An input function indicative of a time-activity curve in pulmonary arterial plasma is produced from a series of PET image frames in lieu of manual blood sampling. Two manually acquired blood samples are input along with pixel values of a blood pool region of interest (ROI) in the PET image frames into a two-parameter model of the ROI's time-activity curve. In an iterative process the model converges to accurately indicate the amount of 18F-FDG tracer in the arterial blood. Pulmonary uptake of the tracer is assessed with the PET image frames and the calculated input function.