Methodology: As a multicenter, prospective, randomized, controlled trial, patients with ST-segment elevation myocardial infarction (STEMI) who have an intermediate stenotic culprit lesion (angiographic diameter stenosis ≤70%) and TIMI flow grade 3 or residual stenosis ≤70% after thrombus aspiration and TIMI grade 3 will be enrolled and randomized to OCT-guided group or angiography-guided group. In OCT-guided group, pre-PCI OCT imaging of the culprit lesion will be performed and revascularization strategy will be determined based on underlying mechanism of culprit lesion morphology. For lesions required stenting, OCT measurements will be used to guide the selection of stent size and to optimize the result of stent implantation. In the angiography-guided group, OCT will not be performed and the PCI procedures will be done according to routine practice by the operators. Primary outcome is patient-level rate of stenting during primary PCI and the incidence of recurrent myocardial ischemic events (unstable angina-induced rehospitalization, recurrent myocardial infarction, target lesion revascularization) or cardiac death within 1 month. Secondary outcomes include the incidence of cardiocerebrovascular events (cardiac death, recurrent myocardial infarction, stroke, target lesion revascularization, malignant arrhythmia, and unstable angina-induced rehospitalization) and the incidence of heart failure event within 1 year.

Patient enrollment and procedure overview: Patients with STEMI\<12h in whom coronary angiography is planned will be screened. Those have a culprit lesion stenosis ≤70% by visual estimation and TIMI blood flow grade 3 at index angiography or after thrombus aspiration will be consented to participate in the study. After informed consent obtained, patients will be assigned to OCT-guided group or angiography-guided group randomly. In the OCT-guided group, OCT imaging of infarcted-related artery will be performed to assess the underlying mechanism of culprit lesion. Reperfusion strategy will be decided by the operators according to the OCT findings. Conservative medical strategy will be recommended if the underlying mechanism is plaque erosions, small ruptures without dissection and hematoma, SCAD without obstructive stenosis. Otherwise, stent implantation may be performed. The procedure of stent implantation will be guided by OCT according to current guideline and consensus documents. Before stent implantation, the landing zone and size of stent will be decided according to the pre-PCI OCT images. After implantation, OCT imaging will be performed to optimize the results of stent implantation including stent expansion, apposition, edge dissection, tissue protrusion. In the angiography-guided group, PCI procedure will be performed according to current guidelines and their daily practice. All patients are required to take dual antiplatelet treatment with aspirin (100 mg/day) and ticagrelor (180 mg/day) or clopidogrel (75mg/day) for at least 12 months. The use of low molecular weight heparin and glycoprotein IIb/IIIa inhibitor are determined by the operators.

Follow-up: Patients will be followed by phone calls or clinical visits by study coordinators at 1 month (30 days) and 1 year (12 months). Severe adverse events including heart failure event, stent failure, cardiac death, recurrent myocardial infarction, stroke, target lesion revascularization, malignant arrhythmia, and unstable angina-induced rehospitalization will be collected in all patients throughout the whole study period until the last patient completes 12 months of follow up. Additional phone follow-up may also be performed subject to executive committee approval.