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Approaches to ultra-low radiation dose coronary artery calcium scoring based on 3rd generation dual-source CT: A phantom study
European Journal of Radiology 85 (2016) 39–47
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European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Approaches to ultra-low radiation dose coronary artery calcium scoring based on 3rd generation dual-source CT: A phantom study Andrew D. McQuiston a , Giuseppe Muscogiuri a,b , U. Joseph Schoepf a,c,∗ , Felix G. Meinel a,d , Christian Canstein e , Akos Varga-Szemes a , Paola M. Cannao’ a,f , Julian L. Wichmann a,g , Thomas Allmendinger h , Rozemarijn Vliegenthart a,i , Carlo N. De Cecco a,b a
Department of Radiology and Radiological Science, Medical University of South Carolina, Charleston, SC, USA Department of Radiological Sciences, Oncology and Pathology, University of Rome “Sapienza”—Polo Pontino, Latina, Italy c Department of Medicine, Division of Cardiology, Medical University of South Carolina, Charleston, SC, USA d Institute for Clinical Radiology, Ludwig-Maximilians-University Hospital, Munich, Germany e Siemens Medical Solutions, Malvern, PA, USA f Scuola di Specializzazione in Radiodiagnostica, University of Milan, Milan, Italy g Department of Diagnostic and Interventional Radiology, University Hospital Frankfurt, Frankfurt, Germany h Siemens Healthcare, Forchheim, Germany i Center for Medical Imaging—North East Netherlands, Department of Radiology, University Medical Center Groningen, Groningen, The Netherlands b
a r t i c l e
i n f o
Article history: Received 2 June 2015 Received in revised form 26 August 2015 Accepted 30 October 2015 Keywords: Dual-source CT Agatston score Ultra-high-pitch acquisition Iterative reconstruction Radiation dose
a b s t r a c t Objectives: To investigate to what extent 3rd generation dual-source computed tomography (DSCT) can reduce radiation dose in coronary artery calcium scoring. Methods: Image acquisition was performed using a stationary calcification phantom. Prospectively electrocardiogram (ECG)-triggered 120 kV sequential, and 120 and Sn100 kV ultra-high pitch (UHP) acquisitions were performed with different tube currents (80, 60, 40, 20 mA). Images were reconstructed using filtered back projection (FBP) and 3rd generation iterative reconstruction (IR). Contrast-to-noise ratio (CNR), Agatston score, calcium volume, and radiation dose were assessed. For statistical analysis Friedman tests and Wilcoxon rank sum tests were used. Results: Even at reduced tube currents, the three acquisition techniques did not show significant differences in Agatston score (p = 0.4) or calcium volume (p = 0.08) with FBP reconstruction. Calcium volumes were significantly lower for 3rd generation IR compared to FBP reconstructions (p < 0.01). CTDIvol for the 120 kV sequential, 120 and Sn100 kV UHP acquisitions at 80 and 20 mA were 1.2–0.37, 0.48–0.17, and 0.07–0.02 mGy, respectively. Conclusion: 3rd generation DSCT enabled a reduction of tube current in both the sequential and UHP acquisitions without significantly affecting coronary calcium scoring. Tin filtered 100 kV scanning may allow accurate quantification of calcium score without correction of the HU threshold. © 2015 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Great efforts have been made in recent years to reduce radiation dose in coronary computed tomography (CT) angiography (CCTA)
Abbreviations: CCTA, coronary computed tomography angiography; DSCT, dualsource CT; UHP, ultra-high-pitch; IC, integrated circuit; HA, hydroxyapatite; FBP, filter back projection. ∗ Corresponding author at: Department of Radiology and Radiological Science, Medical University of South Carolina, Ashley River Tower, 25 Courtenay Drive, Charleston, SC 29425-2260, USA. Fax: +1 843 876 3157. E-mail address: [email protected] (U.J. Schoepf). http://dx.doi.org/10.1016/j.ejrad.2015.10.023 0720-048X/© 2015 Elsevier Ireland Ltd. All rights reserved.
[1]. Advanced dose modulation and acquisition techniques [2,3], use of prospective electrocardiogram (ECG)-triggering and highpitch acquisition [4], and in particular lowering tube voltage have proven to be effective and easy methods to achieve considerable dose reduction at CCTA with a resulting radiation dose less than one mSv [5–7]. Contrarily, the CT scan protocol for coronary calcium scoring (120–130 kV, up to 145 mA, prospective ECG-triggering, filtered back projection, 2.5–3 mm reconstructed slice thickness) is still largely unchanged across various CT platforms, as this is the protocol that has been validated extensively [8]. Decrease in tube voltage is generally not recommended because blooming artifacts from calcium increase at lower kV. Furthermore, lower kV results in
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higher Hounsfield Units, thus leading to overestimation of the calcium score unless the calcium segmentation threshold is increased [9,10]. A recently developed 3rd generation dual-source CT (DSCT) system [11] features a new ultra-high-pitch acquisition mode (UHP) which allows scan speeds up to 737 mm/s [12], potentially reducing dose even further. In addition, the presence of an integrated circuit (IC) detector allows for significant reduction of the electronic image noise, which translates to the possibility of lowering the tube currents without any reduction in image quality [13]. Furthermore, the CT system allows the application of a tin-filtered 100 kV spectrum (Sn100 kV), which selectively absorbs low-energy photons, resembling a 120 kV spectrum but at much lower radiation dose [14,15]. Spectral filtration may be a promising strategy for dose reduction in coronary artery calcium scoring, as already demonstrated in pulmonary nodule detection [16]. Therefore, the purpose of this study was to investigate whether 3rd generation dual-source CT allows performing accurate coronary artery calcium scoring with substantially decreased tube current, comparing the standard 120 kV sequential acquisition with Sn100 and 120 kV UHP acquisitions. In addition we tested the impact of a 3rd generation iterative reconstruction (IR) algorithm on coronary calcium quantification.
