- Email: [email protected]
Impact on Image Quality and Radiation Dose of Third-Generation Dual-Source Computed Tomography of the Coronary Arteries
Impact on Image Quality and Radiation Dose of Third-Generation Dual-Source Computed Tomography of the Coronary Arteries Georg Apfaltrer, MDa, Dieter H. Szolar, MDb, Eric Wurzinger, BScb, Richard A.P. Takx, MD, MSc, PhDc, John W. Nance, MDd, Anja Dutschke, MDe, Sebastian Tschauner, MDa, Christian Loewe, MDf, Helmut Ringl, MDg, Erich Sorantin, MDa, and Paul Apfaltrer, MDg,h,* The aim of this study was to assess the image quality (IQ) and radiation dose of thirdgeneration dual-source computed tomography (CT) coronary angiography (cCTA) in comparison with 64-slice single-source CT. This retrospective study included 140 patients (73 men, mean age 62 – 11 years) with low-to-intermediate probability of coronary artery disease who underwent either third-generation dual-source cCTA using prospectively electrocardiography-triggered high-pitch spiral acquisition (n [ 70) (group 1) or retrospective electrocardiography-gated cCTA on a 64-slice CT system (n [ 70) (group 2). Contrast-to-noise and signal-to-noise ratios were measured within the aorta and coronary arteries. Subjective IQ was assessed using a 5-point Likert scale. Effective dose was estimated using specific conversion factors. The contrast-to-noise ratio of group 1 was significantly higher than group 2 at all levels (all p <0.001). Signal-to-noise ratio of group 1 was also significantly higher than group 2 (p <0.05), except for the distal left circumflex artery. Subjective IQ for group 1 was rated significantly better than for group 2 (median score [25th to 75th percentile]: 1 [1 to 2] vs 2 [2 to 3]; p <0.001). The median effective dose was 1.55 mSv (1.09 to 1.88) in group 1 versus 12.29 mSv (11.63 to 14.36) in group 2 (p <0.001) which corresponds to a mean radiation dose reduction of 87.4%. In conclusion, implementation of third-generation dual-source CT system for cCTA leads to improved IQ with significant radiation dose savings. Ó 2017 Elsevier Inc. All rights reserved. (Am J Cardiol 2017;119:1156e1161) Advancements in computed tomography (CT) technology have allowed coronary CT angiography (cCTA) to become a widely established technique for coronary artery assessment in selected patients with low-to-intermediate probability of coronary artery disease (CAD).1e6 cCTA enables direct visualization of the coronary arteries, providing a fast, noninvasive assessment of atherosclerotic burden and, importantly, reliable exclusion of CAD with its a Division of Pediatric Radiology, Department of Radiology, Medical University of Graz, Graz, Austria; bDiagnostikum Graz-Südwest, Graz, Austria; cDepartment of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands; dDivision of Cardiovascular Imaging, Department of Radiology and Radiological Science, Medical University of South Carolina, Charleston, South Carolina; eDepartment of Diagnostic and Interventional Radiology, University Hospital Heidelberg, Heidelberg, Germany; fDivision of Cardiovascular and Interventional Radiology, Department of Biomedical Imaging and Image-Guided Therapy, Medical University of Vienna, Vienna, Austria; gDepartment of Biomedical Imaging and Image-Guided Therapy, Medical University of Vienna, Vienna, Austria; and hInstitute of Clinical Radiology and Nuclear Medicine, University Medical Center Mannheim, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany. Manuscript received November 4, 2016; revised manuscript received and accepted December 20, 2016. See page 1161 for disclosure information. *Corresponding author: Tel: þ43 (0)1 40400-48180; fax: þ43 (0)1 4040048980. E-mail address: [email protected] (P. Apfaltrer).
0002-9149/17/$ - see front matter Ó 2017 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.amjcard.2016.12.028
high negative predictive value. cCTA utilization continues to increase with the advent of technical innovations that have both increased diagnostic accuracy and reduced radiation doses, the latter being particularly important given increasing recognition of and concern regarding ionizing radiation exposure.7,8 Recently, third-generation dual-source 192-slice CT was introduced, with incremental improvements in detector width, gantry rotation time, and pitch settings which should improve both image quality (IQ) and radiation dose. Many clinicians may still be unfamiliar with the magnitude of radiation exposure arising from cCTA in daily practice and the tremendous progress in radiation dose reduction while maintaining or enhancing diagnostic IQ using high-end CT systems. Consequently, the purpose of this study was to assess the IQ and radiation dose of the third-generation dual-source cCTA in comparison with 64-slice single-source CT. Methods Our institutional review board waived the requirement for informed consent for this retrospective study. From October to December 2015, a total of 70 patients (median age 63.8 years; range 39.2 to 82.8 years), including 34 men (median age 61.5 years; range 39.2 to 75.5 years) and 36 women (median age 65.9 years; range 45.3 to 82.8 years), with low-to-intermediate probability of CAD underwent www.ajconline.org
Coronary Artery Disease/Dual-Source CT of the Coronary Arteries Table 1 Patient demographics and characteristics
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Table 2 Objective image quality
Patient characteristics
Group 1 Somatom Force (n¼70)
Group 2 Sensation 64 (n¼70)
p-value
Objective Image Quality
Male-to-female ratio Age (years) Body weight (kg) Body mass index (kg/m2) Average heart rate (beats/min) Dose-length product (mGycm)* Effective dose (mSv)*
34:36 63.8 8.7 80.2 14 27.5 4.1
39:31 59.9 12.2 78.4 15.5 27.1 4.6
n.s. n.s. n.s. n.s.
CT-Signal (HU)
61.8 7.4
62.3 6.3
n.s.
