Eur Radiol (2012) 22:1271–1277 1275 Fig. 4 Changes in a hyperdense and b hypodense artefacts acquisition with no padding uses the full dose (200 mAs) for image reconstruction [17]. With equal tube voltage setting, the retrospectively gated protocol at 600 mAs with an R-R interval of 1,000 ms (frequency 60/min) and a nominal temporal resolution of 135 ms uses only approximately 80 mAs (0600 mAs× 135/1,000) for the reconstruction of a single ECG interval. At 75 BPM the dose efficiency increases to approximately 100 mAs. At 90 BPM, 120 mAs is used for the reconstruction of a single ECG interval which results in an image noise level comparable to axial acquisition. Axial acquisition reduced hyperdense and hypodense artefacts except hypodense artefacts of the St Jude valve at 75 BPM. The mechanisms responsible for this reduction are not yet fully understood. The important reduction in hyper- and hypodense artefacts for both valves at 60 and 90 BPM are in contrast to the lack of reduction of hypodense arte- facts for the St Jude valve at 75 BPM. Possibly, this seems to be related to lower artefact volumes at 75 BPM with retrospective gating. Also, differences between the two valves may play a role. Earlier in vitro work showed that the opened tilting disc valve led to a sharper increase in hyper- and hypodense artefacts when compared to the St Jude valve [12]. Furthermore, the St Jude valve is composed of a nickel alloy prosthetic ring in contrast to the titanium alloy used for the Medtronic Hall. How these differences would cause a lack of reduction of hypodense artifacts at 75 BPM is not clear. Based on the differences between the two acquisition techniques one may postulate that factors asso- ciated with retrospective gating such as multisegment