2. Materials and methods 2.1. Phantom setup An anthropomorphic thorax phantom (QRM, Moehrendorf, Germany) resembling a human chest was used in this study [17]. The phantom is equipped with a spine insert surrounded by softtissue equivalent material and artificial lungs (resin with a CT density of approximately 800HU at 120 kV). A cylindrical cardiac calcification insert (QRM-CCI, QRM) is positioned in a compartment representing the heart. The phantom measures 30 cm in the lateral axis and 20 cm in the antero-posterior axis. The insert contains cylindrical calcifications, radiating from the center in three lines. Each of the 3 lines represents a different density and each contains two distinct calcifications. The calcifications measure 5, 3, and 1 mm in diameter and have a density of 200, 400 and 800 mg hydroxyapatite (HA) per cm3 .
2.3. Image reconstruction and data analysis All image data were reconstructed using both filtered back projection (FBP) and advanced modeled iterative reconstruction (ADMIRE, Siemens) algorithms using a sharp reconstruction kernel (Qr36). For the ADMIRE algorithm, image data can be reconstructed at various strength levels (from 1 to 5). Following recommendation of the manufacturer, a strength level of 3 was used to optimize noise reduction and image impression. Images were reconstructed with 3 mm section thickness and 3 mm reconstruction increment, and evaluated on a dedicated analysis platform (syngo.via, Siemens). All analyses were performed by a radiologist with 5 years of experience in cardiac imaging blinded to the acquisition mode.
2.4. Quantitative image quality Circular regions of interest (ROI) approximately 0.2 cm2 in size were placed centrally in the large phantom calcifications of each density row to provide mean attenuation and standard deviation (SD) data for that row. A circular ROI approximately 1.0 cm2 was also placed in the region of soft tissue between the high and medium density rows for information on background attenuation and noise, the latter expressed as standard deviation. A single acquisition was performed for each kV setting, but all measurements of CT attenuation and image noise were performed on three different sections and the results were averaged. Contrast-to-noise ratio (CNR) was calculated as (CT attenuation of the large 800 mg calcification—CT attenuation of the soft tissue) divided by the standard deviation of the CT attenuation of soft tissue (image noise).
2.5. Calcium scoring For each tube current setting in the three acquisition modes, the calcium score according to Agatston [18] and calcium volume were measured for each calcification using the standard detection threshold of 130 Hounsfield units (HU) (syngo.via, Siemens). The total calcium score and total calcium volume for all calcifications combined was also calculated.
2.6. Radiation dose 2.2. CT acquisition parameters Images were acquired using a 3rd generation DSCT system (SOMATOM Force, Siemens Healthcare, Forchheim, Germany). Data were acquired according to three sets of parameters:
1. Standard sequential acquisition with prospective ECGtriggering, tube voltage of 120 kV, reference tube current ranging from 20 to 80 mA, 0.25 s gantry rotation time, 44 × 1.2 mm detector collimation. 2. Ultra-high-pitch spiral acquisition, tube voltage of 120 kV, reference tube current ranging from 20 to 80 mA, 0.25 s gantry rotation time, 192 × 0.6 mm detector collimation, pitch 3.2. 3. Ultra-high-pitch spiral acquisition, tube voltage of Sn100 kV, reference tube current ranging from 20 to 80 mA, 0.25 s gantry rotation time, 192 × 0.6 mm detector collimation, pitch 3.2. Images were acquired with reference tube current set at 80, 60, 40, and 20 mA. Automated tube current modulation (CAREDose 4D, Siemens) was enabled for all acquisitions.
Volume computed tomography index (CTDIvol ) values were automatically generated by the system for each acquisition. The CTDIvol for the 120 kV prospective, Sn100 kV UHP and 120 kV UHP protocols at 80, 60, 40 and 20 mA were recorded.
2.7. Statistical analysis For continuous variables, mean values and standard deviations were calculated. Objective image quality parameters, Agatston score, calcium volume, and CTDIvol were plotted against the different tube currents, for FBP and IR. The potential for dose reduction was calculated based on the percentage of the mean value of radiation dose between the three acquisition modes at different tube currents. Statistical difference of Agatston score and calcium volume between the different acquisition modes with the same reconstruction was assessed with the Friedman test. Statistical difference of CNR, Agatston score, and calcium volume with different reconstructions was evaluated with the Wilcoxon rank-sum test. All analyses were performed using MedCalc for Windows, version 13.3.1.0 (MedCalc Software, Ostend, Belgium).
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Fig. 1. Influence of tube current on visual image impression using filtered back projection reconstruction, (A) 120 kV prospective acquisition; (B) 120 kV UHP acquisition; (C) Sn100 kV UHP acquisition.
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Fig. 2. Influence of tube current on visual image impression using iterative reconstruction, (A) 120 kV prospective acquisition; (B) 120 kV UHP acquisition; (C) Sn100 kV UHP acquisition.