111.00 878.00 [77.75-134.00] [836.00-1025.50] 1.55 [1.09-1.88] 12.29 [11.63-14.36]
<0.001 <0.001
Unless otherwise specified, data are mean standard deviations. n.s. ¼ nonsignificant. * Medians [25th-75th-percentile].
cCTA on a third-generation dual-source CT system at our institution (group 1). For comparison, data from a equivalent number of 70 patients with low-to-intermediate probability of CAD were used (median age 59.9 years; range 33.4 to 88.2 years), including 39 men (median age 58.9 years; range 33.4 to 88.2 years) and 31 women (median age 61.1 years; range 36.2 to 78.5 years) who had undergone cCTA on a 64-slice CT system from August to October 2015 at our institution (group 2). For both groups of patients with low-tointermediate probability of CAD, patient selection was only chronological and no further inclusion and exclusion criteria were used. cCTA examinations were performed with a thirdgeneration 2 192-section dual-source CT system (Somatom Force; Siemens Healthineers, Forchheim, Germany) (group 1) at 90 KV or with a 64-slice CT system (Sensation 64; Siemens Healthineers) at 120 KV (group 2). Before the cCTA study, a nonenhanced electrocardiography (ECG)triggered CT acquisition was performed in all patients to obtain the coronary calcium score using scanner-specific scanning parameters as previously described.9 Contrast enhancement was achieved using a bolus-tracking protocol with a threshold of 100 HU within a region of interest placed in the ascending thoracic aorta. A volume of 50 ml of Iomeprol (Iomeron 400; Bracco Altana Pharma, Konstanz, Germany) was used in group 1, and 90 ml of the same contrast agent was used in group 2. In all patients, contrast material was injected at a flow rate of 5 ml/s through an 18gauge intravenous antecubital catheter, followed by 50 ml of saline at the same flow rate. Group 1 data sets were acquired through prospectively ECG-triggered high-pitch spiral acquisitions during diastole as previously described,10 whereas group 2 data sets were acquired through retrospective ECG gating.11 According to our institutional protocol, patients without contraindications and a heart rate >80 beats/min received up to 10 mg of bisoprolol (CONCOR tablets) oral and Beloc (AstraZeneca, London, England) intravenously in repeated 5-mg doses before image acquisition. All patients without contraindications received 0.4 mg of nitroglycerine spray sublingually before cCTA. The calcium score was calculated using dedicated software (CaScore; Siemens Healthineers) according to the Agatston method.9 All contrast-enhanced cCTA data were
Image noise (HU) Contrast-to-noise ratio (CNR)
Signal-to-noise ratio (SNR)
Group 1 Somatom Force (n¼70) Aorta LM pLAD pLCx pRCA dLAD dLCx dRCA
172.9 167.8 167.0 180.1 174.7 138.4 140.3 164.9 7.1 40.1
Group 2 Sensation 64 (n¼70) 380.2 393.1 358.4 390.5 395.2 292.9 323.3 333.1 31.2 26.1
p-value
78.0 76.8 81.6 103.9 74.9 83.9 86.6 84.6 7.4 11.2
<0.001 <0.001 <0.001 <0.001 <0.001 0.002 0.006 <0.001 0.683 <0.001
Aorta
499.8 480.1 464.5 483.5 471.1 354.7 378.2 438.3 31.7 50.0
LM pLAD pLCx pRCA dLAD dLCx dRCA Aorta
48.5 46.9 48.6 48.1 37.4 39.2 45.7 15.9
38.2 36.6 37.9 41.7 26.0 27.7 39.8 4.4
27.0 25.2 26.8 27.1 21.1 22.9 23.6 12.8
12.1 12.0 12.1 11.7 9.8 10.5 10.5 4.0
<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001
LM pLAD pLCx pRCA dLAD dLCx dRCA
15.3 14.8 15.4 15.0 11.3 12.1 14.1
4.5 4.6 4.8 4.7 3.8 3.9 5.1
13.3 12.1 13.2 13.3 9.9 11.0 11.1
4.0 4.0 4.7 3.9 3.6 4.1 3.7
0.006 <0.001 0.009 0.025 0.006 0.117 <0.001
Unless otherwise specified, data are mean standard deviations. dLAD ¼ distal left anterior descending artery; dLCx ¼ distal left circumflex artery; dRCA ¼ distal right coronary artery; HU ¼ Hounsfield units; LM ¼ left main coronary artery; pLAD ¼ proximal left anterior descending artery; pLCx ¼ proximal left circumflex artery; pRCA ¼ the proximal right coronary artery.
