Introduction 1 1 2 3 5 6 7 8 9 The purpose of this study was therefore to assess Ca-scoring on 64-slice MDCT and DSCT versus EBT on a moving cardiac phantom as a function of heart rate, slice thickness and calcification density using 3 different Ca-scoring methods. Methods and materials Cardiac phantom 1 7 10 2 11 12 1 1 Fig. 1 Left Right  Fig. 2 Motion curve of the phantom at 70 bpm. The curve is defined by the time-deflection points T1–T8 and the reconstruction intervals of the DSCT and 64-slice MDCT are indicated by the grey areas. Other heart rates are obtained by a time scaling of the data points. For higher heart rates (>90 bpm) the data point T5 was omitted to reflect the relative larger diminishing of the diastolic phase  Table 1 The three artificial coronary arteries high, medium and low density calcification (HDC, MDC and LDC) with the properties of the inserted calcifications as specified by the manufacturer Artificial artery 3 3 HDC 796 1.58 MDC 401 1.30 LDC 197 1.16 Data acquisition − 2 2 Table 2 Phases used for reconstruction of the images in percentage of the beat time at different heart rates used in beats per minute (bpm) Heart rates (bpm) 50 60 70 80 90 100 110 64-Slice MDCT-phase (%) 76 74 60 58 56 53 51 DSCT-phase (%) 83 82 70 69 69 67 66 4 11 13 15 Data analysis Two root mean square measures were used to analyze the scoring results. The first measure quantifies the susceptibility of the calcium score to cardiac motion. The second measure quantifies the deviation of the calcium score from the reference value. 1 \documentclass[12pt]{minimal}
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$$ {\text{CMS}} = \frac{1} {{N - 1}}{\sqrt {{\sum\limits_{i = 1}^N {(x_{0} - x_{i} )^{2} } }} }\frac{1} {{x_{0} }} $$\end{document} x 0 x i i N 1/x 0 2 \documentclass[12pt]{minimal}
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$$ \Delta = {\sqrt {{\sum\limits_{i = 1}^N {(y_{i} - z_{i} )^{2} } }} }\frac{1} {{y_{{av}} }} $$\end{document} y i i z i i y av av 2 y i z i i Noise levels were measured using a standard Region of Interest (ROI) technique. The ROI was placed in a section of a slice containing only water. The standard deviation of the measured HU-values within the selected ROI was considered to be a measure for the noise level. All measurements are considered to be normally distributed. Mean and standard deviation (sd) are given for each measurement. Results 3 Fig. 3 triangles circles squares  The results show a general underestimation of the Ca-score for Ca-scoring obtained at 3.0 mm slice thickness when comparing CT-data and EBT at all heart rates except for the Agatston and volume score of the high density calcifications at 70 and 80 bpm. In general, the Ca-scores obtained with 0.6 mm slice thickness on CT are overestimated compared to the EBT-data or are similar to the EBT-data at all heart rates. 3 3 3 The 64-slice MDCT with a slice thickness of 3.0 mm (solid lines with triangles) showed increased Ca-scores for the Agatston score at 70–90 bpm and for the volumes score at all heart rates for the high density calcification, whereas the equivalent mass score showed a slight decrease. The medium and low density calcification also showed a decrease in scoring results at higher heart rates. The 64-slice MDCT with a slice thickness of 0.6 mm (dotted lines with circles) showed highly increased Ca-scores above 70 bpm for the high density calcification for all scoring methods. This is also seen for the medium density calcification for the volume score, whereas the equivelnt mass and Agatston score showed a peak in Ca-scores at 80 bpm. The low density calcification showed diminished results at higher heart rates for all scoring methods. The Ca-scores of the medium and low density calcification obtained with DSCT with a slice thickness of 3.0 mm (solid lines with triangles) were decreased at elevated heart rates. The results of the high density calcification were relatively constant over the whole range of heart rates. Finally DSCT at 0.6 mm (dotted lines with circles) showed increased results for Agatston and volume score of the high density calcification. The Agatston score of the medium density calcification showed a small decrease and relatively constant results were observed for the equivalent mass score of the high density calcification and volume and equivalent mass score of the medium density calcification. Diminishing results with increasing heart rate were observed for all methods for the low density calcification. 