3. Results 3.1. Quantitative image quality In FBP reconstructions, noise for the 120 kV sequential acquisition ranged from 18 to 31 HU for 80 mA and 20 mA, respectively, from 19 to 30 HU for the 120 kV UHP, and from 21 to 49 HU for
Sn100 kV UHP (Fig. 1). For ADMIRE reconstructions, image noise ranged from 11 to 21 HU for 120 kV sequential acquisitions, from 12 to 21 HU for 120 kV UHP, and from 13 to 38 HU for Sn100 kV UHP acquisitions (Fig. 2). For the 120 kV sequential acquisitions with FBP, CNR ranged from 57 to 31. For the 120 kV UHP and Sn100 kV UHP acquisitions, CNR varied from 54 to 30 and from 32 to 15 HU, respectively. For IR
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Fig. 3. Trend of CNR in each acquisition using different tube currents in filtered back projection reconstruction (A) and iterative reconstruction (B).
images, CNR ranged from 94 to 44 for the standard 120 kV sequential mode, from 85 to 40 for 120 kV UHP, and from 40 to 17 for Sn100 kV UHP acquisitions. All the IR datasets showed a significantly higher CNR in comparison with FBP (p < 0.01) (Fig. 3). Regardless of reconstruction algorithm, the 120 kV sequential acquisitions showed no significant difference in CNR compared to 120 kV UHP (P=0.12). Additionally, the CNR for Sn100 kV UHP was significantly reduced (p=0.02) compared to other protocols.
3.2. Calcium volume Calcium volumes for the 120 kV sequential acquisitions at 80 and 20 mA using FBP were 669.6 and 663.6 mm3 , respectively. For the 120 kV UHP acquisition, these scores were 636.0 and
641.1 mm3 , and for Sn100 kV UHP 645.1 and 613.9 mm3 . With IR, these scores were 574.5 and 537.6 mm3 for 120 kV sequential, 525.7 and 517.8 mm3 for 120 kV UHP, and 462.7 and 465.6 mm3 for Sn100 kV (Fig. 4). Calcium volumes were significantly lower for third generation IR compared to FBP reconstructions (p < 0.01). No significant difference in calcium volume was observed between acquisition modes using FBP (p = 0.08). However, using third generation IR, calcium volume showed a decrease for the 120 kV UHP and Sn100 kV UHP acquisitions compared to 120 kV sequential acquisition (p = 0.01). 3.3. Agatston score For the 120 kV sequential acquisitions with FBP, Agatston scores of 686 and 701 were calculated at 80 and 20 mA, respectively. These
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Fig. 4. Trend of calcium volume in each acquisition using different tube currents with filtered back projection reconstruction (A) and iterative reconstruction (B).
scores were 688 and 686 for the 120 kV UHP acquisition, and 639 and 672 for Sn100 kV UHP acquisitions. For each acquisition protocol at 80 and 20 mA with the third generation IR algorithm, these scores were 594 and 579, 567 and 554, and 463 and 466, respectively (Fig. 5). Agatston scores were significantly lower with third generation IR compared to FBP reconstructions (p < 0.01). The difference observed in Agatston scores was not statistically significant between the three acquisitions with FBP (p = 0.4). In contrast, a significant difference was observed using IR, with a reduced Agatston score in Sn100 kV compared with the 120 kV sequential and 120 kV UHP (p = 0.01).
3.4. Radiation dose CTDIvol for the 120 kV sequential acquisition at reference tube currents of 80, 60, 40, and 20 mA was 1.2, 0.88, 0.58, and 0.37 mGy, respectively. Thus, reducing the tube current from 80 to 20 mA for the120 kV sequential acquisition can reduce radiation dose by 69%. At the same reference tube current settings, CTDIvol for the 120 kV UHP acquisition mode was 0.48, 0.36, 0.24, and 0.17 mGy, respectively, and 0.07, 0.05, 0.03, and 0.02 mGy for the Sn100 kV UHP acquisition (Fig. 6). Compared to 120 kV sequential at 20 mA, a mean reduction in CTDIvol of 54% was observed for 120 kV UHP at 20 mA and 91.9% for Sn100 kV UHP at 40 mA.
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Fig. 5. Trend of Agatston score in each acquisition using different tube currents with filtered back projection reconstruction (A) and iterative reconstruction (B).
4. Discussion Our phantom study suggests that 3rd generation DSCT has the potential to significantly reduce radiation dose for coronary calcium scoring without affecting calcium assessment by decreasing tube current to reference values as low as 20 mA using the 120 kV sequential and 120 kV UHP acquisitions, and to a reference value as low as 40 mA with the Sn100 kV UHP acquisition. In particular, the Sn100 kV UHP protocol with 40 mA allowed for a radiation dose reduction from 0.37 to 0.03 mGy, compared with the 120 kV sequential protocol at 20 mA. Mean CNR was significantly higher in the 120 kV sequential acquisition, slightly decreasing in the 120 kV UHP protocol and even further with the Sn100 kV UHP acquisition. In all acquisitions, the IC
detector allowed a significant tube current reduction while maintaining a diagnostic CNR in FBP data-sets. For the 120 kV sequential, 120 kV UHP and Sn100 kV UHP acquisitions, a decrease from 80 to 20 mA incurred a CNR reduction of 46%, 44% and 53%, respectively. In each acquisition mode, third generation IR resulted in a significant increase in CNR, mostly attributable to a reduction in image noise. In any case, with both FBP and IR, the Sn100 kV UHP protocol was not feasible at 20 mA due to high noise which impaired the recognition of low density calcifications. Regardless, CNR, calcium volume and Agatston score values showed a tendency to remain stable using FBP reconstruction through all the acquisition protocols and tube currents, the only exception being Sn100 kV UHP at 20 mA, where the calcifications with the lowest density (200 mg/cm3 CaHA) were not visible due to the high image
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Fig. 6. Influence on CTDIvol of different tube currents on each technique of acquisition.
