The present invention concerns a data processing apparatus for locating source cells (25) responsible for an electrical anomaly in a subject's body part (8). The apparatus comprises means for: obtaining one or more electrical signals (19) resulting from electrical activity of one or more source cells (25) in the subject's body part (8); 5 time-reversing the one or more electrical signals (19) for obtaining one or more time-reversed electrical signals (21); feeding the one or more time-reversed electrical signals (21) into a simulation model (17) of the subject's body part, which is configured to model anatomy and/or electrical activity of the subject's body part (8), to cause the time-reversed electrical signals (21) when fed into the simulation model to converge in the 10 simulation model at one or more convergence locations (23); and identifying the one or more convergence locations (23) in the simulation model (17) thereby allowing locating the source cells (25) responsible for the electrical anomaly.