1274 Table 2 Artefacts measured in opened (systolic) valves Valve St Jude Medtronic hall BPM Threshold Median artefact volume (mm3) (interquartile range) 60 75 90 60 75 90 <−45 HU >175 HU <−45 HU >175 HU <−45 HU >175 HU <−45 HU >175 HU <−45 HU >175 HU <−45 HU >175 HU Retrospective 2,056 (1,939–2,263) 9,000 (8,725–10,375) 1,609 (1,567–1,841) 8,400 (7,625–8,775) 2,742 (1,964–2,896) 9,700 (9,100–10,300) 2,271 (1,873–2,630) 8,400 (8,300–9,225) 2,192 (1,788–2,522) 7,550 (7,400–8,075) 3,164 (2,547–3,770) 8,400 (7,575–10,000) Eur Radiol (2012) 22:1271–1277 Prospective 1,168 (1,146–1,201) 7,000 (6,925–7,150) 1,609 (1,558–1,641) 6,900 (6,700–7,275) 1,564 (1,527–1,589) 7,000 (6,800–7,450) 1,571 (1,474–1,724) 6,150 (5,950–6,525) 1,608 (1,446–1,730) 5,800 (5,625–6,075) 1,774 (1,723–1,977) 5,950 (5,725–6,100) radiation exposure. In our model, prospective triggering reduced artefact volumes for both valves and at all three frequencies with the exception of hypodense artefacts asso- ciated with the St Jude valve at 75 BPM. The imaging of PHV with MDCT is hampered by arte- facts that depend on valve material and valve geometry [1– 3, 10, 12]. Because of the radiopaque components, these artefacts have been interpreted as metal-related artefacts and efforts to improve image quality have included the increase in tube voltage and current [3, 7]. Unfortunately, this ap- proach results in sizable dose exposure (and prohibitive exposure for serial evaluations) and therefore limits the usefulness of CT evaluation of PHV. In addition, retrospec- tively gated acquisition with reconstruction of 10 intervals (i.e. the full cardiac cycle) has been used to offset the variation of the PHV image quality during the cardiac cycle [2, 3, 7]. Because of the motion of the radiopaque PHV parts, which occurs due to the leaflet excursions and move- ment of the valve as a whole, reconstructions at each 10% of the ECG interval may generate only two or three image sets that are of diagnostic quality [2, 11, 12]. In vitro, rapid leaflet motion has been shown to increase hyperdense and hypodense artefacts, and rapid motion of the prosthetic ring, which has a higher density than the leaflets, may be expected to further increase artefacts [12]. In this study, we examined how pro- spective triggering and retrospective gating affected artefact behaviour and noise in PHV imaging. In our in vitro model, we tested the effect of prospective triggering at different frequencies. Axial acquisition reduced image noise only for 60 and 75 BPM but not for 90 BPM. This may be explained by differences in the efficiency of dose used for image reconstruction. For example, axial Fig. 3 Multiplanar reformatted images of the St Jude valve in a view perpendicular to the leaflets acquired with retrospective gating (a) and prospective triggering (b) at 60 BPM. Note the increased deformation of the subprosthetic PMMA contours and increased hyper- and hypodense artefacts in (a)