t i o Measurement of absolute myocardial blood flow (MBF) and coronary flow reserve (CFR) is of great importance clinically. To make such measurements quantitatively, one must also measure the arterial input function (AIF). Many studies, including the one reported by Vasquez et al. (1) in this issue of iJACC, which focuses on rubidium-82 (Rb), have sought optimal ways to measure the AIF for a v...