Impact of a Reduced Tube Voltage on CT Angiography and Radiation DoseResults of the PROTECTION I Study
Bernhard Bischoff, MD*,
Franziska Hein, MD*,
Tanja Meyer, MD*,
Martin Hadamitzky, MD*,
Stefan Martinoff, MD ,
Albert Schömig, MD*,
Jörg Hausleiter, MD*,*
* Klinik für Herz- und Kreislauferkrankungen, Deutsches Herzzentrum München, Klinik an der TU München, Munich, Germany
Institut für Radiologie und Nuklearmedizin, Deutsches Herzzentrum München, Klinik an der TU München, Munich, Germany
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Abstract
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Objectives: The aim of this study was to determine the impact of a reduced 100-kV tube voltage on image quality and radiation exposure in a pre-defined subgroup analysis of the international, multicenter radiation dose survey PROTECTION I (Prospective Multicenter Study on RadiaTion Dose Estimates Of Cardiac CT AngIOgraphy I) study.
Background: Cardiac computed tomography angiography (CCTA) has become a frequently used diagnostic tool in clinical practice. Despite continually improving CT technology, there remain concerns regarding the associated radiation exposure. A reduced tube voltage of 100 kV has been proposed as an effective means for dose reduction in nonobese patients.
Methods: The study assessed the relevant radiation dose parameters as well as quantitative and qualitative diagnostic image quality data in a subgroup of 321 patients (100 kV: 82 patients; 120 kV: 239 patients), who were scanned at study sites that applied a 100-kV tube voltage in at least 1 patient. Diagnostic image quality was assessed by an experienced CCTA investigator with a 4-point score (1: nondiagnostic to 4: excellent image quality). Effective radiation dose was estimated from the dose-length-product of each CCTA study.
Results: The use of the 100-kV scan protocol was associated with 53% reduction in CCTA median radiation dose estimates, when compared with the conventional 120-kV scan protocol (p < 0.001). Although image noise significantly increased by 26.3% with 100 kV, signal- as well as contrast-to-noise ratios also increased by 7.9% (p = 0.254) and 10.8% (p = 0.027), respectively. Reduction of tube voltage did not impair diagnostic image quality (median diagnostic score: 3.5 [3.25 to 3.75] vs. 3.5 [3.0 to 3.75] for 100 kV vs. 120 kV; p = 0.22).
Conclusions: In this nonrandomized PROTECTION I dose survey, reducing the CCTA tube voltage to 100 kV in nonobese patients is associated with a significant reduction in radiation exposure while maintaining diagnostic image quality. Thus, the 100-kV scan technique should be considered for CCTA dose reduction in adequately selected patients.
Key Words: computed tomography • coronary artery disease • tube voltage
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Abbreviations and Acronyms
| | BMI = body mass index | | CCTA = cardiac computed tomography angiography | | CT = computed tomography | | DLP = dose-length-product | | ECG = electrocardiography |
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With the introduction of 64-slice computed tomography (CT), cardiac computed tomography angiography (CCTA) has emerged as a useful diagnostic imaging modality for the assessment of coronary artery disease. The main field of application is the examination of patients at intermediate pre-test probability for obstructive coronary artery disease (1–3). Despite continuous improvements in CT technology, there remains concern regarding radiation exposure. When performing 64-slice CCTA, the effective radiation dose averages usually between 10 and 15 mSv, particularly depending on the CT system, the scanning technique, as well as patient-related factors (4). Therefore, various strategies and new scanning methods have been developed to reduce radiation exposure. The international PROTECTION I (Prospective Multicenter Study On RadiaTion Dose Estimates Of Cardiac CT AngIOgraphy I) study analyzed the extent of CCTA radiation dose estimates as well as the impact of different strategies to reduce dose in clinical practice (4).
