A method for selecting a reference gate includes acquiring a four-dimensional (4D) emission dataset of an object of interest, sorting the 4D emission dataset into a plurality of gates, calculating a slice activity value for each slice in the plurality of gates, generating a maximum difference value for each gate using the plurality of slice activity values, and selecting a reference gate based on the locations of the maximum difference values. An imaging system and a non-transitory computer readable medium are also described herein.