reconstructed with a section thickness of 0.5 mm in the axial plane. Third-generation dual-source CT data were reconstructed with a third-generation iterative reconstruction (IR) technique (advanced modeling iterative reconstruction [ADMIRE]; Siemens Healthineers) strength level 3 and a corresponding vascular kernel12,13; 64-slice CT system data were reconstructed from CTA raw data with traditional filtered back projection and corresponding vascular algorithm as previously described.11 Image evaluation was performed on a dedicated imageprocessing workstation (Syngo MMWP VE 36A; Siemens Healthineers). Both objective and subjective IQ were assessed for all patients in a blinded and randomized fashion by 2 blinded observers in consensus (GA, experience in cCTA: 6 years; PA, experience in cCTA: 7 years). All technical and personal identifiers were removed from the images. The CT attenuation (in Hounsfield units) was evaluated on the transverse images by drawing regions of interest (ROI) as large as possible inside the vessel lumen while avoiding arterial walls and artifacts.14 Measures were taken in the mid-ascending aorta, left main coronary artery, proximal left anterior descending artery, proximal left circumflex artery, proximal right coronary artery, distal left
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Table 3 Subjective image quality Score
1 2 3 4 5
Characteristics
Group 1 Somatom Force (n¼70)
no motion artifacts and clear delineation of the segment minor artifacts and mild blurring of the segment moderate artifacts and moderate blurring without structure discontinuity severe artifacts and doubling or discontinuity in the course of the segment image not evaluative and vessel structures not differentiable
37 24 7 2 0
(52.9%) (34.3%) (10%) (2.9%) (0%)
Group 2 Sensation 64 (n¼70) 10 35 13 9 3
(14.3%) (50.0%) (18.6%) (12.9%) (4.3%)
Total
47 59 20 11 3
(33.6%) (42.1%) (14.3%) (7.9%) (2.1%)
Likert scale adapted from Leschka et al.1
anterior descending artery, distal left circumflex artery, and the distal right coronary artery, and the mean of these values was used for further calculations. Image noise was defined as the SD of vessel attenuation at the mid-ascending aorta. Epicardial fat attenuation was measured, and contrastto-noise ratio (CNR) was calculated using the following formula: (ROImean-Fatmean)/FatSD. For subjective IQ assessment, both readers inspected all data sets noting the presence of motion artifacts and assessing IQ using a previously described 5-point Likert scale1 in consensus, in which a score of 1 indicated no motion artifacts and clear delineation of the segment; a score of 2, minor artifacts and mild blurring of the segment; a score of 3, moderate artifacts and moderate blurring without structure discontinuity; a score of 4, severe artifacts and doubling or discontinuity in the course of the segment; and a score of 5, image not evaluative and vessel structures not differentiable. Volume CT dose index values were indicated in the dose report of the CT system provided for each CT study, and individual radiation dose was estimated based on the doselength product (DLP) given by the CT system. For estimating effective dose (ED), a conversion factor was used according to the generic method presented in the European Guidelines for Multislice Computed Tomography (dose conversion coefficients for the chest in mSv/Gy$cm: 0.014).15 Continuous variables were expressed as mean SD and categorical variables as frequencies (percentages). Quantitative and subjective IQ was compared using paired-samples t test and nonparametric Wilcoxon signed-rank test. ED was estimated using specific conversion factors. All p values were 2 sided, with a value <0.05 considered statistically significant. Statistical analyses were performed using commercially available software (SPSS, version 22; IBM, Armonk, New York). Results Detailed characteristics of the patient population of both groups are provided in Table 1. Patients who were investigated on the Somatom Force (group 1) and the Sensation 64 (group 2) did not show significant differences in average heart rate during the scan (61.8 7.4 vs 62.3 6.3 beats/min, p >0.05). Eleven patients (15.7%) in group 1 showed arrhythmia during the scan compared with 12 patients (17.1%) in group 2. Quantitative image analyses are summarized in Table 2. CT attenuation values in group 1 reached significantly
Figure 1. Box plots demonstrate ED distribution for both groups.
higher values at all levels compared with group 2 (p <0.05). There was no significant difference in image noise at any level (p >0.05). CNR of group 1 was significantly higher than group 2 at all levels (all p <0.001). Signal-to-noise ratio in group 1 was also significantly higher than group 2 (p <0.05) except for 1 level, the distal left circumflex artery, where differences did not reach significant differences (p ¼ 0.117). Median IQ scores were significantly higher in group 1 (median score [25th to 75th percentile]: 1 [1 to 2] vs 2 [2 to 3], p <0.001). IQ scores showed no significant differences in patients with irregular heart rates during the scan between both groups (group 1: 11 patients, group 2: 12 patients; p >0.05). Table 3 displays subjective IQ scores for both groups. Volume CT dose index, DLP, and ED were all significantly lower in group 1. Median DLP in group 1 was 111.00 mGycm (77.75 to 134.00), corresponding to an estimated median ED of 1.55 mSv (1.09 to 1.88), whereas median DLP in group 2 was 878.00 mGycm (836.00 to 1,025.50), corresponding to an estimated ED of 12.29 mSv (11.63 to 14.36) (p <0.001). The dosimetric parameters are summarized in Table 1 and displayed in Figures 1 to 3. Discussion Our results show that cCTA using a third-generation dual-source CT system provides excellent IQ while
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Figure 2. Images in a 61-year-old man with chest pain. Coronary CT angiography was performed at 90 kV with a third-generation dual-source CT system (group 1), resulting in an ED of 0.9 mSv. (A) Image at the centerline of the RCA and the LAD coronary artery, (B) curved maximum intensity projections, and (C) 3-dimensional volume-rendered image show patent vessel lumina in the major epicardial vessels. Cx ¼ left circumflex artery; LAD ¼ left anterior descending; RCA ¼ right coronary artery.
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Figure 3. (A) Coronary CT angiography performed at a third-generation dual-source CT system (group 1) in a 51-year-old patient with chest pain. ED was 1.1 mSv, and 50 ml of contrast medium was used. (B) Coronary CT angiography performed 64-slice CT system (group 2) in a 56-year-old patient with chest pain. ED was 13.3 mSv, and 120 ml of contrast medium was used. Both 3-dimensional volume-rendered images show patent vessel lumens with excellent image qualities (score 1).
