Coronary computed tomography angiography (CCTA) has emerged as favorable noninvasive imaging method in cardiology.^1,2 Due to its high accuracy, CCTA clarifies the diagnosis of angina in patients with suspected coronary artery disease in addition to standard clinical care³ and excludes obstructive coronary artery disease with high negative predictive value.⁴ Importantly, the use of CCTA has been associated with a reduction in non-fatal myocardial infarction.⁵ Furthermore, CCTA carries significant prognostic information.⁶ Due to the rapidly increasing volume of CCTAs performed, safety considerations are an important concern, especially for the cohort of younger patients with low likelihood of disease. In this regard, clinicians should aim to reduce radiation dose exposure "as low as reasonably achievable" (ALARA principle) while maintaining diagnostic image quality.

During the last decade, radiation scientists and clinicians have developed and promoted numerous dose-saving techniques in CCTA imaging (Table 1). Low tube current imaging leads to linear reduction of radiation dose, but low tube potential imaging is much more effective due to the exponential reduction of radiation dose with the reduction of tube voltage.⁷ Low tube potential imaging at 100 kVp has been shown to maintain image quality while significantly reducing radiation exposure (PROTECTION II [Prospective Randomized Trial On RadiaTion Dose Estimates Of CT AngIOgraphy In PatieNts Scanned With A 100kV Protocol]).⁸ Automatic dose modulation software was developed to automatically lower tube potential and tube current depending on the patients' size and shape to ensure constant image quality at every anatomic level.⁹ Improved computed tomography (CT) protocols with administration of radiation only at the mid of diastole (prospective electrocardiography [ECG]-triggered axial scanning) or solely during one cardiac cycle (prospective ECG-triggered high-pitch helical scanning) further contribute to saving dose and have been validated to maintain image quality (PROTECTION III [Prospective Randomized Trial On RadiaTion Dose Estimates Of CT AngIOgraphy In PatieNts Scanned With A Sequential Scan Protocol] and PROTECTION IV [Prospective Randomized Trial On RadiaTion Dose Estimates Of CT AngIOgraphy In PatieNts Scanned With A High-Pitch-First Scan Strategy]).^10,11 Another major advancement in CCTA imaging has been the introduction of iterative image reconstruction with advanced raw data processing, leading to decreased image noise in low-contrast areas and giving rise to further reductions of tube current (PROTECTION V [Prospective Randomized Trial On Radiation Dose Estimates Of CT Angiography In Patients Applying Iterative Image Reconstruction Techniques]).¹² Beyond that, CT hardware and design contributes to the level of radiation dose exposure. As an example, powerful X-ray generators with high X-ray output as well as wide detector CT are required for modern high-pitch scanning.

Table 1: Dose-Saving Techniques in CCTA Imaging

CT Adjustment and Software

Low tube current imaging

Low tube potential imaging

Automatic dose modulation

ECG-controlled tube current modulation

Iterative image reconstruction

CT Protocols

Prospective ECG-triggered axial (sequential) scanning

Prospective ECG-triggered high-pitch spiral scanning

CT Hardware and Design

Powerful X-ray generators

Dual-source CT scanning with spectral pre-filtration

Advanced X-ray detectors with antiscatter grids

The implementation of the mentioned dose-saving techniques in clinical practice and the impact on dose reduction was the subject of our recent study, PROTECTION VI (Prospective Multicenter Registry on RadiaTion Dose Estimates of Cardiac CT AngIOgraphy IN Daily Practice in 2017).¹³ In this large prospective multicenter dose survey, we analyzed over 4,000 CCTAs from 61 international study sites of 32 different countries and found considerable reduction of radiation dose during the last decade. Compared with a similar dose survey in 2007 (PROTECTION I [Prospective Multicenter Study on Radiation Dose Estimates of Cardiac CT Angiography in Daily Practice I]),¹⁴ we observed a reduction of the median dose-length-product (DLP) by 78% in current clinical routine CCTA imaging (Figure 1). The median DLP of the current survey added up to only 195 mGy x cm. Using a dose conversion factor for cardiovascular CT (k = 0.026),¹⁵ the median DLP translates to a median estimated dose of only 5.1 mSv. Although low tube potential imaging was rarely used in 2007, a reduced tube potential protocol of 100 kVp or below was applied in 56% of scans in 2017. The implementation of prospective CT protocols has gained popularity and increased from only 6% in 2007 to 78% in 2017. We identified 3 patient-related independent predictors associated with radiation dose of CCTA. An increase in body weight of 10 kg, an increase in heart rate of 10 bpm, and the absence of sinus rhythm were associated with an increase in radiation dose of 7%, 8%, and 21%, respectively. A decrease in the tube potential of 10 kVp and the use of iterative image reconstruction were identified as scan-related independent predictors and were associated with dose reductions of 21% and 30%, respectively. Compared with retrospective ECG-gated helical scanning, the prospective ECG-triggered scan modes (axial or high-pitch) resulted in a 74% reduction of radiation dose. Importantly, the lower radiation exposure in 2017 was not associated with an increase in non-diagnostic CCTAs when compared with the dose survey of 2007.

Figure 1: Reduction of Radiation Dose in CCTA Imaging

A striking 37-fold variability in median DLP was observed between the hospitals with lowest and highest DLP (range of median DLP 57-2090 mGy x cm). This variability implies the potential for further dose reduction in centers at the upper end of this dose survey. From PROTECTION VI, we proposed a new diagnostic reference level for CCTA imaging of 400 mGy x cm. This diagnostic reference level provides a means to document adherence to standards of patient safety and for targeted implementation of quality assurance programs to reduce exposure levels.

