1. A method comprising: binning emission projection data acquired from a cyclically varying element into a plurality of different phases of the cyclical varying; computing pairwise difference emission projection data in projection data space comprising differences between emission projection data in different pairs of phases of the cyclical varying; aggregating the pairwise difference emission projection data computed for the different pairs of phases of the cyclical varying to generate difference emission projection data; and locating an image feature corresponding to the cyclically varying element in the emission projection data based on the difference emission projection data; wherein at least the computing and locating are performed by an electronic device.