allowing significantly lower radiation dose exposure and contrast medium volume compared with 64-slice CT. The relatively recent rise in cCTA is a direct reflection of technical advances in CT scanner technology. cCTA studies are challenging because of intrinsic cardiac motion. This has been addressed through decreasing scan acquisition times, and each generation of MDCT, starting with the 4-slice scanners, resulted in a higher proportion of successfully examined patients. Subsequent development of dual-source CT systems offered incremental advantages in acquisition time as the scanner geometry effectively acquires double the data for each rotation compared with single-source systems, allowing increased pitch acquisitions, often obtained over only a single cardiac cycle. The high pitch, in addition to limiting dose to a single cardiac cycle, eliminates overlapping volume coverage of sequential transverse sections, resulting in submillisievert cCTA examinations.16,17 Initial investigations demonstrated that third-generation dualsource CT provides improved temporal resolution of 66 ms with increased z axis coverage per second, improving IQ compared with previous cCTA image acquisition strategies.18 Innovations in cCTA hardware and techniques have allowed for great reductions in patient radiation exposure over the past several years. Specific techniques include radiation dose modulation through the cardiac cycle (limiting full doses to the diagnostic portion of the cardiac cycle), high pitch imaging, tube voltage reduction, and iterative reconstruction techniques (which can maintain IQ at lower radiation doses). The results of our study highlight the combined impact of these innovations, with an 87.4% reduction in radiation exposure using prospective ECG triggering, increased pitch, decreased kV, and model-based iterative reconstruction. Previous studies have demonstrated that lowering the kilovoltage setting from standard 120 to 100 kV allows for a significant reduction in radiation exposure.19,20 The thirdgeneration dual-source CT system used in the present
study provides increased tube output compared with previous systems, which could overcome some of the previous limitations on low-kV imaging. Indeed, in the present study, patients were scanned at 90 KV with the latest generation scanner independent of their BMI, which was considerably higher than in previous studies and may in fact reflect a more generalizable patient cohort who underwent cCTA. Yet image noise did not preclude diagnostic evaluation, as reflected in the higher qualitative and quantitative IQ scores seen in the low-kV group 1 compared with the higher kV group 2. We also observed significantly higher mean attenuation values of the mid-ascending aorta, left main coronary artery, proximal left anterior descending artery, proximal left circumflex artery, proximal right coronary artery, distal left anterior descending artery, distal left circumflex artery, and the distal right coronary artery at 90 kV in group 1, which is in line with published data on low-kV imaging21 and can be explained by the photoelectric effect.22 Low-dose protocols have previously been limited because of high image noise and consequently low IQ. Increasing computer power has allowed for the reemergence of IR techniques that improve IQ, largely through reduction of image noise. This also allows for dose saving compared with traditional filtered back projection as diagnostic IQ can be maintained while acquiring images with lower doses.23e25 Using a third-generation IR technique (ADMIRE) in the low-kV study group (1) yielded a significant improvement of both signal-to-noise ratio and CNR for almost all coronary arteries and the aorta, which is in line with previous low-kV studies using IR techniques.1,18 Improving temporal resolution in subsequent generations of CT systems have resulted in a higher proportion of diagnostic examinations,26 a trend which also manifested in our investigation, in which we observed a significant higher median IQ score in the most current scanner generation (group 1) median score [25th to 75th percentile]: 1 [1 to 2] than with the institution’s former available scanner
Coronary Artery Disease/Dual-Source CT of the Coronary Arteries
generation in group 2 (2 [2 to 3]), p <0.001. Another advantage of low-tube voltage technique is feasible reduction in requisite contrast agent administration while maintaining diagnostic attenuation values. Cao et al27 described contrast media reduction by 30.5% when cCTA was performed using an 80-kV protocol rather than with a 120-kV protocol. According to our default institutional protocol, 90-kV studies were performed using 50 ml in group 1, corresponding to a 44.4% reduction in administered contrast agent comparison with group 2 (90 ml) while maintaining adequate or even improved results in IQ, as described earlier. This reduction on contrast volume is crucial given the growing number of diagnostic and therapeutic procedures; furthermore, contrast mediaeinduced acute kidney injury has become the third leading cause for hospitalacquired acute kidney injury.28 Our study has several limitations. First, and most importantly, this was a retrospective study with a relatively small patient cohort; larger, prospective studies will be required to confirming our findings. Second, comparisons were made between 2 consecutive patient groups. Intraindividual comparisons with repeat examinations using different protocols would strengthen our claims, but this is not possible for obvious ethical reasons.
11. 12.
13.
14.
15. 16. 17.