In summary, a variety of techniques is available for efficient dose reduction in CCTA. This progress contributed to the establishment of CCTA as a frequently used noninvasive imaging method as supported by national guidelines.¹⁶ The achieved dose reduction can be attributed to some important factors:

1. The increasing awareness about radiation safety and the growing experience and knowledge of CT imagers in cardiovascular CT
2. The publication of and adherence to best practice guidelines for cardiac CT imaging¹⁷
3. The availability of scan protocols in modern CT scanners, which are radiation-dose efficient

However, the great variability of the DLP between study sites highlights the need for further education, spreading of modern CT systems, and adaptation of contemporary cardiac scan protocols.

References

1. Hamilton-Craig CR, Friedman D, Achenbach S. Cardiac computed tomography--evidence, limitations and clinical application. Heart Lung Circ 2012;21:70-81.
2. Marano R, Pirro F, Silvestri V, et al. Comprehensive CT cardiothoracic imaging: a new challenge for chest imaging. Chest 2015;147:538-51.
3. SCOT-HEART investigators. CT coronary angiography in patients with suspected angina due to coronary heart disease (SCOT-HEART): an open-label, parallel-group, multicentre trial. Lancet 2015;385:2383-91.
4. Budoff MJ, Dowe D, Jollis JG, et al. Diagnostic performance of 64-multidetector row coronary computed tomographic angiography for evaluation of coronary artery stenosis in individuals without known coronary artery disease: results from the prospective multicenter ACCURACY (Assessment by Coronary Computed Tomographic Angiography of Individuals Undergoing Invasive Coronary Angiography) trial. J Am Coll Cardiol 2008;52:1724-32.
5. SCOT-HEART Investigators, Newby DE, Adamson PD, et al. Coronary CT Angiography and 5-Year Risk of Myocardial Infarction. N Engl J Med 2018;379:924-33.
6. Cho I, Al'Aref SJ, Berger A, et al. Prognostic value of coronary computed tomographic angiography findings in asymptomatic individuals: a 6-year follow-up from the prospective multicentre international CONFIRM study. Eur Heart J 2018;39:934-41.
7. Bischoff B, Hein F, Meyer T, et al. Impact of a reduced tube voltage on CT angiography and radiation dose: results of the PROTECTION I study. JACC Cardiovasc Imaging 2009;2:940-6.
8. Hausleiter J, Martinoff S, Hadamitzky M, et al. Image quality and radiation exposure with a low tube voltage protocol for coronary CT angiography results of the PROTECTION II Trial. JACC Cardiovasc Imaging 2010;3:1113-23.
9. Park YJ, Kim YJ, Lee JW, et al. Automatic Tube Potential Selection with Tube Current Modulation (APSCM) in coronary CT angiography: Comparison of image quality and radiation dose with conventional body mass index-based protocol. J Cardiovasc Comput Tomogr 2012;6:184-90.
10. Hausleiter J, Meyer TS, Martuscelli E, et al. Image quality and radiation exposure with prospectively ECG-triggered axial scanning for coronary CT angiography: the multicenter, multivendor, randomized PROTECTION-III study. JACC Cardiovasc Imaging 2012;5:484-93.
11. Deseive S, Pugliese F, Meave A, et al. Image quality and radiation dose of a prospectively electrocardiography-triggered high-pitch data acquisition strategy for coronary CT angiography: The multicenter, randomized PROTECTION IV study. J Cardiovasc Comput Tomogr 2015;9:278-85.
12. Deseive S, Chen MY, Korosoglou G, et al. Prospective Randomized Trial on Radiation Dose Estimates of CT Angiography Applying Iterative Image Reconstruction: The PROTECTION V Study. JACC Cardiovasc Imaging 2015;8:888-96.
13. Stocker TJ, Deseive S, Leipsic J, et al. Reduction in radiation exposure in cardiovascular computed tomography imaging: results from the PROspective multicenter registry on radiaTion dose Estimates of cardiac CT angIOgraphy iN daily practice in 2017 (PROTECTION VI). Eur Heart J 2018;39:3715-23.
14. Hausleiter J, Meyer T, Hermann F, et al. Estimated radiation dose associated with cardiac CT angiography. JAMA 2009;301:500-7.
15. Trattner S, Halliburton S, Thompson CM, et al. Cardiac-Specific Conversion Factors to Estimate Radiation Effective Dose From Dose-Length Product in Computed Tomography. JACC Cardiovasc Imaging 2018;11:64-74.
16. Moss AJ, Williams MC, Newby DE, Nicol ED. The Updated NICE Guidelines: Cardiac CT as the First-Line Test for Coronary Artery Disease. Curr Cardiovasc Imaging Rep 2017;10:15.
17. Halliburton SS, Abbara S, Chen MY, et al. SCCT guidelines on radiation dose and dose-optimization strategies in cardiovascular CT. J Cardiovasc Comput Tomogr 2011;5:198-224.

Clinical Topics: Cardiovascular Care Team, Heart Failure and Cardiomyopathies, Invasive Cardiovascular Angiography and Intervention, Noninvasive Imaging, Atherosclerotic Disease (CAD/PAD), Statins, Interventions and Coronary Artery Disease, Interventions and Imaging, Computed Tomography, Nuclear Imaging

Keywords: Diagnostic Imaging, Cardiac Imaging Techniques, Coronary Artery Disease, Heart Rate, X-Rays, Diastole, Prospective Studies, Prognosis, Tomography, X-Ray Computed, Electrocardiography, Antineoplastic Combined Chemotherapy Protocols, Tomography, Spiral Computed, Cytarabine, Myocardial Infarction, Registries, Radiation Dosage, Body Weight, Cohort Studies

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