DESCRIPTION (Adapted from Applicants Abstract): In ischemic heart disease the presence and extent of collateral circulation can prevent myocardial infarction, or at least limit the damage and reduce the size of infarcts. The objective of this proposal is to characterize the development of coronary collaterals in the heart with methods based on magnetic resonance imaging (MRI). MRI-based measures of collateral growth can define the functional capacity of collaterals to preserve myocardial blood flow and contractile function in regions at risk of infarction. This would be a significant advantage over current angiographic methods used to define the extent of collateral circulation in the heart. The proposed studies will also determine to what degree collateral development in heart translates into an increased capacity of the heart to prevent stress-induced ischemia. The studies will be carried out in an experimental animal model of collateral development with slow progressive occlusion of the left circumflex coronary artery ('ameroid constrictor' model). This animal model of collateral development approximates the progression of coronary artery disease as it occurs in humans. The proposed diagnostic MRI techniques for assessment of collateral blood flow in the heart may in the future allow improved prediction of outcome. Understanding how collateral growth can be followed non-invasively also means that one can assess more effectively new therapies that boost collateral development in the heart and which could markedly change the natural history of coronary artery disease.