Disclosures The authors have no conflicts of interest to disclose. 1. Leschka S, Wildermuth S, Boehm T, Desbiolles L, Husmann L, Plass A, Koepfli P, Schepis T, Marincek B, Kaufmann PA, Alkadhi H. Noninvasive coronary angiography with 64-section CT: effect of average heart rate and heart rate variability on image quality. Radiology 2006;241:378e385. 2. Leschka S, Stolzmann P, Desbiolles L, Baumueller S, Goetti R, Schertler T, Scheffel H, Plass A, Falk V, Feuchtner G, Marincek B, Alkadhi H. Diagnostic accuracy of high-pitch dual-source CT for the assessment of coronary stenoses: first experience. Eur Radiol 2009;19: 2896e2903. 3. Apfaltrer P, Schoepf UJ, Vliegenthart R, Rowe GW, Spears JR, Fink C, Nance JW Jr. Coronary computed tomography—present status and future directions. Int J Clin Pract 2011;65(Suppl 173):3e13. 4. Leber AW, Johnson T, Becker A, von Ziegler F, Tittus J, Nikolaou K, Reiser M, Steinbeck G, Becker CR, Knez A. Diagnostic accuracy of dual-source multi-slice CT-coronary angiography in patients with an intermediate pretest likelihood for coronary artery disease. Eur Heart J 2007;28:2354e2360. 5. Husmann L, Schepis T, Scheffel H, Gaemperli O, Leschka S, Valenta I, Koepfli P, Desbiolles L, Stolzmann P, Marincek B, Alkadhi H, Kaufmann PA. Comparison of diagnostic accuracy of 64-slice computed tomography coronary angiography in patients with low, intermediate, and high cardiovascular risk. Acad Radiol 2008;15:452e461. 6. Fine JJ, Hopkins CB, Ruff N, Newton FC. Comparison of accuracy of 64-slice cardiovascular computed tomography with coronary angiography in patients with suspected coronary artery disease. Am J Cardiol 2006;97:173e174. 7. Hausleiter J, Meyer T, Hadamitzky M, Huber E, Zankl M, Martinoff S, Kastrati A, Schomig A. Radiation dose estimates from cardiac multislice computed tomography in daily practice: impact of different scanning protocols on effective dose estimates. Circulation 2006;113: 1305e1310. 8. Smith-Bindman R. Is computed tomography safe? N Engl J Med 2010;363:1e4. 9. Agatston AS, Janowitz WR, Hildner FJ, Zusmer NR, Viamonte M Jr, Detrano R. Quantification of coronary artery calcium using ultrafast computed tomography. J Am Coll Cardiol 1990;15:827e832. 10. Achenbach S, Goroll T, Seltmann M, Pflederer T, Anders K, Ropers D, Daniel WG, Uder M, Lell M, Marwan M. Detection of coronary artery
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stenoses by low-dose, prospectively ECG-triggered, high-pitch spiral coronary CT angiography. JACC Cardiovasc Imaging 2011;4: 328e337. Becker CR. Assessment of coronary arteries with CT. Radiol Clin North Am 2002;40:773e782. Gordic S, Morsbach F, Schmidt B, Allmendinger T, Flohr T, Husarik D, Baumueller S, Raupach R, Stolzmann P, Leschka S, Frauenfelder T, Alkadhi H. Ultralow-dose chest computed tomography for pulmonary nodule detection: first performance evaluation of single energy scanning with spectral shaping. Invest Radiol 2014;49: 465e473. Meyer M, Haubenreisser H, Schoepf UJ, Vliegenthart R, Leidecker C, Allmendinger T, Lehmann R, Sudarski S, Borggrefe M, Schoenberg SO, Henzler T. Closing in on the K edge: coronary CT angiography at 100, 80, and 70 kV-initial comparison of a secondversus a third-generation dual-source CT system. Radiology 2014;273: 373e382. Kim R, Park EA, Lee W, Chung JW. Feasibility of 320-row area detector CT coronary angiography using 40 mL of contrast material: assessment of image quality and diagnostic accuracy. Eur Radiol 2016;26:3802e3810. The 2007 recommendations of the International Commission on Radiological Protection. ICRP publication 103. Ann ICRP 2007;37: 1e332. Deak PD, Langner O, Lell M, Kalender WA. Effects of adaptive section collimation on patient radiation dose in multisection spiral CT. Radiology 2009;252:140e147. Fink C, Krissak R, Henzler T, Lechel U, Brix G, Takx RA, Nance JW, Abro JA, Schoenberg SO, Schoepf UJ. Radiation dose at coronary CT angiography: second-generation dual-source CT versus single-source 64-MDCT and first-generation dual-source CT. AJR Am J Roentgenol 2011;196:W550eW557. Shim SS, Kim Y, Lim SM. Improvement of image quality with betablocker premedication on ECG-gated 16-MDCT coronary angiography. AJR Am J Roentgenol 2005;184:649e654. Bischoff B, Hein F, Meyer T, Hadamitzky M, Martinoff S, Schomig A, Hausleiter J. Impact of a reduced tube voltage on CT angiography and radiation dose: results of the PROTECTION I study. JACC Cardiovasc Imaging 2009;2:940e946. Feuchtner GM, Jodocy D, Klauser A, Haberfellner B, Aglan I, Spoeck A, Hiehs S, Soegner P, Jaschke W. Radiation dose reduction by using 100-kV tube voltage in cardiac 64-slice computed tomography: a comparative study. Eur J Radiol 2010;75:e51ee56. Kalva SP, Sahani DV, Hahn PF, Saini S. Using the K-edge to improve contrast conspicuity and to lower radiation dose with a 16-MDCT: a phantom and human study. J Comput Assist Tomogr 2006;30: 391e397. Huda W, Scalzetti EM, Levin G. Technique factors and image quality as functions of patient weight at abdominal CT. Radiology 2000;217: 430e435. Gosling O, Loader R, Venables P, Roobottom C, Rowles N, Bellenger N, Morgan-Hughes G. A comparison of radiation doses between state-of-the-art multislice CT coronary angiography with iterative reconstruction, multislice CT coronary angiography with standard filtered back-projection and invasive diagnostic coronary angiography. Heart 2010;96:922e926. Prakash P, Kalra MK, Ackman JB, Digumarthy SR, Hsieh J, Do S, Shepard JA, Gilman MD. Diffuse lung disease: CT of the chest with adaptive statistical iterative reconstruction technique. Radiology 2010;256:261e269. Sagara Y, Hara AK, Pavlicek W, Silva AC, Paden RG, Wu Q, Abdominal CT. Comparison of low-dose CT with adaptive statistical iterative reconstruction and routine-dose CT with filtered back projection in 53 patients. AJR Am J Roentgenol 2010;195:713e719. Dewey M, Hoffmann H, Hamm B. CT coronary angiography using 16 and 64 simultaneous detector rows: intraindividual comparison. Rofo 2007;179:581e586. Cao JX, Wang YM, Lu JG, Zhang Y, Wang P, Yang C. Radiation and contrast agent doses reductions by using 80-kV tube voltage in coronary computed tomographic angiography: a comparative study. Eur J Radiol 2014;83:309e314. Seeliger E, Sendeski M, Rihal CS, Persson PB. Contrast-induced kidney injury: mechanisms, risk factors, and prevention. Eur Heart J 2012;33:2007e2015.
