An emission imaging data processing device comprising: an electronic processor; and a non-transitory storage medium storing instructions readable and executable by the electronic processor to perform a respiratory motion signal generation method including: operating a positron emission tomography or single photon emission computed tomography (SPECT) imaging device to acquire emission data of an imaging subject in an imaging field of view; for each region of an array of regions defined in the imaging FOV, computing an activity position versus time curve from the emission data acquired by the PET or SPECT imaging device; and generating at least one respiratory motion signal by combining the activity position versus time curves of at least a sub-set of the regions of the array of regions after frequency-selective filtering of the activity position versus time curves to select content in a respiratory frequency band; wherein the computing of the activity position versus time curve from the emission data acquired by the PET or SPECT imaging device includes: generating an activity map of the region for successive time intervals from the emission data acquired by the PET or SPECT imaging device; and computing values of a statistical activity position descriptor for the activity maps of the region for the successive time intervals; and wherein the statistical activity position descriptor includes a position of the centroid of the activity map along a transverse direction parallel with an axial anatomical axis (z) of the imaging subject, and/or a minimum distance of the centroid of the activity map from an axial anatomical axis (z) of the imaging subject. an electronic processor; and a non-transitory storage medium storing instructions readable and executable by the electronic processor to perform a respiratory motion signal generation method including: operating a positron emission tomography or single photon emission computed tomography (SPECT) imaging device to acquire emission data of an imaging subject in an imaging field of view; for each region of an array of regions defined in the imaging FOV, computing an activity position versus time curve from the emission data acquired by the PET or SPECT imaging device; and generating at least one respiratory motion signal by combining the activity position versus time curves of at least a sub-set of the regions of the array of regions after frequency-selective filtering of the activity position versus time curves to select content in a respiratory frequency band; wherein the computing of the activity position versus time curve from the emission data acquired by the PET or SPECT imaging device includes: generating an activity map of the region for successive time intervals from the emission data acquired by the PET or SPECT imaging device; and computing values of a statistical activity position descriptor for the activity maps of the region for the successive time intervals; and wherein the statistical activity position descriptor includes a position of the centroid of the activity map along a transverse direction parallel with an axial anatomical axis (z) of the imaging subject, and/or a minimum distance of the centroid of the activity map from an axial anatomical axis (z) of the imaging subject. operating a positron emission tomography or single photon emission computed tomography (SPECT) imaging device to acquire emission data of an imaging subject in an imaging field of view; for each region of an array of regions defined in the imaging FOV, computing an activity position versus time curve from the emission data acquired by the PET or SPECT imaging device; and generating at least one respiratory motion signal by combining the activity position versus time curves of at least a sub-set of the regions of the array of regions after frequency-selective filtering of the activity position versus time curves to select content in a respiratory frequency band; wherein the computing of the activity position versus time curve from the emission data acquired by the PET or SPECT imaging device includes: generating an activity map of the region for successive time intervals from the emission data acquired by the PET or SPECT imaging device; and computing values of a statistical activity position descriptor for the activity maps of the region for the successive time intervals; and wherein the statistical activity position descriptor includes a position of the centroid of the activity map along a transverse direction parallel with an axial anatomical axis (z) of the imaging subject, and/or a minimum distance of the centroid of the activity map from an axial anatomical axis (z) of the imaging subject. generating an activity map of the region for successive time intervals from the emission data acquired by the PET or SPECT imaging device; and computing values of a statistical activity position descriptor for the activity maps of the region for the successive time intervals; and wherein the statistical activity position descriptor includes a position of the centroid of the activity map along a transverse direction parallel with an axial anatomical axis (z) of the imaging subject, and/or a minimum distance of the centroid of the activity map from an axial anatomical axis (z) of the imaging subject.