Eur Radiol (2012) 22:1271–1277 Fig. 2 The PMMA cylindrical valve chamber in which a mechanical prosthetic valve was tightly mounted (arrow) under a 45° angle to the gantry (a). Only leaflet motion was possible. The valve chamber was then posi- tioned in a thoracic phantom (b) and connected to the piston pump for leaflet motion These images were used for HU measurements as well as 3D volume rendering. Image acquisition with both protocols was performed at 60, 75 and 90 BPM. All image sets were transferred to a dedicated worksta- tion for analysis (Extended Brilliance Workstation, Philips Medical Systems, Best, The Netherlands). Hyper- and hypo- dense artefact volumes were quantified in 3D-volume ren- dered images using two thresholds based on the densities of the surrounding structures according to a methodology de- scribed elsewhere for neurosurgical clips [13]. The PMMA in the valve chamber had a density of 130 ±15 HU, and water 1 ± 16 HU (mean ± standard deviation). We chose threshold values that were approximately 3 standard devia- tions (SD) above the HU of PMMA measurement and below the HU of water, respectively. The chosen thresholds were 175 HU for hyperdense artefacts and −45 HU for hypodense artefacts. Because the PHVs are composed of various radi- opaque components such as a titanium ring and tungsten impregnated leaflets, which have CT densities higher than 800 HU [12], the 175-HU threshold included the radiopaque components of the PHV. Hence, the percentage change of the volume measured with the 175-HU threshold underesti- mates the change in actual artefacts (i.e. the volume mea- sured with the 175-HU threshold without the volume of radiopaque components with densities over 800 HU) [12]. Areas outside the valve chamber and other unrelated sources of artefacts were digitally excised in an identical manner for all images. Image noise (defined as the SD of CT attenuation) was measured in all images using a circular region of interest (diameter 1 cm) that was placed in an identical section of the PMMA structure of the valve chamber. Statistical analysis Data were analysed using SPSS software (SPSS Statistics Version 16.0, SPSS Inc, Chicago, IL, USA). Non-parametric data were presented as medians with interquartile range (IQR). A Mann–Whitney U test was used for the compari- son of artefact volumes and image noise measurements. Statistical significance was defined as P< 0.05. 1273 Prospective One ECG interval Retrospective All ECG intervals 128×0.625 mm 120 200 0 270 Cardiac B 80 13.3 106.5 128×0.625 mm 120 600 0.2 270 Cardiac B 80.1 39.8 517.8 Results The median artefact volumes for the two thresholds (−45 and 175 HU) at different frequencies are summarised in Table 2. For the St Jude valve, prospective triggering reduced hypo- dense artefacts (all volume <−45 HU) at 60 BPM (P0 0.001), 90 BPM (P00.001) but not 75 BPM (P0 0.6). Hyperdense artefacts (all volume >175 HU) were reduced with prospective triggering at 60 BPM (P0 0.001), 75 BPM (P0 0.001) and 90 BPM (P0 0.001). For the Medtronic Hall valve, hypodense artefacts were reduced with prospective triggering at 60 BPM (P0 0.009), 75 BPM (P0 0.016) and 90 BPM (P0 0.001). Hyperdense artefacts were also reduced at all frequen- cies (P0 0.001 for 60, 75, and 90 BPM, see Figs. 3 and 4). Volume rendered images illustrate the reduction of artefacts emanating from the leaflet and artefacts related to the pros- thetic ring (Fig. 5). The image noise measurements are pre- sented in Table 3. Although significant differences were found between retrospective gating and prospective triggering at 60 and 75 BPM, the image noise did not differ at 90 BPM. Our findings demonstrate that axial imaging of PHVs using prospective triggering reduces artefacts with much lower Discussion Table 1 CT parameters Collimation kV mAs Pitch Rotation time (ms) Filter Anatomical length (mm) CTDIvol (mGy) DLP