Usually, CCTA is performed with a tube voltage of 120 kV. However, CCTA acquisition with 100-kV tube voltage is also possible and has been suggested as an effective means to lower radiation dose in nonobese patients (5). So far, the effect of a reduced tube voltage on image quality and radiation dose has only been investigated in single-center studies with a small number of patients (5–7). These studies suggested that decreasing tube voltage significantly reduces radiation dose without impairing image quality. The aim of this analysis was to determine the effect of lowering the tube voltage from 120 kV to 100 kV in a pre-defined subgroup analysis of a large prospective multicenter study concerning: 1) image quality; and 2) radiation dose.
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Methods
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Study protocol.
The methods of the PROTECTION I study have been described in detail elsewhere (4). In brief, 50 international study sites provided image data and scanning protocols of consecutive CCTAs performed during 1 month with the use of 16- or 64-slice CT systems. Image data as well as CCTA study details were collected and analyzed in a central CCTA core laboratory. A total of 1,965 patients were enrolled in the PROTECTION I study. The indications comprised the visualization of coronary arteries or bypass grafts, combined examination of the coronary and pulmonary arteries in chest pain patients, or visualization of the cardiac anatomy before or after electrophysiologic procedures. In the current analysis only patients examined at study sites that applied a reduced tube voltage of 100 kV in at least 1 patient (8 of 50 study sites) for visualization of coronary arteries with a 64-slice CT system and with either a 100- or 120-kV tube voltage were included. The scan protocol including the selection of the tube voltage was at the discretion of the performing physician. The study was approved by the ethics committee, and all patients gave written informed consent as required at the individual study sites.
Image quality.
To obtain objective indexes of image quality, the image noise, signal-to-noise ratio, and contrast-to-noise ratio were determined for all CCTAs. The image noise was derived from the averaged SDs of the CT attenuation values (Hounsfield units) inside of 2 large regions of interest in the proximal segments of the left and right coronary arteries (Fig. 1). The signal intensity was defined as the mean attenuation values derived from the same 2 regions of interest. The signal-to-noise ratio was calculated as mean CT attenuation values of the left and right coronary arteries divided by image noise. The contrast-to-noise ratio was defined as the difference between the mean CT attenuation values of the proximal coronary arteries and the mean density of the left ventricular wall, which was divided by image noise.
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Figure 1 Measuring of Image Quality Data in a Proximal Left Main Coronary Artery
Region of interest (circle) in the left main coronary artery example illustrating determination of signal intensity (425 HU) and image noise (28 HU), as performed in every patient in the left main and proximal right coronary artery.
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Furthermore, a diagnostic image quality score was assessed by an experienced CCTA core lab reader who was blinded to the CCTA acquisition details in all 82 patients in the 100-kV group and in all 239 patients of the 120-kV group who were scanned at study sites that applied a reduced tube voltage of 100 kV in at least 1 patient. The image quality score was determined from the mean score of the 4 main coronary arteries (left main, left anterior descending, left circumflex, and right coronary artery) on the basis of a 4-point grading system: Score 1. Nondiagnostic. Reduced image quality that precludes appropriate evaluation of the coronary arteries.
Score 2. Adequate. Reduced image quality because of motion, noise, or low contrast with an obvious affected assessment of the vessel but still sufficient to rule out significant stenosis.
Score 3. Good. Nonlimiting artifacts, reduced attenuation of vessel lumen, and/or calcification with preserved evaluability of luminal stenosis as well as plaque characteristics.
Score 4. Excellent. Complete absence of motion artifacts, good attenuation of vessel lumen, and clear delineation of vessel walls, with the ability to assess luminal stenosis as well as plaque characteristics.
Estimation of radiation dose.
The collected parameters relevant to radiation dose included the volume CT dose index (CTDIvol) and dose-length-product (DLP), which were both obtained from the CT scan protocol of each CCTA study. The DLP was the primary study outcome parameter. The calculation of the effective dose is based on a method proposed by the European Working Group for Guidelines on Quality Criteria in CT (8), deriving radiation dose estimates from the product of the DLP and an organ weighting factor for the chest as the investigated anatomic region (k = 0.014 mSv x [mGy x cm]–1 averaged between male and female models).