1 4 4 Fig. 4 a b c  2 5 5 5 Fig. 5 a b c c  5 The Δ-indexes were 53.2 for DSCT and 72.0 for 64-slice MDCT both with a slice thickness of 0.6 mm averaged over the scoring methods and densities. The Δ-indexes at 3.0 mm were 102.0 for DSCT and 96.9 for 64-slice MDCT averaged over the scoring methods and densities. Noise levels were as follows: 64-slice MDCT showed 36.1 ± 2.9 HU and 13.2 ± 1.2 HU for 0.6 and 3.0 mm slice thickness, respectively. DSCT showed 43.0 ± 1.6 HU and 16.1 ± 1.0 HU for 0.6 and 3.0 mm slice thickness, respectively. EBT with a slice thickness of 3.0 mm showed a noise level of 20.5 ± 0.8 HU. The noise did not vary at different heart rates. Discussion An assessment was made of Ca-scoring on 64-slice multi-detector computed tomography and dual-source computed tomography versus electron beam tomography on a moving cardiac phantom as a function of heart rate, slice thickness and calcification density using 3 different Ca-scoring methods. From the results it can be concluded that the Agatston, volume and equivalent mass scores depend on heart rate, slice thickness and the CT-system used. Furthermore DSCT is approximately 50% less susceptible to cardiac motion as 64-slice MDCT in Ca-scoring. 16 14 17 2 3 11 6 3 15 Fig. 6 black solid grey dotted grey  The susceptibility of calcium score on heart rate has been assessed by the CMS-index using the 3 different scoring methods available. The results show that the CMS-index of EBT is the lowest for all methods. Therefore it can be concluded that EBT is the least susceptible to cardiac motion. The CMS-index of DSCT is approximately half the CMS-index of 64-slice MDCT, showing a reduction of 50% of the influence of cardiac motion on Ca-scoring on DSCT with respect to 64-slice MDCT. These results can be explained with the improved temporal resolution of DSCT compared to 64-slice MDCT (83 vs. 165 ms). A reduction of the slice thickness also results in a lower CMS-index. Therefore we conclude that the use of a small slice thickness reduces the susceptibility to cardiac motion for both 64-slice MDCT and DSCT. The difference between CT-data and EBT-data has been assessed by the Δ-index using the Agatston and volume score, the equivalent mass results have been compared to the physical amount of calcium. The results show the lowest Δ-index for DSCT with a slice thickness of 0.6 mm for Agatston and volume score. The CT modalities at 0.6 mm and EBT showed similar Δ-indexes for the approximation to the physical mass. A reduction of the Δ-index was observed comparing the two CT-modalities at 0.6 mm and 3.0 mm. The best resemblance between EBT and CT was observed for DSCT with a slice thickness of 0.6 mm. 18 21 Limitations 22 o 11 The coronary artery we used for our simulation, the LAD, exhibits lesser motion than the LCX and especially the RCA, which exhibits very large motion swings especially in systole. In our study we, however, wanted to show the influence of motion on the coronary calcium score independent of a specific major coronary artery. We therefore have used motion curves with velocities similar to the LAD to simulate the motion, because if a dependency of calcium score on coronary motion could be proven for the lowest velocity of the LAD, we expect an even stronger dependence for the higher velocities of the LCX and RCA. In our study we have shown that for higher heart rates the under- or overestimation of the calcium score increases as a function of calcification density, independently of the absolute velocity of the artery, but depending on the relative heart rate difference from 0 bpm. Because this motion dependent effect is pronounced visible for the relative low velocity of the LAD, we expect that the results can also be applied to the vaster moving other major arteries. Conclusion The results of Ca-scoring are influenced by heart rate, slice thickness and modality used. DSCT is approximately 50% less susceptible to cardiac motion than 64-slice MDCT using a robot phantom. Susceptibility is further reduced with a smaller slice thickness. DSCT gives a better approximation of the absolute calcium score on EBT than results obtained with 64-slice MDCT when using a smaller slice thickness (0.6 mm). The two modalities show similar results when using larger slice thicknesses (3.0 mm). In general, the use of a smaller slice thickness further reduces the difference between CT-data and EBT-data. The best approximation to the physical amount of calcium was found using a small slice thickness, where 64-slice MDCT and DSCT show similar results. The best approximation of Ca-scoring on EBT is observed for DSCT with a slice thickness of 0.6 mm.