Downloaded from http://journals.lww.com/jcat by BhDMf5ePHKbH4TTImqenVGLGjjqParD6qRnsxo3ve0fYvAFKupZE4N9Fah8EpeqFjfsauUKZxvs= on 05/02/2020 Potential of Radiation Dose Reduction by Optimizing Z-Axis Coverage in Coronary Computed Tomography Angiography on a Latest-Generation 256-Slice Scanner Elia von Felten, MD, Michael Messerli, MD, Andreas A. Giannopoulos, MD, PhD, Dominik C. Benz, MD, Moritz Schwyzer, MD, Georgios Benetos, MD, Georgios Rampidis, MD, Dimitri Patriki, MD, Christel H. Kamani, MD, Christoph Gräni, MD, PhD, Tobias A. Fuchs, MD, Aju P. Pazhenkottil, MD, Catherine Gebhard, MD, PhD, Philipp A. Kaufmann, MD, and Ronny R. Buechel, MD Objective: The purpose of this study was to quantify the reduction in ra- diation dose achievable by using the optimal z-axis coverage in coronary computed tomography (CT) angiography (CCTA) on a latest-generation 256-slice scanner. Methods: A total of 408 scans were reviewed that were performed on a wide-range detector scanner allowing up to 16-cm z-axis coverage (adjust- able in 2-cm increments). For each CCTA study, we assessed the radiation dose (ie, dose-length product and volume CT dose index) and measured the minimum z-axis coverage necessary to cover the complete cardiac anat- omy. We calculated the potential radiation dose savings achievable through reduction of the z-axis coverage to the minimum necessary. Results: The majority of the CCTA scans were performed with a z-axis coverage of 16 cm (n = 285, 69.9%), followed by 14 cm (n = 121, 29.7%) and 12 cm (n = 2, 0.5%). In the group that was scanned with a col- limation of 16 cm, radiation dose could have been reduced by 12.5% in 55 patients, 25% in 195 patients, and 37.5% in 33 patients when using op- timal z-axis coverage for CCTA. In the group that was scanned with a col- limation of 14 cm, radiation dose could have been reduced by 14.3% in 90 patients, and 28.6% in 30 patients, whereas in the group that was scanned with a collimation of 12 cm, dose could have been reduced by 16.7% in 2 patients. Conclusions: Using correct z-axis coverage in CCTA on a latest- generation 256-slice scanner yields average dose reductions of 22.0% but may be as high as 37.5%. Key Words: cardiac CT, coronary artery disease, coronary CT angiography, dose optimization, wide-area CT detectors (J Comput Assist Tomogr 2020;44: 289–294) C ardiac computed tomography (CT) has evolved to be an im- portant tool for the assessment of a variety of cardiac pathol- ogies, including coronary artery disease (CAD), which can be easily and noninvasively assessed with coronary CT angiography (CCTA). 1 Following the principle of ALARA (as low as reason- ably achievable), 2 technical advances from all manufacturers have led to tremendous dose reductions of CCTA over the past decade with reported effective doses of 4.5 mSv on average. 3,4 Reducing radiation exposure remains of utmost importance, especially be- cause the individual risk for each patient is difficult to quantify. 5 Coronary CT angiography dose reduction is achieved through nu- merous strategies, including prospective electrocardiogram (ECG) triggering, 6 the use of topogram and/or ECG-based tube current modulation, lowering of the tube peak kilovoltage, and the use of iterative reconstruction algorithms. 7,8 Regarding dosimetry in CT, the dose-length product (DLP) and the volume CT dose index (CTDI vol ) are the universal param- eters used for exposure assessment and comparison of different CT protocols and devices. They allow an estimation of the total ra- diation dose received by a patient and are automatically logged by CT devices. There is a linear relationship between z-axis coverage and the total radiation dose delivered to the patient, as measured by the DLP. 9 Hence, optimizing the z-axis coverage is one of the simplest radiation dose reduction techniques that can be used. Wide-area CT detectors allow covering up to 16 cm in the z axis during one single rotation without a need to move the patient table during image acquisition. A previous study indicated that reduc- ing z-axis coverage in CCTA with a 320-detector-row scanner can reduce DLP by approximately 27%. 10 The purpose of this study was therefore to quantify the re- duction in radiation dose achievable by using optimal z-axis coverage on a latest-generation 256-slice scanner including subanalysis regarding sex- and age-specific influences. MATERIALS AND METHODS Study Population Patients were included in this retrospective study as a second- ary study of a larger prospective study with a different purpose than our study (KEK-ZH-Nr. 214–0632). All patients gave written informed consent prior to inclusion into the study. Thus, the analysis presented here is based on a retrospective review of prospectively acquired data. Data of a total of 408 consecutive patients who were referred for exclusion of CAD with CCTA due to stable symptoms from March 2015 to November 2016 were reviewed. Patients with additional clinical questions requiring extension of the scan cover- age beyond the heart (eg, patients with suspected aortic vessel dis- ease) were excluded (n = 42). The study was approved by the local ethics committee. From the Department of Nuclear Medicine, Cardiac Imaging, University Hospi- tal Zurich, University Zurich, Zurich, Switzerland. Received for publication August 2, 2019; accepted December 3, 2019. Correspondence to: Ronny R. Buechel, MD, Department of Nuclear Medicine, Cardiac Imaging, University Hospital Zurich, Ramistrasse, 8091 Zurich, Switzerland (e‐mail: ronny.buechel@usz.ch). The University Hospital of Zurich holds a research contract with GE Healthcare. M.M. received a research grant from the Iten-Kohaut Foundation, Switzerland. C.G. received research grants from the Swiss National Science Foundation; the Olga Mayenfisch Foundation, Switzerland; the OPO Foundation, Switzerland; the Novartis Foundation, Switzerland; and the Swissheart Foundation. No further specific grants from funding agencies in the public, commercial, or not-for-profit sectors were received for this study. The other authors declare no conflict of interest. Elia von Felten and Michael Messerli share first authorship. Institutional review board approved the study, and all patients gave written informed consent prior to inclusion into the study. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved. DOI: 10.1097/RCT.0000000000000993 ORIGINAL ARTICLE J Comput Assist Tomogr • Volume 44, Number 2, March/April 2020 www.jcat.org 289 Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.