0002-9149/17/$ - see front matter Ó 2017 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.amjcard.2016.12.028
high negative predictive value. cCTA utilization continues to increase with the advent of technical innovations that have both increased diagnostic accuracy and reduced radiation doses, the latter being particularly important given increasing recognition of and concern regarding ionizing radiation exposure.7,8 Recently, third-generation dual-source 192-slice CT was introduced, with incremental improvements in detector width, gantry rotation time, and pitch settings which should improve both image quality (IQ) and radiation dose. Many clinicians may still be unfamiliar with the magnitude of radiation exposure arising from cCTA in daily practice and the tremendous progress in radiation dose reduction while maintaining or enhancing diagnostic IQ using high-end CT systems. Consequently, the purpose of this study was to assess the IQ and radiation dose of the third-generation dual-source cCTA in comparison with 64-slice single-source CT. Methods Our institutional review board waived the requirement for informed consent for this retrospective study. From October to December 2015, a total of 70 patients (median age 63.8 years; range 39.2 to 82.8 years), including 34 men (median age 61.5 years; range 39.2 to 75.5 years) and 36 women (median age 65.9 years; range 45.3 to 82.8 years), with low-to-intermediate probability of CAD underwent www.ajconline.org
Coronary Artery Disease/Dual-Source CT of the Coronary Arteries Table 1 Patient demographics and characteristics
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Table 2 Objective image quality
Patient characteristics
Group 1 Somatom Force (n¼70)
Group 2 Sensation 64 (n¼70)
p-value
Objective Image Quality
Male-to-female ratio Age (years) Body weight (kg) Body mass index (kg/m2) Average heart rate (beats/min) Dose-length product (mGycm)* Effective dose (mSv)*
34:36 63.8 8.7 80.2 14 27.5 4.1
39:31 59.9 12.2 78.4 15.5 27.1 4.6
n.s. n.s. n.s. n.s.
CT-Signal (HU)
61.8 7.4
62.3 6.3
n.s.
111.00 878.00 [77.75-134.00] [836.00-1025.50] 1.55 [1.09-1.88] 12.29 [11.63-14.36]
<0.001 <0.001
Unless otherwise specified, data are mean standard deviations. n.s. ¼ nonsignificant. * Medians [25th-75th-percentile].
cCTA on a third-generation dual-source CT system at our institution (group 1). For comparison, data from a equivalent number of 70 patients with low-to-intermediate probability of CAD were used (median age 59.9 years; range 33.4 to 88.2 years), including 39 men (median age 58.9 years; range 33.4 to 88.2 years) and 31 women (median age 61.1 years; range 36.2 to 78.5 years) who had undergone cCTA on a 64-slice CT system from August to October 2015 at our institution (group 2). For both groups of patients with low-tointermediate probability of CAD, patient selection was only chronological and no further inclusion and exclusion criteria were used. cCTA examinations were performed with a thirdgeneration 2 192-section dual-source CT system (Somatom Force; Siemens Healthineers, Forchheim, Germany) (group 1) at 90 KV or with a 64-slice CT system (Sensation 64; Siemens Healthineers) at 120 KV (group 2). Before the cCTA study, a nonenhanced electrocardiography (ECG)triggered CT acquisition was performed in all patients to obtain the coronary calcium score using scanner-specific scanning parameters as previously described.9 Contrast enhancement was achieved using a bolus-tracking protocol with a threshold of 100 HU within a region of interest placed in the ascending thoracic aorta. A volume of 50 ml of Iomeprol (Iomeron 400; Bracco Altana Pharma, Konstanz, Germany) was used in group 1, and 90 ml of the same contrast agent was used in group 2. In all patients, contrast material was injected at a flow rate of 5 ml/s through an 18gauge intravenous antecubital catheter, followed by 50 ml of saline at the same flow rate. Group 1 data sets were acquired through prospectively ECG-triggered high-pitch spiral acquisitions during diastole as previously described,10 whereas group 2 data sets were acquired through retrospective ECG gating.11 According to our institutional protocol, patients without contraindications and a heart rate >80 beats/min received up to 10 mg of bisoprolol (CONCOR tablets) oral and Beloc (AstraZeneca, London, England) intravenously in repeated 5-mg doses before image acquisition. All patients without contraindications received 0.4 mg of nitroglycerine spray sublingually before cCTA. The calcium score was calculated using dedicated software (CaScore; Siemens Healthineers) according to the Agatston method.9 All contrast-enhanced cCTA data were
Image noise (HU) Contrast-to-noise ratio (CNR)
Signal-to-noise ratio (SNR)
Group 1 Somatom Force (n¼70) Aorta LM pLAD pLCx pRCA dLAD dLCx dRCA
172.9 167.8 167.0 180.1 174.7 138.4 140.3 164.9 7.1 40.1
Group 2 Sensation 64 (n¼70) 380.2 393.1 358.4 390.5 395.2 292.9 323.3 333.1 31.2 26.1
p-value
78.0 76.8 81.6 103.9 74.9 83.9 86.6 84.6 7.4 11.2
<0.001 <0.001 <0.001 <0.001 <0.001 0.002 0.006 <0.001 0.683 <0.001
Aorta
499.8 480.1 464.5 483.5 471.1 354.7 378.2 438.3 31.7 50.0
LM pLAD pLCx pRCA dLAD dLCx dRCA Aorta
48.5 46.9 48.6 48.1 37.4 39.2 45.7 15.9
38.2 36.6 37.9 41.7 26.0 27.7 39.8 4.4
27.0 25.2 26.8 27.1 21.1 22.9 23.6 12.8
12.1 12.0 12.1 11.7 9.8 10.5 10.5 4.0
<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001
LM pLAD pLCx pRCA dLAD dLCx dRCA
15.3 14.8 15.4 15.0 11.3 12.1 14.1
4.5 4.6 4.8 4.7 3.8 3.9 5.1
13.3 12.1 13.2 13.3 9.9 11.0 11.1
4.0 4.0 4.7 3.9 3.6 4.1 3.7
0.006 <0.001 0.009 0.025 0.006 0.117 <0.001
Unless otherwise specified, data are mean standard deviations. dLAD ¼ distal left anterior descending artery; dLCx ¼ distal left circumflex artery; dRCA ¼ distal right coronary artery; HU ¼ Hounsfield units; LM ¼ left main coronary artery; pLAD ¼ proximal left anterior descending artery; pLCx ¼ proximal left circumflex artery; pRCA ¼ the proximal right coronary artery.
