1. A method for diagnosing a risk of cardiac disease, the method comprising the steps of: identifying a luminal cross-sectional area of a side branch vessel; identifying a luminal cross-sectional area of a main artery most proximal to the side branch vessel; determining a myocardial region at risk for infarct from side branch occlusion relative to a mass perfused by the most proximal artery, wherein such region is based on a relationship between the cross-sectional areas of the side branch and the most proximal main artery; and diagnosing a risk of cardiac disease based on a diameter of the side branch vessel and the size of the myocardial region at risk for infarct perfused by the side branch.