High Risk for Distal Embolism in Acute Coronary Syndrome (ACS)

Inclusion Criteria: Patients who were diagnosed as ACS and meet the following criteria. * Patients with acute myocardial infarction or unstable angina within 2 months from onset. * Patients who have severe coronary stenosis which require PCI and in whom attenuated plaque ≥ 5mm (longitudinally) by IVUS before PCI is observed prior to balloon dilatation and stent deployment. Exclusion Criteria: * Patients who need balloon expansion (including small diameter balloon) for IVUS catheter crossing. * Patients who were resuscitated after dead on arrival * Graft lesion or in-stent restenosis lesion * Patients on dialysis or renal failure (Cr\>1.5mg/dl). * Left main trunk lesion * Target vessel size is \<2.5mm or \>5mm * Ineligible for PCI * Lactating and (possibly) pregnant woman or having possibility of pregnant * Patients who are considered ineligible by the attending physician