1. A method of reconstructing time-varying positions of an individual radioactive cell, comprising: a. transplanting to a human or animal host a radioactive cell; b. acquiring a positron emission tomography (PET) scan, using a PET scanner, of said human or said animal host to obtain a list of coincidence events containing lines and corresponding timestamps; c. using a computer to: i. model a time-varying position of said radioactive cell as a 3D function of a temporal variable; ii. determining an average distance for different parameters of said 3D function, wherein said average distance is determined by computing a geometrical distance between one said line in said list of coincidence events and a 3D position given by said 3D function for said timestamp corresponding to said one line, wherein an average of all said computed geometrical distances is computed; iii. iteratively repeating said modeling step i) and said average distance determination of step ii) until said average distance converges to a minimum distance; and d. reconstructing a time-varying position of said radioactive transplanted cell, using said PET scanner, by outputting said 3D function of said temporal variable of said reconstructed time-varying position of said transplanted radioactive cell, wherein said reconstructed time-varying position provides an estimate of individual cell migration in real time inside said human or animal host.