noise. In a report by Voros et al. regarding guidelines for minimizing radiation exposure during acquisition of coronary artery calcium scans, the authors suggested a target image noise of <23 HU [19]. In our study, we observed that in FBP reconstructions using the 120 kV sequential protocol, the noise was below the target for all tube currents except 20 mAs; using the 120 kV UHP protocol, the target image noise was exceeded only at tube currents of 20 and 40 mA; with the Sn100 kV UHP protocol, this threshold was exceed for all the tube currents. With the application of the ADMIRE algorithm, the image noise was significantly reduced in all the reconstructions. All acquisitions under the 120 kV sequential and 120 kV UHP protocols demonstrated image noise measurements within the suggested range. The threshold was only exceeded in the Sn100 kV UHP acquisition at 20, 40, and 60 mA. Marwan et al. investigated calcium score assessment utilizing a prospectively ECG-triggered high-pitch acquisition protocol at 100 kV with a fixed tube current of 80 mA in 150 patients [10].
The authors demonstrated that using a threshold value of 147HU, instead of the standard 130HU used at 120 kV, allowed for accurate assessment of coronary artery calcium in comparison with the standard kV acquisition typically used in clinical practice. Similarly, Nakazato et al. [9] demonstrated in 60 patients that using a threshold of 147 HU for 100 kV acquisitions produced equivalent results at reduced radiation exposure when compared to the 120 kV protocol. In our phantom study, however, the application of tin filtration in combination with the 100 kV acquisition yielded no significant difference in calcium scores compared with the 120 kV sequential acquisition, even using the standard 130 HU threshold and FBP reconstruction. This result may be attributable to the fact that the Sn prefiltration eliminates lower energy photons, hardening the x-ray spectrum and obtaining attenuation values closer to a 120 kV acquisition. This is coupled with a significant reduction in radiation dose [20]. Although clinical confirmation of our results is needed, this
Fig. 7. Influence of third-generation iterative reconstruction algorithm in reducing blooming artifacts (A) compared with standard filtered back projection reconstruction (B) image.
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opens the door for using a Sn100 kV protocol for calcium scoring without correction of the HU threshold. We further demonstrated that the application of a third generation IR algorithm significantly lowers image noise thereby increasing CNR. Additionally, this algorithm reduces calcium score and volume measurements compared to FBP. Similar findings have been reported in previous patient investigations analyzing first and second generation IR algorithms [21,22]. However, a recent study by Schindler et al. using first and second generation iterative reconstruction and the same phantom used in our study, as well as in patients, and a study by Matsuura et al. performed on patients concluded that IR techniques have no substantial effect on reproducible quantification of coronary calcium when compared to FBP [23,24]. Our findings could be explained by the fact that the third generation iterative reconstruction algorithm seems to reduce blooming artifacts present in FBP image reconstructions, as shown in Fig. 7. This would lead to a reduction in calcium volume and Agatston score compared with FBP images. Further investigations should be conducted in a clinical setting to evaluate the extent of this blooming artifact reduction and to demonstrate whether the application of a third generation IR algorithm could affect the quantification of calcium volume and whether an adaptation of the commonly used calcium scoring thresholds and/or conversions factors might be necessary for use with the latest IR methods. Our study and its design present several limitations. First of all, we used a stationary phantom without taking into account the influence of cardiac motion. In the clinical setting, implementing an UHP protocol could have a substantial impact on image quality and calcium evaluation in patients with faster heart rates. Furthermore, the CT system currently does not permit using the Sn100 kV setting with ECG-synchronization, and thus it is not yet feasible to perform a calcium score acquisition with tin filtration in clinical practice as part of a cardiac CT protocol. However, the implementation of an ECG-synchronized Sn100 kV acquisition mode is planned as part of future software enhancements for this CT platform. In addition, we performed the experiment using a single phantom size mimicking a normal-sized patient. In obese patients, reducing tube voltage in a fashion similar to our phantom study could result in non-diagnostic image quality. Finally, we did not evaluate the effect of the different densities of calcifications on outcomes using IR, UHP methods, or tin filtration. Further research should be conducted in order to address these important aspects before translating the findings of our study into clinical practice. 5. Conclusions In conclusion, our phantom experiment suggests that using third-generation DSCT allows for a significant reduction in tube current both in the 120 kV sequential and UHP acquisitions without affecting coronary calcium assessment. Furthermore, the tin filtration allows for accurate quantification of calcium score at 100 kV without any correction of the HU threshold. References [1] C.N. De Cecco, F.G. Meinel, S.A. Chiaramida, P. Costello, F. Bamberg, U.J. Schoepf, Coronary artery computed tomography scanning, Circulation 129 (2014) 1341–1345. [2] M.J. Siegel, J.C. Ramirez-Giraldo, C. Hildebolt, D. Bradley, B. Schmidt, Automated low-kilovoltage selection in pediatric computed tomography angiography: phantom study evaluating effects on radiation dose and image quality, Invest. Radiol. 48 (2013) 584–589. [3] M. Soderberg, M. Gunnarsson, The effect of different adaptation strengths on image quality and radiation dose using Siemens Care Dose 4D, Radiat. Prot. Dosim. 139 (2010) 173–179. [4] L.J. Zhang, L. Qi, J. Wang, C.X. Tang, C.S. Zhou, X.M. Ji, et al., Feasibility of prospectively ECG-triggered high-pitch coronary CT angiography with 30 mL iodinated contrast agent at 70 kVp: initial experience, Eur. Radiol. 24 (2014) 1537–1546.
