Most heart attacks occur because a clot forms in a coronary artery blocking blood flow. Without blood heart muscle dies. Untreated, clots can cause a specific type of heart attack -ST-elevation myocardial infarction (STEMI). STEMI patients are treated immediately by finding the blocked artery (culprit lesion) using a dye injected into the coronary arteries and then by unblocking the artery using balloons and stents. This procedure - primary angioplasty - is offered 24/7 and limits the size of heart attacks and saves lives. Cardiologists know how to treat STEMI patients but it's less clear what to do about narrowings in other coronary arteries (bystander disease). This is important - if they're left alone some bystander lesions can cause future events including heart attacks or angina. Recent trials compared stenting ALL the bystander narrowings after primary angioplasty, with stenting none and showed some benefit from stenting all of them (complete revascularisation). However, complete revascularisation carries extra risk, putting patients through more complicated procedures and using up resource. A blanket strategy of complete revascularisation of ALL bystander narrowings in ALL STEMI patients is unlikely to be the correct answer as only a small minority of these patients have further events. In PICNIC the investigators want to identify bystander narrowings most likely to cause a future event, and those unlikely to do so. The study can then test the hypothesis that only the high-risk bystander narrowings need stenting, and the others can be treated with tablets only. Investigators will study patients using specialised imaging techniques from coronary artery CT scans and levels of inflammation to see which narrowings cause future events and which do not. If this can be done, a case can be made to test complete revascularisation only in bystander narrowings that look high risk.