Statistical analysis.
All parameters of the 100-kV protocol were compared with the standard 120-kV protocol. Continuous variables were expressed as median (interquartile range) and compared with the Wilcoxon rank sum test. Categorical variables were expressed as frequencies or percentages. The chi-square test was used to test differences in frequency of categorical variables. We performed a binary logistic regression analysis including weight and tube voltage as variables influencing image quality, due to differences in patients' weight between both groups. For this purpose we dichotomized the diagnostic image quality score at the median of 3.5 (<3.5 vs.  3.5). A p value <0.05 was considered statistically significant. All statistical analyses were performed with the software SPSS (version 16.0.1, SPSS Inc., Chicago, Illinois).
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Results
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In the PROTECTION I study a total of 1,965 patients underwent CCTA at 50 study sites. Among these, 82 patients (4.2%) were scanned with a tube voltage of 100 kV. The current analysis comprised 321 patients undergoing 64-slice CCTA at study sites, that applied a reduced tube voltage of 100 kV in at least 1 patient (8 of 50 study sites). Of these, 82 patients were scanned with a tube voltage of 100 kV, whereas 239 patients were studied with a tube voltage of 120 kV. Table 1
summarizes patient and scanning characteristics of both groups. The frequency of sinus rhythm and electrocardiography (ECG)-dependent tube current modulation as well as median heart rate and scan length did not differ significantly between groups. Although patient height was comparable in both groups, the weight was significantly lower in patients examined with a 100-kV protocol, resulting in a significantly reduced body mass index (BMI) (24.2 [22.8 to 25.9] kg/m2 vs. 26.6 [24.3 to 29.2] kg/m2 for 100-kV vs. 120-kV tube voltage, p < 0.001).
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Table 1 Patient and Scanning Characteristics of Patients Scanned at Study Sites That Applied a Reduced Tube Voltage of 100 kV in at Least 1 Patient
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When using a tube voltage of 120 kV, the CTDIvol added up to 52.2 [39.2 to 61.9] mGy. In contrast, in patients examined with 100-kV tube voltage, the CTDIvol was significantly reduced to 23.6 [18.9 to 29.7] mGy (p < 0.001). Consequently, the DLP was significantly lower with the use of the 100-kV tube voltage protocol (376 [298 to 460] mGy x cm vs. 808 [666 to 991] mGy x cm for 100-kV vs. 120-kV tube voltage, p < 0.001). Median estimated radiation dose was lowered from 14 mSv to 6 mSv when using the 100-kV protocol compared with a tube voltage of 120 kV (p < 0.001).
Table 2
displays the quantitative image quality data, including image noise, signal-to-noise ratio, and contrast-to-noise ratio depending on tube voltage. Lowering tube voltage from 120 kV to 100 kV resulted in an increased image noise by 26.3% (p < 0.001) and increased signal intensity by even 35.6% (p < 0.001). Furthermore, the median signal-to-noise ratio (p = 0.254) and contrast-to-noise ratio (p = 0.027) increased by 7.9% and 10.8%, respectively, when using the 100-kV protocol compared with the 120-kV protocol.
The diagnostic image quality score did not differ between the 100-kV and 120-kV protocol. Data acquisition with a 120-kV tube voltage resulted in a median image quality score of 3.5 [3.0 to 3.75], whereas image quality was rated comparable in patients examined with 100 kV (median image quality score 3.5 [3.25 to 3.75], p = 0.22). Figure 2
displays the diagnostic image quality score in both groups. In the binary logistic regression analysis, neither patients' weight (p = 0.856) nor tube voltage (p = 0.096) had a significant impact on the diagnostic image quality score. Figure 3
displays the image quality of 2 representative CCTAs acquired with a 100- and 120-kV tube voltage protocol, respectively.
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Figure 2 Diagnostic Image Quality Score Depending on Tube Voltage
Boxplots indicating a comparable image quality score in the 120-kV and 100-kV study groups (p = 0.22). Diagnostic image quality was assessed by an experienced coronary computed tomography angiography investigator with a 4-point score (1: nondiagnostic to 4: excellent image quality).
