Microvascular Obstruction

The goal of this observational study to measure the heart's microvascular function in the setting of a myocardial infarction (MI), or heart attack, using a method called continuous saline thermodilution (CST). The participants will include people who are experiencing MI from sudden and complete blockage of a coronary artery requiring immediate balloon and/or stent therapy. After getting the balloon and/or stent therapy, participants will have their heart's microvascular system tested using CST. The main questions it aims to answer are: * What measurements using CST can we expect from the heart's microvascular system during a treated MI? * Can CST measurements during a treated MI predict the amount of heart muscle that is injured and that recovers? For this study, participants will undergo measurement of their heart's microvascular function after balloon and/or stent therapy for the MI. They will then receive an MRI scan of the heart several days after the MI.