a) Input curve derived from radial myocardial velocity measurements in healthy volunteers. Three peaks are seen in systole (S), diastole (D) and atrial systole (AS). End systole (ES) is marked as a vertical line. For a given RR duration the curve is split into systole and diastole and each section is stretched according to the given formula. Curves from each of the 13 RR durations that make up a full acquisition are then combined according to the ECG gating algorithm being simulated. Simulated curves in the presence of RR variation are shown for prospective (b) and retrospective (c) gating.