Methods, devices, and software for selecting image slices. The image slices are preferably taken during a quiet part of the patient's heart cycle in which the heart is relatively motionless so as to reduce blurring and the introduction of artifacts into the resultant reconstructed projection image of the heart. The methods of the present invention can analyze information regarding the duration of the patient's R-R cycles and allow the user to choose from a plurality of selection criteria, typically an absolute time or a percentage of the R-R cycle, so as to select slice images for inclusion in a projection image. The ability to choose from a plurality of selection criteria provides for flexibility to account for irregular heartbeats or a change in the duration of the patient's R-R cycles. In exemplary embodiments, the resultant projection image can be used for coronary calcium scoring or 3-D rendering of the heart.