However, both PET and current CMR techniques are cumbersome, and in the case of PET require radiation as well as on-site Rubidium-82 generators. Therefore, they are challenging for routine clinical practice and have been limited to specialized research centers. An alternative, simple CMR method for measurement of myocardial perfusion reserve by quantifying change in coronary sinus flow has been described. The coronary sinus drains approximately 96% of total myocardial blood flow and provides a potentially convenient location for measurement of global myocardial blood flow. This method has been validated against both invasive and PET techniques. The investigators hypothesized that measurement of coronary sinus flow at stress and rest may provide a simple and rapid assessment of myocardial perfusion reserve during regadenoson stress perfusion CMR.