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Figure 3 Representative Coronary Computed Tomography Angiograms Demonstrating the Image Quality in Both Study Groups
Curved-planar maximum intensity projections of the right coronary artery (A, D), the left descending artery (B, E), and the left circumflex artery (C, F) demonstrating the image quality of the 100-kV (A to C) and the 120-kV protocol (D to F) at identical window levels and hence illustrating the higher attenuation in the 100-kV scan protocol.
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Discussion
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In recent years, CCTA has evolved as a useful noninvasive imaging modality with a very high diagnostic accuracy for the detection of obstructive coronary artery disease (9–11). In addition, CCTA has been shown to have a prognostic impact in the evaluation of patients with chest pain symptoms (12). However, the associated exposure to ionizing radiation has raised concerns. Consequently, several techniques to reduce radiation exposure during CCTA have been developed, in pursuit of the ultimate goal of obtaining diagnostic CCTA images with the lowest possible radiation dose.
The international PROTECTION I study, which is a prospective multicenter survey study on radiation dose of CCTA in daily practice, revealed a median dose of 12 mSv (interquartile range: 8 to 18 mSv) for CCTA with a large 6-fold variation in dose between study sites (4). In addition, this study identified independent predictors for the extent of CCTA radiation exposure. Among these factors, data acquisition with the use of a 100-kV tube voltage protocol was an independent predictor for reduced estimates of radiation exposure. Therefore, the current report investigates the impact of a 100-kV scan protocol for CCTA on radiation dose and image quality in more detail in a pre-specified subgroup analysis of the PROTECTION I study.
In the PROTECTION I study, the use of the 100-kV tube voltage protocol was associated with 53% reduction in radiation dose, when compared with the conventional 120-kV scan protocol. Reducing tube voltage is that effective, because radiation dose changes approximately with the square of the tube voltage (13). The use of the 100-kV tube voltage was associated with a 26.3% increase in image noise, but signal-to-noise as well as contrast-to-noise ratios also increased by 7.9% and 10.8%, respectively, due to an increased signal intensity by 35.6%. These findings are in concordance with a recent publication of Leschka et al. (7), who described significantly higher image noise and contrast-to-noise ratio with a reduced tube voltage in a small group of patients, while estimated effective radiation dose was lowered by 25% to 51%. The improvements in signal- and contrast-to-noise ratios might be more relevant for CCTA reading when compared with the increased image noise, which has primarily an impact on the aesthetic rather than on the diagnostic image quality. In fact, it might be speculated that the higher signal- and contrast-to-noise ratios with the 100-kV scan protocol might even improve the delineation of coronary lumen stenosis (e.g., in calcified lesions). However, this could not be investigated in the current study.
More importantly, the qualitative assessment of diagnostic image quality did not differ between patients scanned with 100-kV tube voltage and those scanned with 120-kV tube voltage. A comparable median score of 3.5 [3.25 to 3.75] versus 3.5 [3.0 to 3.75] was obtained for 100-kV vs. 120-kV scans, respectively (p = 0.22), applying the sensitive grading score for diagnostic image quality. Certainly, the present analysis is limited by the nonrandomized study design of this dose survey study and a potential selection bias; however, the consistency of these results with previous publications still supports a broader applicability of a reduced 100-kV tube voltage protocol in daily practice until prospective randomized data become available.
