Therefore, in this study, we try to evaluate the impact of clopidogrel loading dose 300mg vs. ticagrelor 180 mg on myocardial injury as measured by contrast-enhanced magnetic resonance imaging (CE-MRI) in STEMI patients undergoing primary PCI.

All STEMI patients will be loaded with dual oral antiplatelet therapy of 300 mg aspirin combined with either 300 mg clopidogrel or 180 mg ticagrelor before PCI, followed by maintenance dose of 100 mg aspirin plus 75 mg clopidogrel QD or 90 mg ticagrelor BID. The duty interventional cardiologists will be responsible for patient randomization based on a pre-defined randomization chart. As it is a usual practice in Taiwan for door-to-balloon time to be within 90 minutes, we hence do not expect any impact on study results due to D2B time variation.

All STEMI patients will receive loading dose of dual oral antiplatelet therapy with 300 mg aspirin combined with either 300 mg clopidogrel or 180 mg ticagrelor before PCI, followed by maintenance dose of 100 mg aspirin plus 75 mg clopidogrel QD or 90 mg ticagrelor BID.

The primary variable is to compare myocardial salvage index (in %) as assessed by CE-MRI between 300-mg clopidogrel group and ticargrelor group. Enzymatic infarct size will also be assessed, and two methods will be used to decide that. The first is cardiac troponinI 72 hours after pain onset , while the other is peak CK level after pain onset. Sampling timing of CK will be judged by the caring physicians.

The secondary objectives include: myocardial infarct size (% of LV mass), the extent of MVO, the number of segments with >75% of infarct transmurality.

Inclusion Criteria:

1. Provision of informed consent prior to any study specific procedures
2. Adult patients aged 20 years or older
3. STEMI within 12 hours of onset, defined as:presence of chest pain for >30 minutes and <12 hours after symptom onset; ST-segment elevation >1 mm in ≥2 contiguous leads or presumably a new-onset left bundle-branch block on electrocardiogram.
4. STEMI patients planned for PPCI

Exclusion Criteria:

1. STEMI > 12 h of onset
2. History of renal dysfunction requiring dialysis
3. Evidence of malignant diseases
4. Unwillingness to give out consent
5. Contraindicated for ticagrelor or clopidogrel