Non-invasive Coronary Thrombus Imaging to Define the Cause of Acute Myocardial Infarction

1.1 BACKGROUND

1.1.1 Background of coronary artery disease and acute myocardial infarction

Coronary artery disease remains the leading cause of death worldwide, with more than 23 million annual deaths estimated to occur by 2030. The introduction of high-sensitivity cardiac troponin measurement has revolutionised the identification and management of patients presenting with acute chest pain and myocardial infarction. However, elevated cardiac troponin concentrations can result from both ischaemic (myocardial infarction) or non-ischaemic (myocardial injury) causes. Moreover, the further subclassification of myocardial infarction relies on the underlying aetiology of infarction which is often not readily apparent and remains a diagnostic challenge.

1.1.2 Universal definition of myocardial infarction

Acute myocardial infarction is defined as cardiomyocyte injury and necrosis in a clinical setting consistent with acute myocardial ischaemia. The fourth universal definition of myocardial infarction describes five subtypes of acute infarction. Type 1 myocardial infarction is characterised by underlying atherosclerotic plaque disruption (rupture, ulceration, fissuring, erosion, or calcific nodules) resulting in intraluminal thrombosis, distal coronary embolisation and intraplaque haemorrhage that ultimately cause myocyte necrosis. In contrast, type 2 myocardial infarction is classified as an ischaemic myocardial injury secondary to oxygen supply and demand mismatch, and importantly is not a consequence of atherosclerotic plaque rupture and coronary atherothrombosis. However, some causes of type 2 myocardial infarction do involve intracoronary thrombus including coronary thromboembolism and spontaneous coronary artery dissection. Currently, invasive coronary angiography remains the primary modality to identify patients presenting with plaque related disruption and coronary thrombosis, ultimately providing the distinction between type 1 and type 2 myocardial infarction. However, it is not always definitive and only 10-20% of patients presenting with type 2 myocardial infarction undergo further investigations to identify the presence of underlying coronary artery disease or to exclude coronary atherothrombosis. Indeed, we recently demonstrated comprehensive cardiac imaging and coronary angiography that identified two thirds of patients had previously unrecognised underlying coronary artery disease, and this resulted in diagnostic reclassification of many patients. These standard techniques were however unable to determine whether this plaque was implicated in the presentation or simply a bystander finding. A non-invasive imaging technique to reliably detect intracoronary thrombus and facilitate this distinction would therefore be of major clinical value.