1. A method for calculating an input function for use in the assessment of lung activity in a subject, the steps comprising: a) administering a radionuclide tracer to the subject; b) acquiring a series of image frames with a positron emission tomographic (PET) system; c) acquiring two blood samples from the subject during the performance of step b); d) designating a blood pool region of interest (ROI) in the image frames; and e) calculating the input function using information in the two blood samples, using values in the image frames corresponding to the designated blood pool ROI, and using a two-parameter model of the ROI time-activity curve.