A nuclear imaging system includes a scanner (8), such as a PET scanner. A patient is injected with a [13N]ammonia radioisotope tracer which is contaminated with a small percent of 18F contamination. The scanner receives radiation from the injected tracer and a reconstruction processor (28) reconstructs the detected radiation into image representations. A calibration processor (16) generates an estimated decay curve based on the proton bombardment and a priori information about the tracer. An activity meter (42) measures radiation emitted from a sample of the tracer and a dose calibrator (44) determines a decay curve from the measured radiation. The detected radiation is corrected with one of the decay curves during reconstruction or a correction processor (50) corrects reconstructed images with one or both of the decay curves. A display (14) displays uncorrected reconstructed images and the decay curve and/or the corrected images.