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Contents lists available at ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Approaches to ultra-low radiation dose coronary artery calcium scoring based on 3rd generation dual-source CT: A phantom study Andrew D. McQuiston a , Giuseppe Muscogiuri a,b , U. Joseph Schoepf a,c,∗ , Felix G. Meinel a,d , Christian Canstein e , Akos Varga-Szemes a , Paola M. Cannao’ a,f , Julian L. Wichmann a,g , Thomas Allmendinger h , Rozemarijn Vliegenthart a,i , Carlo N. De Cecco a,b a
Department of Radiology and Radiological Science, Medical University of South Carolina, Charleston, SC, USA Department of Radiological Sciences, Oncology and Pathology, University of Rome “Sapienza”—Polo Pontino, Latina, Italy c Department of Medicine, Division of Cardiology, Medical University of South Carolina, Charleston, SC, USA d Institute for Clinical Radiology, Ludwig-Maximilians-University Hospital, Munich, Germany e Siemens Medical Solutions, Malvern, PA, USA f Scuola di Specializzazione in Radiodiagnostica, University of Milan, Milan, Italy g Department of Diagnostic and Interventional Radiology, University Hospital Frankfurt, Frankfurt, Germany h Siemens Healthcare, Forchheim, Germany i Center for Medical Imaging—North East Netherlands, Department of Radiology, University Medical Center Groningen, Groningen, The Netherlands b
a r t i c l e
i n f o
Article history: Received 2 June 2015 Received in revised form 26 August 2015 Accepted 30 October 2015 Keywords: Dual-source CT Agatston score Ultra-high-pitch acquisition Iterative reconstruction Radiation dose
a b s t r a c t Objectives: To investigate to what extent 3rd generation dual-source computed tomography (DSCT) can reduce radiation dose in coronary artery calcium scoring. Methods: Image acquisition was performed using a stationary calcification phantom. Prospectively electrocardiogram (ECG)-triggered 120 kV sequential, and 120 and Sn100 kV ultra-high pitch (UHP) acquisitions were performed with different tube currents (80, 60, 40, 20 mA). Images were reconstructed using filtered back projection (FBP) and 3rd generation iterative reconstruction (IR). Contrast-to-noise ratio (CNR), Agatston score, calcium volume, and radiation dose were assessed. For statistical analysis Friedman tests and Wilcoxon rank sum tests were used. Results: Even at reduced tube currents, the three acquisition techniques did not show significant differences in Agatston score (p = 0.4) or calcium volume (p = 0.08) with FBP reconstruction. Calcium volumes were significantly lower for 3rd generation IR compared to FBP reconstructions (p < 0.01). CTDIvol for the 120 kV sequential, 120 and Sn100 kV UHP acquisitions at 80 and 20 mA were 1.2–0.37, 0.48–0.17, and 0.07–0.02 mGy, respectively. Conclusion: 3rd generation DSCT enabled a reduction of tube current in both the sequential and UHP acquisitions without significantly affecting coronary calcium scoring. Tin filtered 100 kV scanning may allow accurate quantification of calcium score without correction of the HU threshold. © 2015 Elsevier Ireland Ltd. All rights reserved.
1. Introduction Great efforts have been made in recent years to reduce radiation dose in coronary computed tomography (CT) angiography (CCTA)
Abbreviations: CCTA, coronary computed tomography angiography; DSCT, dualsource CT; UHP, ultra-high-pitch; IC, integrated circuit; HA, hydroxyapatite; FBP, filter back projection. ∗ Corresponding author at: Department of Radiology and Radiological Science, Medical University of South Carolina, Ashley River Tower, 25 Courtenay Drive, Charleston, SC 29425-2260, USA. Fax: +1 843 876 3157. E-mail address: [email protected] (U.J. Schoepf). http://dx.doi.org/10.1016/j.ejrad.2015.10.023 0720-048X/© 2015 Elsevier Ireland Ltd. All rights reserved.
[1]. Advanced dose modulation and acquisition techniques [2,3], use of prospective electrocardiogram (ECG)-triggering and highpitch acquisition [4], and in particular lowering tube voltage have proven to be effective and easy methods to achieve considerable dose reduction at CCTA with a resulting radiation dose less than one mSv [5–7]. Contrarily, the CT scan protocol for coronary calcium scoring (120–130 kV, up to 145 mA, prospective ECG-triggering, filtered back projection, 2.5–3 mm reconstructed slice thickness) is still largely unchanged across various CT platforms, as this is the protocol that has been validated extensively [8]. Decrease in tube voltage is generally not recommended because blooming artifacts from calcium increase at lower kV. Furthermore, lower kV results in
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higher Hounsfield Units, thus leading to overestimation of the calcium score unless the calcium segmentation threshold is increased [9,10]. A recently developed 3rd generation dual-source CT (DSCT) system [11] features a new ultra-high-pitch acquisition mode (UHP) which allows scan speeds up to 737 mm/s [12], potentially reducing dose even further. In addition, the presence of an integrated circuit (IC) detector allows for significant reduction of the electronic image noise, which translates to the possibility of lowering the tube currents without any reduction in image quality [13]. Furthermore, the CT system allows the application of a tin-filtered 100 kV spectrum (Sn100 kV), which selectively absorbs low-energy photons, resembling a 120 kV spectrum but at much lower radiation dose [14,15]. Spectral filtration may be a promising strategy for dose reduction in coronary artery calcium scoring, as already demonstrated in pulmonary nodule detection [16]. Therefore, the purpose of this study was to investigate whether 3rd generation dual-source CT allows performing accurate coronary artery calcium scoring with substantially decreased tube current, comparing the standard 120 kV sequential acquisition with Sn100 and 120 kV UHP acquisitions. In addition we tested the impact of a 3rd generation iterative reconstruction (IR) algorithm on coronary calcium quantification.