reconstructed with a section thickness of 0.5 mm in the axial plane. Third-generation dual-source CT data were reconstructed with a third-generation iterative reconstruction (IR) technique (advanced modeling iterative reconstruction [ADMIRE]; Siemens Healthineers) strength level 3 and a corresponding vascular kernel12,13; 64-slice CT system data were reconstructed from CTA raw data with traditional filtered back projection and corresponding vascular algorithm as previously described.11 Image evaluation was performed on a dedicated imageprocessing workstation (Syngo MMWP VE 36A; Siemens Healthineers). Both objective and subjective IQ were assessed for all patients in a blinded and randomized fashion by 2 blinded observers in consensus (GA, experience in cCTA: 6 years; PA, experience in cCTA: 7 years). All technical and personal identifiers were removed from the images. The CT attenuation (in Hounsfield units) was evaluated on the transverse images by drawing regions of interest (ROI) as large as possible inside the vessel lumen while avoiding arterial walls and artifacts.14 Measures were taken in the mid-ascending aorta, left main coronary artery, proximal left anterior descending artery, proximal left circumflex artery, proximal right coronary artery, distal left
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Table 3 Subjective image quality Score
1 2 3 4 5
Characteristics
Group 1 Somatom Force (n¼70)
no motion artifacts and clear delineation of the segment minor artifacts and mild blurring of the segment moderate artifacts and moderate blurring without structure discontinuity severe artifacts and doubling or discontinuity in the course of the segment image not evaluative and vessel structures not differentiable
37 24 7 2 0
(52.9%) (34.3%) (10%) (2.9%) (0%)
Group 2 Sensation 64 (n¼70) 10 35 13 9 3
(14.3%) (50.0%) (18.6%) (12.9%) (4.3%)
Total
47 59 20 11 3
(33.6%) (42.1%) (14.3%) (7.9%) (2.1%)
Likert scale adapted from Leschka et al.1
anterior descending artery, distal left circumflex artery, and the distal right coronary artery, and the mean of these values was used for further calculations. Image noise was defined as the SD of vessel attenuation at the mid-ascending aorta. Epicardial fat attenuation was measured, and contrastto-noise ratio (CNR) was calculated using the following formula: (ROImean-Fatmean)/FatSD. For subjective IQ assessment, both readers inspected all data sets noting the presence of motion artifacts and assessing IQ using a previously described 5-point Likert scale1 in consensus, in which a score of 1 indicated no motion artifacts and clear delineation of the segment; a score of 2, minor artifacts and mild blurring of the segment; a score of 3, moderate artifacts and moderate blurring without structure discontinuity; a score of 4, severe artifacts and doubling or discontinuity in the course of the segment; and a score of 5, image not evaluative and vessel structures not differentiable. Volume CT dose index values were indicated in the dose report of the CT system provided for each CT study, and individual radiation dose was estimated based on the doselength product (DLP) given by the CT system. For estimating effective dose (ED), a conversion factor was used according to the generic method presented in the European Guidelines for Multislice Computed Tomography (dose conversion coefficients for the chest in mSv/Gy$cm: 0.014).15 Continuous variables were expressed as mean SD and categorical variables as frequencies (percentages). Quantitative and subjective IQ was compared using paired-samples t test and nonparametric Wilcoxon signed-rank test. ED was estimated using specific conversion factors. All p values were 2 sided, with a value <0.05 considered statistically significant. Statistical analyses were performed using commercially available software (SPSS, version 22; IBM, Armonk, New York). Results Detailed characteristics of the patient population of both groups are provided in Table 1. Patients who were investigated on the Somatom Force (group 1) and the Sensation 64 (group 2) did not show significant differences in average heart rate during the scan (61.8 7.4 vs 62.3 6.3 beats/min, p >0.05). Eleven patients (15.7%) in group 1 showed arrhythmia during the scan compared with 12 patients (17.1%) in group 2. Quantitative image analyses are summarized in Table 2. CT attenuation values in group 1 reached significantly
Figure 1. Box plots demonstrate ED distribution for both groups.
higher values at all levels compared with group 2 (p <0.05). There was no significant difference in image noise at any level (p >0.05). CNR of group 1 was significantly higher than group 2 at all levels (all p <0.001). Signal-to-noise ratio in group 1 was also significantly higher than group 2 (p <0.05) except for 1 level, the distal left circumflex artery, where differences did not reach significant differences (p ¼ 0.117). Median IQ scores were significantly higher in group 1 (median score [25th to 75th percentile]: 1 [1 to 2] vs 2 [2 to 3], p <0.001). IQ scores showed no significant differences in patients with irregular heart rates during the scan between both groups (group 1: 11 patients, group 2: 12 patients; p >0.05). Table 3 displays subjective IQ scores for both groups. Volume CT dose index, DLP, and ED were all significantly lower in group 1. Median DLP in group 1 was 111.00 mGycm (77.75 to 134.00), corresponding to an estimated median ED of 1.55 mSv (1.09 to 1.88), whereas median DLP in group 2 was 878.00 mGycm (836.00 to 1,025.50), corresponding to an estimated ED of 12.29 mSv (11.63 to 14.36) (p <0.001). The dosimetric parameters are summarized in Table 1 and displayed in Figures 1 to 3. Discussion Our results show that cCTA using a third-generation dual-source CT system provides excellent IQ while
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Figure 2. Images in a 61-year-old man with chest pain. Coronary CT angiography was performed at 90 kV with a third-generation dual-source CT system (group 1), resulting in an ED of 0.9 mSv. (A) Image at the centerline of the RCA and the LAD coronary artery, (B) curved maximum intensity projections, and (C) 3-dimensional volume-rendered image show patent vessel lumina in the major epicardial vessels. Cx ¼ left circumflex artery; LAD ¼ left anterior descending; RCA ¼ right coronary artery.
