Abstract: According to a preferred embodiment, the invention provides a method for extracting internal organ motion from positron emission tomography (PET) coincidence data, the method comprising the following steps: generating a data stream of PET coincidence data using the list mode capability of a PET scanner; dividing the data stream into time frames of a given length; computing a histogram A(i, t) of an axial coincidence distribution for a set of time frames; computing the axial center of mass z(t) for each of the time frames in the set of time frames based on the histogram A(i, t); transforming z(t) into the frequency domain; determining either the frequency contribution caused by respiratory motion, given by fresp, or the frequency contribution caused by heart contractions, given by fcard and ?f, identified in the frequency spectrum |Z(f)|; and carrying out further processing of Z(f) leading to curves zresp(t) and zcard(t) with which a gating sequence is established.