A method for operating a cardiac CT imaging system (200) in coronary CT with prospective electrocardiography-triggering of the imaging system (200) is proposed. The method comprises the steps of determining a plurality of R-peak times (ti) from a dataset of electrocardiography data, determining a variability of a heart rate by statistically analyzing the determined R-peak times (ti), and automatically adjusting an acquisition time period (ΔTac) of the CT imaging system (200) for acquiring projection data based on the determined variability of the heart rate.