1272 Echocardiography, which is the conventional method of monitoring PHV function, may not always identify the cause of dysfunction because of acoustic shadowing [4–6]. In regions obscured by acoustic shadowing, MDCT has been shown to identify obstructive masses, thrombus, veg- etations and degenerated thickened leaflets of biological prostheses [1–4, 7]. The identification of these causes of dysfunction may guide clinical management and may even form an indication for the surgical replacement of dysfunc- tional valves [1, 2, 4, 8, 9]. These imaging results have been achieved mostly with 64-slice systems using retrospectively ECG-gated coronary CT angiography protocols. With retrospectively gated acqui- sition, the in vitro and in vivo image quality of modern mechanical PHV was found to be generally good despite variable amounts of artefacts [1, 10–12]. The artefacts seemed to be related to the radiopaque parts of the PHV, such as the metal alloy of the prosthetic ring and the tungsten impregnated carbon leaflets, and consisted of low attenua- tion (hypodense) and high attenuation (hyperdense) compo- nents [10, 12]. Because of their resemblance to metal artefacts, probably similar causal mechanisms such as scat- ter, photon starvation, edge effects, beam hardening and motion may play a role [13–15]. Some of these interactions can be influenced by modifying acquisition parameters, oth- er factors (such as motion) are patient-dependent. A prag- matic approach has been the use of modified coronary protocols with retrospective gating but with increased tube currents and voltages [3, 7]. Although these reports yielded good image quality, it was at the expense of sizable radiation exposure, which may curtail the use of CT for PHV evalua- tion and withhold the potential advantages of the technique to all but highly selected patients. Other adaptations for enhanced PHV imaging have not been studied and no spe- cific PHV MDCT protocols have been developed to date. With the introduction of 256- and 320-detector systems, PHV can be imaged within one rotation using prospective ECG-triggering. Experiments with an in vitro model of coronary in-stent restenosis suggested that prospective trig- gering may be associated with improved image quality compared with retrospective gating [16]. In addition, pro- spective triggering may achieve this with a fraction of the radiation dose compared with modified coronary protocols that have been used for PHV imaging. Because PHV also appear as high density objects on CT, a similar improvement may be possible for prospectively triggered imaging of PHV. In order to test this hypothesis we compared a retro- spectively gated and a prospectively triggered protocol using a 256-detector system for imaging the two most common bileaflet and tilting disc mechanical PHVs in a pulsatile in vitro model. Eur Radiol (2012) 22:1271–1277 Materials and methods Two mechanical PHVs, St Jude bileaflet (SJ, St Jude Medical Inc., St Paul, MN, USA, valve size 27 mm) and Medtronic Hall tilting disc (MH, Medtronic Inc., Minne- apolis, MN, USA, valve size 27 mm, Fig. 1), were inserted into a pulsatile in vitro model, which was described by Symersky et al. [10]. In brief, the valves were mounted in a polymethylmethacrylate (PMMA) valve chamber (Fig. 2) that was placed in a thoracic phantom (QRM GmbH, Möhrendorf, Germany). The valve chamber was connected to a computer controlled piston pump and to an afterload reservoir. Water was used as a perfusate. The flow pulses were at a frequency of 60, 75 and 90 beats per minute (BPM) and produced identical pulsation cycles. The valves were opened from 30 to 50% of the artificial ECG interval generated by the computer. Imaging was performed using 256 MDCT (iCT, Philips Medical Systems, Best, The Netherlands). Two imaging protocols were used: 1) a standard helical retrospectively ECG-gated protocol based on coronary CT angiography, which has been used for PHV imaging [2, 3, 7], and 2) a prospectively ECG-triggered protocol. CT parameters are presented in Table 1. Prospective triggering was performed with 128× 0.625 mm collimation and axial imaging with no table movement and 8 cm exposed at the centre of the gantry. The maximal range that could be reconstructed from these data was 70 mm. No padding was used. Retrospective gating was also performed with 128 ×0.625 mm collimation at a pitch of 0.2. We chose the smallest possible anatomical range (80.1 mm), which could be covered with the use of this protocol. CT data acquisition only occurred during the 40% ECG interval (opened PHV) with prospective trigger- ing and reconstructed with retrospective gating. Each valve was imaged 8 times using each protocol, yielding a total of 16 acquisitions per valve. Images were reconstructed at 0.9 mm thick slices with 0.45 mm reconstruction increment. Fig. 1 Photograph of a St Jude (a) and Medtronic Hall aortic valve (b), both pictured from the inflow (ventricular) side. (Images courtesy of St Jude Medical Inc., St Paul, MN, USA, and Medtronic Inc., Minneapolis, MN, USA)