2. Materials and methods 2.1. Phantom setup An anthropomorphic thorax phantom (QRM, Moehrendorf, Germany) resembling a human chest was used in this study [17]. The phantom is equipped with a spine insert surrounded by softtissue equivalent material and artificial lungs (resin with a CT density of approximately 800HU at 120 kV). A cylindrical cardiac calcification insert (QRM-CCI, QRM) is positioned in a compartment representing the heart. The phantom measures 30 cm in the lateral axis and 20 cm in the antero-posterior axis. The insert contains cylindrical calcifications, radiating from the center in three lines. Each of the 3 lines represents a different density and each contains two distinct calcifications. The calcifications measure 5, 3, and 1 mm in diameter and have a density of 200, 400 and 800 mg hydroxyapatite (HA) per cm3 .
2.3. Image reconstruction and data analysis All image data were reconstructed using both filtered back projection (FBP) and advanced modeled iterative reconstruction (ADMIRE, Siemens) algorithms using a sharp reconstruction kernel (Qr36). For the ADMIRE algorithm, image data can be reconstructed at various strength levels (from 1 to 5). Following recommendation of the manufacturer, a strength level of 3 was used to optimize noise reduction and image impression. Images were reconstructed with 3 mm section thickness and 3 mm reconstruction increment, and evaluated on a dedicated analysis platform (syngo.via, Siemens). All analyses were performed by a radiologist with 5 years of experience in cardiac imaging blinded to the acquisition mode.
2.4. Quantitative image quality Circular regions of interest (ROI) approximately 0.2 cm2 in size were placed centrally in the large phantom calcifications of each density row to provide mean attenuation and standard deviation (SD) data for that row. A circular ROI approximately 1.0 cm2 was also placed in the region of soft tissue between the high and medium density rows for information on background attenuation and noise, the latter expressed as standard deviation. A single acquisition was performed for each kV setting, but all measurements of CT attenuation and image noise were performed on three different sections and the results were averaged. Contrast-to-noise ratio (CNR) was calculated as (CT attenuation of the large 800 mg calcification—CT attenuation of the soft tissue) divided by the standard deviation of the CT attenuation of soft tissue (image noise).
2.5. Calcium scoring For each tube current setting in the three acquisition modes, the calcium score according to Agatston [18] and calcium volume were measured for each calcification using the standard detection threshold of 130 Hounsfield units (HU) (syngo.via, Siemens). The total calcium score and total calcium volume for all calcifications combined was also calculated.
2.6. Radiation dose 2.2. CT acquisition parameters Images were acquired using a 3rd generation DSCT system (SOMATOM Force, Siemens Healthcare, Forchheim, Germany). Data were acquired according to three sets of parameters:
1. Standard sequential acquisition with prospective ECGtriggering, tube voltage of 120 kV, reference tube current ranging from 20 to 80 mA, 0.25 s gantry rotation time, 44 × 1.2 mm detector collimation. 2. Ultra-high-pitch spiral acquisition, tube voltage of 120 kV, reference tube current ranging from 20 to 80 mA, 0.25 s gantry rotation time, 192 × 0.6 mm detector collimation, pitch 3.2. 3. Ultra-high-pitch spiral acquisition, tube voltage of Sn100 kV, reference tube current ranging from 20 to 80 mA, 0.25 s gantry rotation time, 192 × 0.6 mm detector collimation, pitch 3.2. Images were acquired with reference tube current set at 80, 60, 40, and 20 mA. Automated tube current modulation (CAREDose 4D, Siemens) was enabled for all acquisitions.
Volume computed tomography index (CTDIvol ) values were automatically generated by the system for each acquisition. The CTDIvol for the 120 kV prospective, Sn100 kV UHP and 120 kV UHP protocols at 80, 60, 40 and 20 mA were recorded.
2.7. Statistical analysis For continuous variables, mean values and standard deviations were calculated. Objective image quality parameters, Agatston score, calcium volume, and CTDIvol were plotted against the different tube currents, for FBP and IR. The potential for dose reduction was calculated based on the percentage of the mean value of radiation dose between the three acquisition modes at different tube currents. Statistical difference of Agatston score and calcium volume between the different acquisition modes with the same reconstruction was assessed with the Friedman test. Statistical difference of CNR, Agatston score, and calcium volume with different reconstructions was evaluated with the Wilcoxon rank-sum test. All analyses were performed using MedCalc for Windows, version 13.3.1.0 (MedCalc Software, Ostend, Belgium).
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Fig. 1. Influence of tube current on visual image impression using filtered back projection reconstruction, (A) 120 kV prospective acquisition; (B) 120 kV UHP acquisition; (C) Sn100 kV UHP acquisition.
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Fig. 2. Influence of tube current on visual image impression using iterative reconstruction, (A) 120 kV prospective acquisition; (B) 120 kV UHP acquisition; (C) Sn100 kV UHP acquisition.
