ESTABLISHING DIAGNOSTIC REFERENCE LEVELS FOR CARDIAC COMPUTED TOMOGRAPHY ANGIOGRAPHY IN SAUDI ARABIA

ESTABLISHING DIAGNOSTIC REFERENCE LEVELS FOR CARDIAC COMPUTED TOMOGRAPHY ANGIOGRAPHY IN SAUDI ARABIA Abstract Cardiac computed tomography angiography (CCTA) is a commonly used diagnostic imaging tool for cardiovascular disease. Despite constant improvements to imaging technologies, the radiation dose to patients remains a concern when using this procedure. Diagnostic reference levels (DRLs) are used as a trigger to identify and alert individual facilities that are using high doses during CT. This study aims to assess patient radiation dose and establish new national DRLs (NDRL) associated with CCTA in Saudi clinical practices. A structured booklet survey was designed for recording patient and scanning protocols during CCTA procedures. The data were collected retrospectively from the participating centres. NDRLs for CCTA were defined as the 75th and 25th of volumetric CT dose index (CTDIvol) and dose length product (DLP). Specific DRLs based on two main ECG-gating modes were also proposed. Data sets related to 197 CCTAs with a mean weight of 77 kg were analysed in detail. The DRL values for CTDIvol and DLP for prospective gating mode and retrospective gating mode were 29 and 62 mGy and 393 and 1057 mGy cm, respectively. NDRLs for CCTA in Saudi Arabia are comparable or slightly lower than European DRLs due to the current use of dose-saving technology. There are major variations in patient doses during CCTA due to differences in CT scanners, scanning modes and departmental CCTA protocols. INTRODUCTION Cardiac computed tomography angiography (CCTA) has long played an important role in both the diagnosis and management of coronary artery disease (CAD). Current advances in CT scanners have allowed CCTA to directly visualise coronary anatomy, which has led to considerably improved diagnostic accuracy(1). Compared to conventional invasive coronary angiography (ICA), CCTA has high accuracy in the detection of coronary atherosclerosis through a non-invasive method, has shorter examination times and is more cost-effective than conventional coronary angiography(2). Therefore, the number of medical imaging centres that perform CCTA examinations is continuously increasing(3). One 2009 study that compared utilisation of ICA before and after introduction of CCTA in a large urban cardiology practice in the United States showed increased utilisation of CCTA, which was associated with a 45% decrease in ICA procedures(4). Several studies, however, have reported large variability in patient doses from CCTA, with diagnostic reference levels (DRLs) ranging from 671 to 1510 mGy cm(5). Those variations have been found to be due to differences in both scanning techniques and dose-reduction technologies available in multi-detector CT (MDCT) scanners, which can be customised to individual patient characteristics. DRLs represent a valuable tool for the continuous improvement of dose optimisation in medical imaging practice(6). They help identify and alert individual facilities using high doses during routine radiological procedures so that they can take corrective action regarding dose optimisation(7). They also provide a standard comparison that allows CT centres to compare the quantities of their local CT doses with national DRLs (NDRLs) for most CT examinations(6). Thus, it is important to establish DRLs for CCTA to enable doses from different scanning techniques to be assessed and compared. DRLs are based on determining the 75th percentile values of the distribution of reported median facility volume CT dose index (CTDIvol) and dose–length product (DLP) for routine CT examinations(8, 9). A literature review found noticeable variations in the sampling methodology used to identify patients and scanning protocols for setting CCTA NDRLs worldwide(5). In addition, there are no published studies on either doses or technologies used for CCTA procedures in Saudi Arabia. The purpose of this study was to assess CT dose levels in Saudi Arabia and establish DRLs for CCTA. The findings can provide a baseline for optimisation of CCTA examinations in Saudi Arabia. MATERIALS AND METHODS Ethical approval was granted by the University of Sydney HREC (2015/605) and by the Institutional Review Boards (IRB) of the individual hospitals recruited for the study. The Saudi health statistics annual report and a geographic information system for the Saudi Ministry of Health portal were reviewed to identify centres that provide a high proportion of cardiac imaging services to the whole population(10). Based on that information, 18 public and private hospitals and CT centres across Saudi Arabia were invited, on a voluntary basis, to participate in this study. Data sets for at least 20 most recent CCTA patients were collected retrospectively from the Radiology Information System (RIS) and Picture Archiving and Communication System (PACS). These included data on recent CCTAs performed for the assessment of CAD in adult patients. All CCTA examinations for the evaluation of bypass graft patency were excluded as they require a longer scan length than a standard CCTA, which would skew the results. No exclusion criteria based on patient weight were applied. Copies of the annual quality assurance (QA) reports were also collected from participating CT centres to confirm that all the CT scanners had a QA programme. Baseline information related to CCTA scanning parameters and patient characteristics were collected from each centre, for each patient scan included in the study data. This consisted of scan mode, tube potential (kVp), tube current–time product (mAs), rotation time, pitch, slice thickness, type and level of iterative reconstruction (IR). Patient characteristics, namely, age, sex, weight, height and cross-sectional area (CSA), were also recorded. The CT dose descriptor, DLP and CTDIvol were recorded from the dose report, including the total DLP from the scanogram, calcium score (CS), test bolus scan and angiographic phases. The scan length for a complete CCTA examination was calculated as shown in the