This study demonstrates the role of careful examination of the physiology of coronary motion in understanding the optimal timing of cardiac CT within diastole and the impact of image acquisition speed on motion artifact. The study also demonstrates the difficulties of cardiac imaging timing given the inherent physiological variability in coronary motion. While we primarily assessed the effects of coronary motion on cardiac CT, the research may also have a role in other gated imaging modalities such as cardiovascular MRI.
The temporal resolution of half-scan reconstruction of current generation single source cardiac CT lies between 140 and 175 ms. While current dual-source imaging reduces the temporal resolution to 70 ms-83 ms [1, 13], in its lowest dose, high-pitch exemplification, the time taken to image the entire axial cardiac volume remains in the order of 280 ms. Retrospective gating of cardiac CT, and increasing the window of acquisition can go some way towards reducing the effects of cardiac motion at higher heart rates, but at the cost of greatly increased radiation dose. Retrospective gated cardiac CT may deliver 4 or 5 times the ionizing radiation of prospective CT  and even when prospective cardiac CT protocols are used, for every extra 100 ms of the cardiac cycle acquired, radiation dose is increased by up to 45% . On the other hand, very low dose prospectively gated cardiac CT acquired during minimal phase acquisition enables only one or a very narrow band of phases to be analyzed, compromising diagnosis in the case of motion artifact. Accurate timing of cardiac CT is therefore vital for both radiation dose reduction and prevention of imaging artifacts.
High temporal resolution imaging of coronary artery motion reveals interesting insights about coronary motion, which may be useful for effective cardiac imaging. Previous measurements of coronary artery motion using electron beam CT [16, 17] or coronary angiography  are inadequate for the precise definition of the cardiac rest period within mid-diastole. This is because the low temporal resolution of these techniques can only provide information as to which broad band of phase acquisition is most appropriate according to heart rate. The issue as to whether end-systole or diastole is superior at a given heart rate is clinically important and has been previously studied [3, 4, 18–20]. Likewise the interaction of gantry rotation time, heart rate and image artifact requires careful attention in order to optimize image quality [21, 22]. The current research adds to these prior works by further elucidating the nature, effects and variability of coronary motion during the imaging phase of mid-diastole.
The equations and tables provided are intended as a guide for gated cardiac imaging across all CT types. They indicate the heart rates at which diastolic imaging becomes viable, the window available for imaging and the chance of motion artifact for given heart rates. In the validation cohort, a strategy of varying the targeted phase according to heart rate was statistically superior to a fixed percentage acquisition in terms of proximity to the ideal imaging phase, although the extent of benefit was small. These tools should prove useful for the achievement of high quality cardiac CT imaging. Nevertheless, there are several limitations to the formulas and conclusions provided and they should not be applied without consideration.
Firstly, it should be noted that even small differences in absolute timing make a large difference in the optimal phase selection. Variation can occur due to triggering parameters, QRS measurement and biological variability. Beat to beat variation of more than 50 ms can be expected at most heart rates and no system of CT triggering can ever account for the unpredictability associated with ectopic beats or atrial fibrillation. The wide variation in the ideal imaging target within the CT validation cohort may have reflected ectopy during the period of heart-rate assessment, although as we were only able to capture the two heartbeats immediately prior to imaging we were unable to assess the impact of ectopy in this study. While measurement of time intervals on tissue Doppler has generally good reliability , the use of a single measurement in our study should also be considered a limitation and may have added to variability of tissue Doppler measurements.
Secondly, the definition of motion artifact used in this study was conservative and it is likely that a degree of motion artifact may be tolerated without affecting clinical interpretability. The effect of motion artifact on diagnosis is inherently subjective and the precise heart-rate boundaries of CT interpretability may vary from the predictions of this study.
Lastly, the difference between a fixed 75% phase trigger and a heart rate adjusted strategy was small, and both strategies benefit from a degree of extra phase acquisition at most heart rates and gantry speeds. CT coronary angiography performed as a part of this study was designed to ensure coverage of mid-diastole and analysis of systolic or end-systolic phases could not be performed. Adjustment of the precise imaging target within mid-diastole should be a consideration secondary to the more fundamental issue of whether end-systolic or diastolic imaging is required.