The goal of acute coronary syndrome (ACS) therapy is to successfully restore both epicardial blood flow and myocardial perfusion. Percutaneous coronary intervention (PCI) has been documented as being the most effective method for restoration of epicardial blood flow. However, epicardial blood flow does not necessarily equate to myocardial perfusion. Clopidogrel binds irreversibly to platelet P 2 Y 12 receptors to inhibit platelet aggregation, with main limitations of slow onset, prevention of recovery of platelet functions, and interindividual variability. Clinical pharmacology and early dose-finding studies suggested a faster onset and greater and more consistent inhibition of platelet aggregation (IPA) with ticagrelor compared with clopidogrel. Two currently main methods of angiographic assessment of myocardial perfusion includes thrombolysis in myocardial infarction(TIMI) myocardial perfusion grading (TMPG) and myocardial blush grading (MBG). These established myocardial perfusion parameters have been widely used in various important trials and are reported to be highly useful in predicting clinical outcomes. However, visual assessment of these methods is categorical, subjective, and operator dependent of contrast in the myocardium using cine-angiographic frame-counting, was developed by the investigators' center to quantify myocardial tissue- level perfusion and was proved to be a predictive value on clinical prognosis.

Thus, the investigators aim to initiate an open-label study evaluating the acute efficacy of treatment with ticagrelor versus clopidogrel on myocardial tissue-level perfusion assessed by Myocardial Perfusion Frame Count(TMPFC) and magnetic resonance imaging (MRI) in patients with high-risk non-ST elevation acute coronary syndrome (NSTE-ACS) undergoing early percutaneous coronary intervention (PCI) .

The investigators hypothesize that compared with clopidogrel, ticagrelor can significantly improve myocardial perfusion assessed by Myocardial Perfusion Frame Count(TMPFC) in high-risk non-ST elevation acute coronary syndrome (NSTE-ACS) patients undergoing early percutaneous coronary intervention (PCI), without additional increased major bleeding.

The goal of acute coronary syndrome (ACS) therapy is to successfully restore both epicardial blood flow and myocardial perfusion. Percutaneous coronary intervention (PCI) has been documented as being the most effective method for restoration of epicardial blood flow. However, epicardial blood flow does not necessarily equate to myocardial perfusion; not every patient with thrombolysis in myocardial infarction(TIMI) 3 flow after successful percutaneous coronary intervention (PCI) achieves effective myocardial tissue-level perfusion. Although epicardial thrombolysis in myocardial infarction(TIMI) 3 flow could be restored in >90% of acute coronary syndrome (ACS) patients undergoing percutaneous coronary intervention (PCI) , normalization of myocardial perfusion was achieved less frequently, with detrimental impacts on survival.

Clopidogrel, the most widely used antiplatelet agent in acute coronary syndrome (ACS), is a thienopyridine prodrug, which is inactive until in undergoes biotransformation into its active metabolite, which then binds irreversibly to platelet P 2 Y 12 receptors. This irreversible binding means that the receptors are inhibited for the lifespan of the platelet. The main limitations of clopidogrel administration include slow onset, prevention of recovery of platelet functions, and interindividual variability.

Clinical pharmacology and early dose-finding studies suggested a faster onset and greater and more consistent inhibition of platelet aggregation (IPA) with ticagrelor compared with clopidogrel. ONSET/OFFSET study showed that in patients with stable coronary artery disease on aspirin, ticagrelor demonstrates a rapid onset of pharmacological effect, as demonstrated by a mean platelet aggregation (IPA) for ticagrelor at 0.5 h after 180 mg loading dose of about 41%, with the maximum platelet aggregation (IPA) effect of 87.9-89.6% by 2-4 hours post dose. A total of 90% of patients had final extent Inhibition of platelet aggregation (IPA) >70% by 2 h post-dose. The high inhibition of platelet aggregation (IPA) effect of ticagrelor (between 87% and 89%) was maintained for 2-8 hours. Ticagrelor might overcome the slow-onset limitation of clopidogrel and bring extra benefit for improving myocardial perfusion in the acute phase of acute coronary syndrome (ACS) when undergoing early percutaneous coronary intervention (PCI).

Currently, there are two main methods of angiographic assessment of myocardial perfusion: Thrombolysis in myocardial infarction (TIMI) myocardial perfusion grading (TMPG), described by Gibson et al. and myocardial blush grading (MBG), described by Van't Hof et al. These established myocardial perfusion parameters, myocardial perfusion grading (TMPG), and myocardial blush grading (MBG), have been widely used in various important trials and are reported to be highly useful in predicting clinical outcomes. However, visual assessment of these methods is categorical, subjective, and operator dependent. Thrombolysis in myocardial infarction(TIMI) Myocardial Perfusion Frame Count (TMPFC), a novel and objective method that measures the filling and clearance of contrast in the myocardium using cine-angiographic frame-counting, was developed by the investigators' center to quantify myocardial tissue- level perfusion and was proved to be a predictive value on clinical prognosis.

Thus, the investigators aim to initiate an open-label study evaluating the acute efficacy of treatment with ticagrelor versus clopidogrel on myocardial tissue-level perfusion assessed by Myocardial Perfusion Frame Count (TMPFC) and magnetic resonance imaging (MRI) in patients with high-risk non-ST elevation acute coronary syndrome (NSTE-ACS) undergoing early percutaneous coronary intervention (PCI).

Inclusion Criteria:

For inclusion in the study subjects should fulfill the following criteria:

1. Provision of informed consent prior to any study specific procedures;
2. Men or women > 18 years of age, with documented evidence of non-ST segment elevation Acute Coronary Syndrome(ACS) in the 24 hours before randomisation;
3. Hospitalized for high-risk non-ST segment elevation Acute Coronary Syndrome(ACS)(GRACE risk score>140) with indication for early percutaneous coronary intervention (PCI) according to 2012 Chinese non-S T segment elevation Acute Coronary Syndrome(ACS) guideline recommendation.

Exclusion Criteria:

Subjects should not enter the study if any of the following exclusion criteria are fulfilled:

1. Evidence of cardiac rupture;
2. History of major hemorrhage (intracranial, gastrointestinal, etc.);
3. Active pathological bleeding;
4. Acute or chronic hematologic disorder including a Hemoglobin less than 10 g/L or a platelet count less than 10×109/L before procedure;
5. Contraindication against the use of clopidogrel and ticagrelor;
6. Subject has any condition for which, in the opinion of the investigator, participation would not be in the best interest of the subject (eg, compromise the well-being) or that could prevent, limit, or confound the protocol-specified assessments.
7. Severe complication 7.1 Other diseases with life expectancy ≤12 months; 7.2 Any history of Severe renal or hepatic dysfunction(hepatic failure, cirrhosis, portal hypertension and active hepatitis); Neutropenia, thrombocytopenia ; Known acute pancreatitis; 7.3 Arterial aneurysm, arterial/venous malformation and aorta dissection;
8. Complex heart condition 8.1 PCI within previous 1 month or Previous coronary-artery bypass surgery(CABG); 8.2 History of myocardial infarction; 8.3 Previously known multivessel coronary artery disease not suitable for percutaneous coronary intervention (PCI);
9. Previous enrolment in this study or treatment with an investigational drug or device under another study protocol in the past 30 days;
10. Treatment with anticoagulants;
11. Pregnancy or lactating;
12. Body weight <40kg or >125kg;
13. Known hypersensitivity to any drug that may appear in the study;
14. Inability to follow the protocol and comply with follow-up requirements or any other reason that the investigator feels would place the patient at increased risk;