This is a prospective, open-label, single center registry. The study is designed to characterize plaque (i.e., assess vulnerable plaque features such as plaque tears, plaque thickness, plaque volume, and lipid content in plaque) in heart arteries in patients who have suffered a recent acute heart attack, and who have blockages >50% in one or more of the other arteries in the heart. The procedure to do this will not be the same procedure that was done to open the artery that caused the heart attack, but will be scheduled 2-40 days after the first procedure, depending on the urgency to evaluate the other arteries and the stability of the patient after the first procedure. The study will specifically evaluate the plaque characteristics in the arteries that didn't cause the heart attack, to see if these arteries have vulnerable plaque and is at risk for future heart attacks. If FFR shows abnormal blood flow, these arteries will be treated with stents. All patients will be followed for 12 months after the second catheterization. The purpose of the study is to determine how commonly vulnerable plaques are identified in patients with acute heart attacks, and how often these lead to future cardiac problems. Identification of plaque features that lead to future problems may allow the development of new medications and devices to prevent future heart attacks and death. These same plaque features are also thought to be operative in patients with blockages in the arteries to the brain, so this study may have important implications for stroke prevention as well. All patients will be considered eligible for participation if they had a recent acute heart attack resulting from complete occlusion of an artery to the heart, if the stent procedure to open the artery was successful, and if they have blockages > 50% in at least one other major heart artery. Patients will not be eligible for participation if they are too unstable with regard to heart, lung, brain, or kidney function, or if they have previous bypasses to the heart.