3. Results 3.1. Quantitative image quality In FBP reconstructions, noise for the 120 kV sequential acquisition ranged from 18 to 31 HU for 80 mA and 20 mA, respectively, from 19 to 30 HU for the 120 kV UHP, and from 21 to 49 HU for
Sn100 kV UHP (Fig. 1). For ADMIRE reconstructions, image noise ranged from 11 to 21 HU for 120 kV sequential acquisitions, from 12 to 21 HU for 120 kV UHP, and from 13 to 38 HU for Sn100 kV UHP acquisitions (Fig. 2). For the 120 kV sequential acquisitions with FBP, CNR ranged from 57 to 31. For the 120 kV UHP and Sn100 kV UHP acquisitions, CNR varied from 54 to 30 and from 32 to 15 HU, respectively. For IR
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Fig. 3. Trend of CNR in each acquisition using different tube currents in filtered back projection reconstruction (A) and iterative reconstruction (B).
images, CNR ranged from 94 to 44 for the standard 120 kV sequential mode, from 85 to 40 for 120 kV UHP, and from 40 to 17 for Sn100 kV UHP acquisitions. All the IR datasets showed a significantly higher CNR in comparison with FBP (p < 0.01) (Fig. 3). Regardless of reconstruction algorithm, the 120 kV sequential acquisitions showed no significant difference in CNR compared to 120 kV UHP (P=0.12). Additionally, the CNR for Sn100 kV UHP was significantly reduced (p=0.02) compared to other protocols.
3.2. Calcium volume Calcium volumes for the 120 kV sequential acquisitions at 80 and 20 mA using FBP were 669.6 and 663.6 mm3 , respectively. For the 120 kV UHP acquisition, these scores were 636.0 and
641.1 mm3 , and for Sn100 kV UHP 645.1 and 613.9 mm3 . With IR, these scores were 574.5 and 537.6 mm3 for 120 kV sequential, 525.7 and 517.8 mm3 for 120 kV UHP, and 462.7 and 465.6 mm3 for Sn100 kV (Fig. 4). Calcium volumes were significantly lower for third generation IR compared to FBP reconstructions (p < 0.01). No significant difference in calcium volume was observed between acquisition modes using FBP (p = 0.08). However, using third generation IR, calcium volume showed a decrease for the 120 kV UHP and Sn100 kV UHP acquisitions compared to 120 kV sequential acquisition (p = 0.01). 3.3. Agatston score For the 120 kV sequential acquisitions with FBP, Agatston scores of 686 and 701 were calculated at 80 and 20 mA, respectively. These
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Fig. 4. Trend of calcium volume in each acquisition using different tube currents with filtered back projection reconstruction (A) and iterative reconstruction (B).
scores were 688 and 686 for the 120 kV UHP acquisition, and 639 and 672 for Sn100 kV UHP acquisitions. For each acquisition protocol at 80 and 20 mA with the third generation IR algorithm, these scores were 594 and 579, 567 and 554, and 463 and 466, respectively (Fig. 5). Agatston scores were significantly lower with third generation IR compared to FBP reconstructions (p < 0.01). The difference observed in Agatston scores was not statistically significant between the three acquisitions with FBP (p = 0.4). In contrast, a significant difference was observed using IR, with a reduced Agatston score in Sn100 kV compared with the 120 kV sequential and 120 kV UHP (p = 0.01).
3.4. Radiation dose CTDIvol for the 120 kV sequential acquisition at reference tube currents of 80, 60, 40, and 20 mA was 1.2, 0.88, 0.58, and 0.37 mGy, respectively. Thus, reducing the tube current from 80 to 20 mA for the120 kV sequential acquisition can reduce radiation dose by 69%. At the same reference tube current settings, CTDIvol for the 120 kV UHP acquisition mode was 0.48, 0.36, 0.24, and 0.17 mGy, respectively, and 0.07, 0.05, 0.03, and 0.02 mGy for the Sn100 kV UHP acquisition (Fig. 6). Compared to 120 kV sequential at 20 mA, a mean reduction in CTDIvol of 54% was observed for 120 kV UHP at 20 mA and 91.9% for Sn100 kV UHP at 40 mA.
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Fig. 5. Trend of Agatston score in each acquisition using different tube currents with filtered back projection reconstruction (A) and iterative reconstruction (B).
4. Discussion Our phantom study suggests that 3rd generation DSCT has the potential to significantly reduce radiation dose for coronary calcium scoring without affecting calcium assessment by decreasing tube current to reference values as low as 20 mA using the 120 kV sequential and 120 kV UHP acquisitions, and to a reference value as low as 40 mA with the Sn100 kV UHP acquisition. In particular, the Sn100 kV UHP protocol with 40 mA allowed for a radiation dose reduction from 0.37 to 0.03 mGy, compared with the 120 kV sequential protocol at 20 mA. Mean CNR was significantly higher in the 120 kV sequential acquisition, slightly decreasing in the 120 kV UHP protocol and even further with the Sn100 kV UHP acquisition. In all acquisitions, the IC
detector allowed a significant tube current reduction while maintaining a diagnostic CNR in FBP data-sets. For the 120 kV sequential, 120 kV UHP and Sn100 kV UHP acquisitions, a decrease from 80 to 20 mA incurred a CNR reduction of 46%, 44% and 53%, respectively. In each acquisition mode, third generation IR resulted in a significant increase in CNR, mostly attributable to a reduction in image noise. In any case, with both FBP and IR, the Sn100 kV UHP protocol was not feasible at 20 mA due to high noise which impaired the recognition of low density calcifications. Regardless, CNR, calcium volume and Agatston score values showed a tendency to remain stable using FBP reconstruction through all the acquisition protocols and tube currents, the only exception being Sn100 kV UHP at 20 mA, where the calcifications with the lowest density (200 mg/cm3 CaHA) were not visible due to the high image
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Fig. 6. Influence on CTDIvol of different tube currents on each technique of acquisition.