Because image noise increases with body weight, the 100-kV scan protocol was applied in patients with a significantly lower body weight. On the basis of the current results and our own experience with a 100-kV tube voltage, this scan technique can be recommended for nonobese patients (e.g., body weight  85 to 90 kg or BMI 30 kg/m2), depending on the individual chest physiognomy. In the PROTECTION I study, the 100-kV tube voltage was applied in only 4.2% of the entire study population. However, when applying a weight threshold <90 kg or BMI threshold <30 kg/m2, 78.9% or 81.8% of the PROTECTION I patients would have qualified for a 100-kV CCTA scan protocol, respectively. This underlines the enormous potential for radiation dose reduction with the use of a reduced tube voltage. Recent data from a small group of patients suggest that tube voltage can even be reduced to 80 kV in slim patients with a body weight <60 kg (6). With a tube voltage of 80 kV, radiation dose was reduced by 88% when compared with a 120-kV scan protocol without impairing image quality. However, an 80-kV tube voltage was not applied in any of the PROTECTION I patients; accordingly no conclusions can be made from the PROTECTION I study in this respect.
The low frequency of 100-kV tube voltage CCTA studies in the PROTECTION I study can be explained by 3 main reasons: 1) the scan protocols recommended by CT-manufacturers usually apply 120 kV; 2) some CT scanners do not support the use of a 100-kV scan protocol for CCTA; and 3) supposedly, many CT radiographers did not consider lowering the tube voltage. Thus, it could be an imaginable approach in the future that the CT scanning software would propose optimal scan protocols for individual patients. The scanning software could suggest, on the basis of entered data of body weight and height or on the basis of sophisticated analysis of attenuation data during topogram acquisition, ideal settings for tube voltage and current to maintain the diagnostic image quality and an acceptable level of image noise at the lowest possible radiation dose. In addition, future scanner software might even suggest scan protocols for further radiation dose reductions on the basis of heart rate and heart rate variability, such as prospective ECG-triggered sequential scan algorithms as compared with conventional ECG-gated spiral data acquisition. The combination of both ECG-triggered sequential scanning with a 100-kV tube voltage is a conceivable approach, which might result in even further reduced radiation dose estimates for CCTA (e.g., below 3 mSv) in many patients (14).
Study limitations.
The PROTECTION I study was a nonrandomized survey study; accordingly, the scanning protocol including selection of the tube voltage was at the discretion of the performing physician, introducing a potential selection bias. Besides, the stated radiation doses comprise the estimated dose of the CCTA scan, thus without taking into account the contributions of the topogram, scan for calcium scoring, and test bolus to the total dose. Furthermore, the diagnostic accuracy of CCTA for the detection of obstructive coronary stenosis when compared with invasive coronary angiography was not investigated. However, as pointed out before, it might be that the improved signal- and contrast-to-noise ratios with the 100-kV scan protocol result in an improved assessment of lesion severity. For further investigation of the 100-kV scan protocol, prospective randomized data are needed.
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Conclusions
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The 100-kV tube voltage scan protocol is a feasible approach to reduce radiation exposure of CCTA in appropriately selected nonobese patients. When compared with conventional data acquisition with 120-kV, the 100-kV scan protocol was associated with a 53% reduction in radiation exposure with a median radiation dose estimate of only 6 mSv. Importantly, the current data provide further support that the diagnostic image quality is maintained. Thus, the subgroup analysis of the PROTECTION I study demonstrates that radiation exposure of CCTA can be reduced substantially by applying the currently available 100-kV tube voltage scan protocol.
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Appendix
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For supplementary lists of clinical studies, please see the online version of this article.
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Acknowledgments
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The authors are indebted to Markus Krebs for his invaluable contributions to the PROTECTION I study.
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Footnotes
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Dr. Hausleiter reports receiving research grants and speaker honoraria from Siemens Medical Solutions unrelated to the current study. Drs. Bischoff and Hein contributed equally to this work.
* Reprint requests and correspondence: Dr. Jörg Hausleiter, Deutsches Herzzentrum, Lazarettstrasse 36, 80636 München, Germany (Email: hausleiter{at}dhm.mhn.de).
Manuscript received December 16, 2008;
revised manuscript received February 17, 2009,
accepted February 20, 2009.
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- Fox K, Garcia MA, Ardissino D, et al. Guidelines on the management of stable angina pectoris: executive summary: the Task Force on the Management of Stable Angina Pectoris of the European Society of Cardiology Eur Heart J 2006;27:1341-1381.[Free Full Text]
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