Heart attacks caused by the complete blockage of a heart artery are treated by opening it with a stent. However, most people will also have 'non-culprit' narrowings found in their other arteries at this time. Although in general people do better if these non-culprit narrowings are also treated with stents if they look severe, this process has problems. This is because narrowings that look severe may be stable and not cause any trouble. For these people a stent is a wasted procedure and unnecessary risk. On the other hand, narrowings that are currently left alone because they appear mild, may progress and cause a heart attack. Participants who have had a heart attack will have a scan from inside the heart arteries during an angiogram (optical coherence tomography, OCT) and a magnetic resonance angiogram (MRA). If the investigators can show that it is possible to accurately predict which non-culprit narrowings are going to progress and which are going to stabilise, medical professionals may be able to better target their treatments after a heart attack.