noise. In a report by Voros et al. regarding guidelines for minimizing radiation exposure during acquisition of coronary artery calcium scans, the authors suggested a target image noise of <23 HU [19]. In our study, we observed that in FBP reconstructions using the 120 kV sequential protocol, the noise was below the target for all tube currents except 20 mAs; using the 120 kV UHP protocol, the target image noise was exceeded only at tube currents of 20 and 40 mA; with the Sn100 kV UHP protocol, this threshold was exceed for all the tube currents. With the application of the ADMIRE algorithm, the image noise was significantly reduced in all the reconstructions. All acquisitions under the 120 kV sequential and 120 kV UHP protocols demonstrated image noise measurements within the suggested range. The threshold was only exceeded in the Sn100 kV UHP acquisition at 20, 40, and 60 mA. Marwan et al. investigated calcium score assessment utilizing a prospectively ECG-triggered high-pitch acquisition protocol at 100 kV with a fixed tube current of 80 mA in 150 patients [10].
The authors demonstrated that using a threshold value of 147HU, instead of the standard 130HU used at 120 kV, allowed for accurate assessment of coronary artery calcium in comparison with the standard kV acquisition typically used in clinical practice. Similarly, Nakazato et al. [9] demonstrated in 60 patients that using a threshold of 147 HU for 100 kV acquisitions produced equivalent results at reduced radiation exposure when compared to the 120 kV protocol. In our phantom study, however, the application of tin filtration in combination with the 100 kV acquisition yielded no significant difference in calcium scores compared with the 120 kV sequential acquisition, even using the standard 130 HU threshold and FBP reconstruction. This result may be attributable to the fact that the Sn prefiltration eliminates lower energy photons, hardening the x-ray spectrum and obtaining attenuation values closer to a 120 kV acquisition. This is coupled with a significant reduction in radiation dose [20]. Although clinical confirmation of our results is needed, this
Fig. 7. Influence of third-generation iterative reconstruction algorithm in reducing blooming artifacts (A) compared with standard filtered back projection reconstruction (B) image.
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opens the door for using a Sn100 kV protocol for calcium scoring without correction of the HU threshold. We further demonstrated that the application of a third generation IR algorithm significantly lowers image noise thereby increasing CNR. Additionally, this algorithm reduces calcium score and volume measurements compared to FBP. Similar findings have been reported in previous patient investigations analyzing first and second generation IR algorithms [21,22]. However, a recent study by Schindler et al. using first and second generation iterative reconstruction and the same phantom used in our study, as well as in patients, and a study by Matsuura et al. performed on patients concluded that IR techniques have no substantial effect on reproducible quantification of coronary calcium when compared to FBP [23,24]. Our findings could be explained by the fact that the third generation iterative reconstruction algorithm seems to reduce blooming artifacts present in FBP image reconstructions, as shown in Fig. 7. This would lead to a reduction in calcium volume and Agatston score compared with FBP images. Further investigations should be conducted in a clinical setting to evaluate the extent of this blooming artifact reduction and to demonstrate whether the application of a third generation IR algorithm could affect the quantification of calcium volume and whether an adaptation of the commonly used calcium scoring thresholds and/or conversions factors might be necessary for use with the latest IR methods. Our study and its design present several limitations. First of all, we used a stationary phantom without taking into account the influence of cardiac motion. In the clinical setting, implementing an UHP protocol could have a substantial impact on image quality and calcium evaluation in patients with faster heart rates. Furthermore, the CT system currently does not permit using the Sn100 kV setting with ECG-synchronization, and thus it is not yet feasible to perform a calcium score acquisition with tin filtration in clinical practice as part of a cardiac CT protocol. However, the implementation of an ECG-synchronized Sn100 kV acquisition mode is planned as part of future software enhancements for this CT platform. In addition, we performed the experiment using a single phantom size mimicking a normal-sized patient. In obese patients, reducing tube voltage in a fashion similar to our phantom study could result in non-diagnostic image quality. Finally, we did not evaluate the effect of the different densities of calcifications on outcomes using IR, UHP methods, or tin filtration. Further research should be conducted in order to address these important aspects before translating the findings of our study into clinical practice. 5. Conclusions In conclusion, our phantom experiment suggests that using third-generation DSCT allows for a significant reduction in tube current both in the 120 kV sequential and UHP acquisitions without affecting coronary calcium assessment. Furthermore, the tin filtration allows for accurate quantification of calcium score at 100 kV without any correction of the HU threshold. References [1] C.N. De Cecco, F.G. Meinel, S.A. Chiaramida, P. Costello, F. Bamberg, U.J. Schoepf, Coronary artery computed tomography scanning, Circulation 129 (2014) 1341–1345. [2] M.J. Siegel, J.C. Ramirez-Giraldo, C. Hildebolt, D. Bradley, B. Schmidt, Automated low-kilovoltage selection in pediatric computed tomography angiography: phantom study evaluating effects on radiation dose and image quality, Invest. Radiol. 48 (2013) 584–589. [3] M. Soderberg, M. Gunnarsson, The effect of different adaptation strengths on image quality and radiation dose using Siemens Care Dose 4D, Radiat. Prot. Dosim. 139 (2010) 173–179. [4] L.J. Zhang, L. Qi, J. Wang, C.X. Tang, C.S. Zhou, X.M. Ji, et al., Feasibility of prospectively ECG-triggered high-pitch coronary CT angiography with 30 mL iodinated contrast agent at 70 kVp: initial experience, Eur. Radiol. 24 (2014) 1537–1546.
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