Systems and methods to obtain a myocardial mass index indicative of an at-risk myocardial region. In at least one embodiment, a method for diagnosing a risk of cardiac disease is provided, 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 and the size of the myocardial region at risk for infarct perfused by the side branch.