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