1. A system that facilitates cross-calibrating a nuclear scanner to a gamma counter, including: a nuclear scanner configured to scan: a radioactive calibration phantom comprising a radioactive material to acquire scan data; and a patient to acquire list mode data; a gamma counter that measures a radioactivity level of a sample of the radioactive material to acquire measurement data; and a processor that executes computer-executable instructions stored in a memory, the instructions including: generating one or more cross-calibration factors from the scan data and the measurement data; generating a reconstructed series of nuclear images by reconstructing the list mode data in each of a series of temporal sampling windows; generating an initial time-activity curve (TAC) based on nuclear image data; and determining a sampling window in which early bolus activity in the TAC provides a predetermined level of accuracy.