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Figure 3. (A) Coronary CT angiography performed at a third-generation dual-source CT system (group 1) in a 51-year-old patient with chest pain. ED was 1.1 mSv, and 50 ml of contrast medium was used. (B) Coronary CT angiography performed 64-slice CT system (group 2) in a 56-year-old patient with chest pain. ED was 13.3 mSv, and 120 ml of contrast medium was used. Both 3-dimensional volume-rendered images show patent vessel lumens with excellent image qualities (score 1).
allowing significantly lower radiation dose exposure and contrast medium volume compared with 64-slice CT. The relatively recent rise in cCTA is a direct reflection of technical advances in CT scanner technology. cCTA studies are challenging because of intrinsic cardiac motion. This has been addressed through decreasing scan acquisition times, and each generation of MDCT, starting with the 4-slice scanners, resulted in a higher proportion of successfully examined patients. Subsequent development of dual-source CT systems offered incremental advantages in acquisition time as the scanner geometry effectively acquires double the data for each rotation compared with single-source systems, allowing increased pitch acquisitions, often obtained over only a single cardiac cycle. The high pitch, in addition to limiting dose to a single cardiac cycle, eliminates overlapping volume coverage of sequential transverse sections, resulting in submillisievert cCTA examinations.16,17 Initial investigations demonstrated that third-generation dualsource CT provides improved temporal resolution of 66 ms with increased z axis coverage per second, improving IQ compared with previous cCTA image acquisition strategies.18 Innovations in cCTA hardware and techniques have allowed for great reductions in patient radiation exposure over the past several years. Specific techniques include radiation dose modulation through the cardiac cycle (limiting full doses to the diagnostic portion of the cardiac cycle), high pitch imaging, tube voltage reduction, and iterative reconstruction techniques (which can maintain IQ at lower radiation doses). The results of our study highlight the combined impact of these innovations, with an 87.4% reduction in radiation exposure using prospective ECG triggering, increased pitch, decreased kV, and model-based iterative reconstruction. Previous studies have demonstrated that lowering the kilovoltage setting from standard 120 to 100 kV allows for a significant reduction in radiation exposure.19,20 The thirdgeneration dual-source CT system used in the present
study provides increased tube output compared with previous systems, which could overcome some of the previous limitations on low-kV imaging. Indeed, in the present study, patients were scanned at 90 KV with the latest generation scanner independent of their BMI, which was considerably higher than in previous studies and may in fact reflect a more generalizable patient cohort who underwent cCTA. Yet image noise did not preclude diagnostic evaluation, as reflected in the higher qualitative and quantitative IQ scores seen in the low-kV group 1 compared with the higher kV group 2. We also observed significantly higher mean attenuation values of the mid-ascending aorta, left main coronary artery, proximal left anterior descending artery, proximal left circumflex artery, proximal right coronary artery, distal left anterior descending artery, distal left circumflex artery, and the distal right coronary artery at 90 kV in group 1, which is in line with published data on low-kV imaging21 and can be explained by the photoelectric effect.22 Low-dose protocols have previously been limited because of high image noise and consequently low IQ. Increasing computer power has allowed for the reemergence of IR techniques that improve IQ, largely through reduction of image noise. This also allows for dose saving compared with traditional filtered back projection as diagnostic IQ can be maintained while acquiring images with lower doses.23e25 Using a third-generation IR technique (ADMIRE) in the low-kV study group (1) yielded a significant improvement of both signal-to-noise ratio and CNR for almost all coronary arteries and the aorta, which is in line with previous low-kV studies using IR techniques.1,18 Improving temporal resolution in subsequent generations of CT systems have resulted in a higher proportion of diagnostic examinations,26 a trend which also manifested in our investigation, in which we observed a significant higher median IQ score in the most current scanner generation (group 1) median score [25th to 75th percentile]: 1 [1 to 2] than with the institution’s former available scanner
Coronary Artery Disease/Dual-Source CT of the Coronary Arteries
generation in group 2 (2 [2 to 3]), p <0.001. Another advantage of low-tube voltage technique is feasible reduction in requisite contrast agent administration while maintaining diagnostic attenuation values. Cao et al27 described contrast media reduction by 30.5% when cCTA was performed using an 80-kV protocol rather than with a 120-kV protocol. According to our default institutional protocol, 90-kV studies were performed using 50 ml in group 1, corresponding to a 44.4% reduction in administered contrast agent comparison with group 2 (90 ml) while maintaining adequate or even improved results in IQ, as described earlier. This reduction on contrast volume is crucial given the growing number of diagnostic and therapeutic procedures; furthermore, contrast mediaeinduced acute kidney injury has become the third leading cause for hospitalacquired acute kidney injury.28 Our study has several limitations. First, and most importantly, this was a retrospective study with a relatively small patient cohort; larger, prospective studies will be required to confirming our findings. Second, comparisons were made between 2 consecutive patient groups. Intraindividual comparisons with repeat examinations using different protocols would strengthen our claims, but this is not possible for obvious ethical reasons.
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13.
14.
15. 16. 17.
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