Image Quality in Low Radiation Exposure Protocol for ECG-Gated Coronary CTA
Image Quality in Low Radiation Exposure Protocol for ECG-Gated Coronary CTA
Objective: The purpose of our study was to systematically compare the image quality of dual-source CT coronary angiography using 100 kV instead of 120 kV.
Subjects and Methods: One hundred patients with a body weight ≤ 85 kg were included. A dual-source CT scanner was used (330-milliseconds rotation, 0.6-mm collimation, 56 ± 7 mL of IV contrast agent at 5 mL/s). Each patient was randomized either to scanning protocol group 1 (120 kV and 330 mAs) or protocol group 2 (100 kV and 330 mAs). ECG pulsing was used for all patients. Data sets were assessed by two independent observers for image quality, signal-to-noise ratio, and contrast-to-noise-ratio. Effective dose was determined based on dose-length product.
Results: There were no significant differences in body weight or heart rate between the two groups (70 ± 10 kg and 57 ± 8 bpm [beats per minute] vs 70 ± 9 kg and 59 ± 8 bpm). Use of 100 kV led to significant reduction of radiation exposure (group 1: 12.7 ± 1.7 mSv; volume CT dose index [CTDIvol], 47.8 ± 6.1 mGy and group 2: 7.8 ± 2.0 mSv; CTDIvol, 28.6 ± 6.3 mGy; p < 0.001). Interobserver agreement in assessing image quality (κ = 0.71) was close. Mean patient-based image quality scores were not significantly different (group 1, 2.7 ± 0.5 and group 2, 2.6 ± 0.4; p = 0.75). Also, vessel-based scores showed no significant differences. Beyond the level of significance, group 1 and group 2 showed one and two nonassessable patients and two and three nonassessable vessels, respectively. Mean intraluminal attenuation, contrast enhancement, and image noise were significantly higher for 100 kV, whereas signal-to-noise and contrast-to-noise-ratios were not different between the two scanning protocols.
Conclusion: The use of lower tube voltage leads to significant reduction in radiation exposure in noninvasive coronary CT angiography. Image quality in nonobese patients is not negatively influenced.
MDCT allows reliable detection of coronary artery stenoses with high sensitivity and high specificity. According to recent guidelines, its use is considered "appropriate" for symptomatic patients at intermediate risk for coronary heart disease. Despite its noninvasive nature, the associated radiation exposure of coronary CT angiography has been reason for concern.
Radiation exposure of diagnostic invasive coronary angiography is usually estimated to be between 2 and 6 mSv. The radiation dose of coronary CT angiography is typically higher. Because of differences in scanner technology and data acquisition protocols, radiation doses for coronary CT angiography vary, but effective dose values between 6 and 16 mSv have been reported. To put this in context, the estimated additional lifetime risk for developing fatal cancer after a radiation exposure of 10 mSv is quoted at about one in 1,000 as an average value for all age groups (according to the BEIR VII reports). The review of the same study supports a so-called "linear no-threshold risk model," indicating that any extent of dose reduction is accompanied by a corresponding decrease in lifetime risk for cancer incidence but without showing a lower threshold. In a more recent study, the estimated attributable lifetime cancer risks after an ECG-gated coronary CT angiography varied between one in 219 and one in 5,017 for a 20-year-old woman and an 80-year-old man, respectively.
Different approaches are available to lower radiation exposure during coronary CT angiography. The use of ECG pulsing leads to a reduction of tube current (mAs) by 80% outside the pulsing window, thus allowing the radiation dose to be lowered by 37-40%, depending on the scanner generation. The use of prospectively triggered instead of retrospectively gated coronary CT scanning provides another effective tool to reduce radiation exposure. In selected patients with reasonably low and stable heart rates, the use of prospective ECG gating preserved adequate image quality and led to a significant reduction of radiation dose to 2.1-4.2 mSv. Lowering tube voltage is an additional way to decrease radiation dose during coronary CT angiography. However, it also leads to increased use of iodinated contrast agents and to higher image noise.
Despite these numerous possibilities for reducing radiation exposure in coronary CT angiography, maintaining adequate and diagnostic image quality is crucial. Because scanning protocols using lower tube voltage could potentially impair image quality and diagnostic accuracy, we systematically evaluated image quality of coronary dual-source CT angiography using 100 kV compared with the traditionally used value of 120 kV.
