Quantifying the Arterial Input Function*

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 variety of tracers. Knowledge of the AIF is critical, not just to determine MBF and CFR, but also to measure other parameters of myocardial physiology (e.g., glucose and oxidative metabolic rates). It is worthwhile, therefore, to further reflect on the AIF. What is it? How is it used? How can it be measured? How might errors in its measurement affect clinical decisions?