A method of reconstructing time-varying position of individual radioactive point sources directly from Positron Emission Tomography (PET) measurements is provided that includes using a PET scanner to acquire list-mode coincidence events of a moving radioactive point source, using an appropriately programmed computer to model a trajectory of the moving radioactive point source as a 3D function of a temporal variable, then apply an optimization procedure to find the trajectory that minimizes a distance between the trajectory and the recorded list-mode coincidence events, and using the PET scanner to output a real time position of the radioactive point source.