Abstract and Introduction
Abstract
Objective: The purpose of our study was to systematically compare the image quality of dual-source CT coronary angiography using 100 kV instead of 120 kV.
Subjects and Methods: One hundred patients with a body weight ≤ 85 kg were included. A dual-source CT scanner was used (330-milliseconds rotation, 0.6-mm collimation, 56 ± 7 mL of IV contrast agent at 5 mL/s). Each patient was randomized either to scanning protocol group 1 (120 kV and 330 mAs) or protocol group 2 (100 kV and 330 mAs). ECG pulsing was used for all patients. Data sets were assessed by two independent observers for image quality, signal-to-noise ratio, and contrast-to-noise-ratio. Effective dose was determined based on dose-length product.
Results: There were no significant differences in body weight or heart rate between the two groups (70 ± 10 kg and 57 ± 8 bpm [beats per minute] vs 70 ± 9 kg and 59 ± 8 bpm). Use of 100 kV led to significant reduction of radiation exposure (group 1: 12.7 ± 1.7 mSv; volume CT dose index [CTDIvol], 47.8 ± 6.1 mGy and group 2: 7.8 ± 2.0 mSv; CTDIvol, 28.6 ± 6.3 mGy; p < 0.001). Interobserver agreement in assessing image quality (κ = 0.71) was close. Mean patient-based image quality scores were not significantly different (group 1, 2.7 ± 0.5 and group 2, 2.6 ± 0.4; p = 0.75). Also, vessel-based scores showed no significant differences. Beyond the level of significance, group 1 and group 2 showed one and two nonassessable patients and two and three nonassessable vessels, respectively. Mean intraluminal attenuation, contrast enhancement, and image noise were significantly higher for 100 kV, whereas signal-to-noise and contrast-to-noise-ratios were not different between the two scanning protocols.
Conclusion: The use of lower tube voltage leads to significant reduction in radiation exposure in noninvasive coronary CT angiography. Image quality in nonobese patients is not negatively influenced.
Introduction
MDCT allows reliable detection of coronary artery stenoses with high sensitivity and high specificity. According to recent guidelines, its use is considered "appropriate" for symptomatic patients at intermediate risk for coronary heart disease. Despite its noninvasive nature, the associated radiation exposure of coronary CT angiography has been reason for concern.
Radiation exposure of diagnostic invasive coronary angiography is usually estimated to be between 2 and 6 mSv. The radiation dose of coronary CT angiography is typically higher. Because of differences in scanner technology and data acquisition protocols, radiation doses for coronary CT angiography vary, but effective dose values between 6 and 16 mSv have been reported. To put this in context, the estimated additional lifetime risk for developing fatal cancer after a radiation exposure of 10 mSv is quoted at about one in 1,000 as an average value for all age groups (according to the BEIR VII reports). The review of the same study supports a so-called "linear no-threshold risk model," indicating that any extent of dose reduction is accompanied by a corresponding decrease in lifetime risk for cancer incidence but without showing a lower threshold. In a more recent study, the estimated attributable lifetime cancer risks after an ECG-gated coronary CT angiography varied between one in 219 and one in 5,017 for a 20-year-old woman and an 80-year-old man, respectively.
Different approaches are available to lower radiation exposure during coronary CT angiography. The use of ECG pulsing leads to a reduction of tube current (mAs) by 80% outside the pulsing window, thus allowing the radiation dose to be lowered by 37-40%, depending on the scanner generation. The use of prospectively triggered instead of retrospectively gated coronary CT scanning provides another effective tool to reduce radiation exposure. In selected patients with reasonably low and stable heart rates, the use of prospective ECG gating preserved adequate image quality and led to a significant reduction of radiation dose to 2.1-4.2 mSv. Lowering tube voltage is an additional way to decrease radiation dose during coronary CT angiography. However, it also leads to increased use of iodinated contrast agents and to higher image noise.
Despite these numerous possibilities for reducing radiation exposure in coronary CT angiography, maintaining adequate and diagnostic image quality is crucial. Because scanning protocols using lower tube voltage could potentially impair image quality and diagnostic accuracy, we systematically evaluated image quality of coronary dual-source CT angiography using 100 kV compared with the traditionally used value of 120 kV.
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