Evaluation of fractional flow reserve (FFR) is a key method for assessing ischemia with a view to guiding revascularization strategy following acute coronary syndrome. A stenosis that appears to be severe by angiography may cause limited ischemia (with an FFR value \>0.80) due to the incapacity of the necrotic zone to achieve physiological hyperemia, i.e. maximal coronary flow. Recently, it has been demonstrated that absolute coronary flow, and micro- and macrovascular resistance, as measured by a thermodilution technique, using the Rayflow microcatheter (Hexacath) are strongly associated with myocardial mass. In extensive necrosis, there is a loss of myocardial mass, and these tools could be of potential interest in measuring myocardial viability, which reflects the extent of remaining viable myocardial mass. Therefore, this study aims to investigate the relationship between both absolute coronary flow and microvascular resistance, and myocardial viability assessed by MRI. In a prospective, single-centre, interventional study, we will compare absolute coronary flow and microvascular resistance in the left anterior descending artery, in patients with and without a history of ST segment elevation MI.