Introduction 1 2 6 7 8 9 10 11 11 13 14 15 14 15 The purpose of our study was to assess the lumen visibility of coronary artery stents at various heart rates with an automated objective method using a moving heart phantom on a 64-MDCT system. Theory 16 17 18 1 \documentclass[12pt]{minimal}
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IS = g*MS{\left( {HR} \right)} + {\left( {1 - g} \right)}*AT{\left( {HR} \right)}
$$\end{document} 1 Materials and methods 2 1 left Fig. 1 Left: Right:  1 right Measurements have been performed on a 64-detector CT-scanner (Somatom Sensation 64, Siemens, Forchheim, Germany). The scan parameters were 120 kV, 120 mAs, 370 ms rotation time and 64 × 0.6 mm collimation. Image reconstruction was performed using a sharp convolution kernel (Siemens B46f) with a reconstruction slice width of 0.75 mm and a 0.5-mm increment. ECG-gating was used during scanning and the images were retrospectively reconstructed at 25% of the RR-interval corresponding to maximum expansion of the heart phantom. 2 2 2 Fig. 2 Left: Right:  3 left 3 right Fig. 3 left: right: left  4 p 2 \documentclass[12pt]{minimal}
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W_{p}  = {\left( {{FWHM} \mathord{\left/
 {\vphantom {{FWHM} {TW}}} \right.
 \kern-\nulldelimiterspace} {TW}} \right)}*100\% 
$$\end{document} The percentual width is used instead of the absolute width to compensate for different magnifications of the stent images. p 3 \documentclass[12pt]{minimal}
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LV = {{\left( {D*W_{p} } \right)}} \mathord{\left/
 {\vphantom {{{\left( {D*W_{p} } \right)}} {1000}}} \right.
 \kern-\nulldelimiterspace} {1000}
$$\end{document} If the left and right edge of the attenuation profile are at different heights, D is calculated from the average height. The stent images at individual heart rates were ranked according to their CT attenuation (7=least attenuation, 1=most attenuation), FWHM (7=largest width, 1=smallest width) and D (7=largest depth, 1=smallest depth) in the axial and longitudinal plane. The difference between the CT attenuation values in the axial and longitudinal plane were compared using the Wilcoxon test at a significance level of 5%. The correlation between heart rate and CT attenuation, and the correlation between heart rate and LV, were analysed by calculating the Spearman rank correlation coefficient at the significance level of 5%. Results CT attenuation in the stent lumen 3 4 5 6 All stents showed increased attenuation with increasing heart rate in the longitudinal plane. The CT attenuation in the longitudinal plane showed an increase from 0 to 100 bpm between 75 and 221 HU for the Taxus and Bx Sonic stent, respectively. The average increase was 139 ± 49 HU. The average Spearman coefficient for all stents was 0.66. All stents showed increased attenuation with increasing heart rates in the axial plane. The CT attenuation in the axial plane showed an increase from 0 to 100 bpm between 76 and 252 HU for the Bx Sonic, Multi-link Zeta and the Jostent, respectively. The average increase was 148 ± 69 HU from 0 to 100 bpm. The average Spearman coefficient in the axial plane was 0.58. The average increase of CT attenuation over both planes was 144 ± 59 HU. 5 6 7 8 2 3 7 8 All stents showed decreasing LV with increasing heart rate in the longitudinal plane. The LV decreased from 0 to 100 bpm between 7.5% and 48.6% for the Taxus and Cypher stent respectively. The average decrease in LV was 26.8 ± 13.4%. The average Spearman coefficient was 0.47. All stents showed decreasing LV with increasing heart rates in the axial plane. The LV decreased from 0 to 100 bpm between 19.8% and 48.8% for the Lekton and Multi-link Zeta stent respectively. The average decrease in LV was 30.9 ± 10.4%. The average Spearman coefficient was 0.58. The average decrease of LV over both planes was 29 ± 12%. Image scoring 9 Discussion 19 20 21 22 23 24 9 25 Influence of increasing heart rate 14 15 26 29 8 30 23 30 ASVC 9 Image scoring 9 6 18 32 9 31 31 32 33 Reducing blurring Cardiac movement and limited temporal resolution will introduce blurring in CT images of coronary arteries. The result for image scoring showed that the influence of cardiac movement is almost twice as large as the influence of the temporal resolution. To reduce blurring, it is therefore more efficient to reduce the heart rate than to increase the temporal resolution. Limitations 18 Screenshots captured with the visualisation software were used to perform LV measurements. A loss of information is expected due to image compression used in the .jpg format of the screenshots. However, a comparison between the screenshot method and direct analysis using dicom files showed no observable difference in results. Conclusion We have shown that at increasing heart rates the CT attenuation in the stent increases and the lumen visibility decreases. A new approach to asses the stent lumen has been described which shows similar results to CT attenuation measurements and previous studies. The cardiac movement during data acquisition causes approximately twice as much blurring compared with the influence of temporal resolution. We conclude that a lowering of the heart rate is more beneficial to image quality than using a multi-sector reconstruction technique. In addition, we conclude that it is beneficial to image quality to acquire data in one cardiac cycle. Electronic Supplementary Material Below is the link to the electronic supplentary material. 
 Fig. 4 dotted blue line TW D FWHM Fig. 5 CT attenuation measured in the stent lumen at various heart rates in the longitudinal plane (DOC 76 kb) Fig. 6 CT attenuation measured in the stent lumen at various heart rates in the axial plane (DOC 77 kb) Fig. 7 Lumen visibility plotted versus heart rate in the longitudinal plane (DOC 75 kb) Fig. 8 Lumen visibility plotted versus heart rate in the axial plane (DOC 74 kb) Fig. 9 green blue dotted red dotted orange Table 1 AT MS Table 2 Commercial name, manufacturer, material, length and diameter of the stents used in the study (DOC 33 kb) Table 3 HU-value measurements in the longitudinal plane (DOC 34 kb) Table 4 HU-value measurements in the axial plane (DOC 34 kb) Table 5 Results of the measured FWHM in the longitudinal plane (DOC 34 kb) Table 6 Results of the measured FWHM in the axial plane (DOC 34 kb) Table 7 Results of the measured profile depth in the longitudinal plane (DOC 34 kb) Table 8 Results of the measured profile depth the axial plane (DOC 34 kb)