1276 Fig. 5 Volume rendered images of the 3D volume of a St Jude PHV at 60 BPM measured with the −45 HU threshold (including all volumes less than −45 HU) and with the 175 HU threshold (including all volumes with a density higher than 175 HU). Top panel artefact configurations with retrospective gating. Bottom panel images with prospective triggering. The 175 HU threshold volume includes all radiopaque parts of the valve such as the leaflets and the prosthetic ring reconstructions and helical interpolation may play a role [14– 20]. Although increased metal artefacts have been associated Table 3 Image noise (median and interquartile range) for retrospective gating and prospective triggering a The P values result from Mann–Whitney U test Valve St Jude Medtronic hall Eur Radiol (2012) 22:1271–1277 with increased noise [14, 15], we found a sharp reduction of hyper- and hypodense artefacts at 90 BPM for both valves despite equal image noise. This suggests that reduction of image noise may not be the primary mechanism responsible for artefacts reduction. However, small variations in leaflet position due to a fluttering motion of the leaflets may cause image discrepancies between consecutive cardiac cycles that increase artefacts with multisegment reconstructions [17, 18]. Furthermore, helical interpolation has been suggested as a mechanism for metal artefacts [16, 18–20]. Other experimental work comparing retrospective gating with prospective triggering for metal objects is scarce. Two reports using in vitro models of coronary in-stent restenosis found improved image quality and a reduction in artefacts [16, 20]. Because of the discrepancy in size and composition of coronary stents, our results are not readily comparable to these reports but similar mechanisms may explain the arte- fact reduction. There are several limitations to our study. Prospectively triggered acquisition allows reconstruction of only one pre- selected cardiac phase and therefore does not allow dynamic imaging. We used only two mechanical prostheses. Possibly, other prostheses consisting of other metal compounds might yield different results. However, the St Jude prosthesis is the most commonly implanted prosthesis worldwide and is the most likely to be encountered clinically. We used the Med- tronic Hall valve because it is the most widely implanted tilting disc valve. Both the Medtronic Hall and St Jude valves have been associated with dysfunction due to tissue ingrowth [2, 8, 21] and for both prostheses CT imaging has been shown to be of additional value for the diagnosis [2, 4, 8]. Also, we used a phantom in which the valve was fixed and only the leaflets moved. Movement of the valve as a whole may induce more artefacts. Furthermore, the use of different reconstruction filters may change the amount of PHV-induced artefacts. Our results suggest a reduction of most PHV-related artefacts at different frequencies that appears independent of the image noise reduction with axial acquisition. Prereq- uisites, however, are a regular heart rate and the correct timing relative to the cardiac cycle. P valuesa 0.001 0.001 0.198 0.001 0.026 0.269 Beats per minute Retrospective gating Prospective triggering 60 75 90 60 75 90 16.0 (14.5–17.0) 18.0 (17.0–18.8) 14.0 (12.3–16.0) 16.0 (14.3–17.0) 17.5 (15.0–19.8) 15.0 (14.3–15.8) 11.0 (10.3–12.0) 13.5 (12.3–14.8) 13.0 (12.3–13.8) 11.0 (10.3–11.8) 14.0 (12.5–16.0) 14.5 (13.0–15.0)