following equation. Scanlength(cm)=DLP/CTDIvol. (1) A booklet was designed to record both the patient and scanner parameters from each centre. This booklet was adapted from that used for the United Kingdom NDRL surveys in 2011(11). Feedback from four registered radiographers was incorporated into a revised format. Instructions to assist radiographers to complete the survey successfully were also provided. Data sets were analysed using IBM SPSS Statistics v22.0 (IBM Corp. Armonk, NY). The median CTDIvol and DLP for CCTA and CS examinations were calculated and NDRL was reported as the 75th, median and 25th percentile. Those percentiles are most commonly and recently used as an indicator of the DRL(5). RESULTS Eleven of 18 hospitals (seven public, two university and two private) completed and returned the survey data booklets. The CT scanners ranged from 64 to 128 slices, and data sets from 11 different CT scanners were collected. Two types of MDCT systems by four different CT equipment manufacturers were included: 64 slice systems by General Electric (GE Healthcare, Milwaukee, USA), Siemens (Siemens Healthcare, Erlangen, Germany) and Philips (Philips Healthcare, Best, Netherlands); and a 128 slice dual source system by Siemens (Siemens Healthcare, Erlangen, Germany). Ten hospitals submitted complete CCTA dose information for at least 20 patients, and one hospital submitted data for only 12 patients due to the recent installation of a new CCTA scanner and PACS. After standardising for weight, this resulted in 197 CCTA data sets with the average hospital submitting of 18 ± 4 data sets. To standardise weight for the sampled population, which is necessary to establish CCTA DRLs, the survey data were filtered to match commonly accepted requirements(12). Thus, data sets for patients with weight above 105 kg were removed, resulting in standard weights for the sample of 70 ± 10 kg. This resulted in a sample of 197 CCTAs with mean weight of 77 kg. Of these, 108 CCTAs were performed using prospective ECG-gating mode (PGM) and 89 using retrospective ECG-gating mode (RGM). Some 63.5% of patients were male, reflecting the higher rates of cardiovascular disease and mortality in males compared to females in Saudi Arabia(13, 14). The characteristics of patients and CCTA protocols and equipment are shown in Table 1. Table 1. Patient and CCTA scanning characteristics (n = 197). Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) AP width, anterior posterior diameter; CS, calcium score; DLP, dose–length product; IQR, interquartile range; MDCT, multi-detector computed tomography; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 1. Patient and CCTA scanning characteristics (n = 197). Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) AP width, anterior posterior diameter; CS, calcium score; DLP, dose–length product; IQR, interquartile range; MDCT, multi-detector computed tomography; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Saudi national DRLs The distribution of CT dose quantities for each centre is presented in Figures 1 and 2. From these data, combined DRL values and DRL values for each gating mode were established. The 75th percentile, median and 25th percentile for CTDIvol and DLP related to CCTA and CS for the current study are summarised in Table 2. Comparison of the results with DRLs reported from other countries is shown in Table 3. Table 2. DRLs for CCTA and Calcium score test in Saudi Arabia. Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; mixed modes, all scanning modes were inclusive; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 2. DRLs for CCTA and Calcium score test in Saudi Arabia. Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; mixed modes, all scanning modes were inclusive; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 3. CCTA DRLs in Saudi Arabia compared with other international DRLs. Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 3. CCTA DRLs in Saudi Arabia compared with other international DRLs. Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Figure 1. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using prospective ECG-gating mode (PGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for PGM. Centres 3, 5, 6, 10 and 11 did not provide data sets for CCTAs using PGM. Figure 1. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using prospective ECG-gating mode (PGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for PGM. Centres 3, 5, 6, 10 and 11 did not provide data sets for CCTAs using PGM. Figure 2. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using retrospective ECG-gating mode (RGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for RGM. Centres 2, 7 and 8 did not provide data sets for CCTAs using RGM. Figure 2. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using retrospective ECG-gating mode (RGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for RGM. Centres 2, 7 and 8 did not provide data sets for CCTAs using RGM. DISCUSSION Compared to other recently reported NDRL values, relatively low values were observed in this study (Table 3). The 75th percentile of the DLP value is 40% lower than the NDRL values from Italy(15) and Japan(16), but comparable to those from the Netherlands(17) and France(18). This low value can be attributed to the use of current dose saving technologies. For example, a PGM protocol was used for 55% of CCTAs in Saudi Arabia; this protocol was either not the routine scanning protocol employed by radiographers or was used in <23% of cases in Iran and Italy(15, 19). PGM has been shown to be the most effective recently developed dose-saving technology used in CCTA examinations(20, 21). The current study reports that PGM allowed a significant dose reduction with CTDIvol 54% lower than that for RGM. These wide dose variations between the ECG-gating modes emphasise the need to analyse the CTDIvol and DLP individually and, therefore, establish DRLs for each mode. The current work shows wide dose variations across the hospitals surveyed for CCTA; the highest was a 5-fold difference in median DLP between centres (169–984 mGy cm). These differences were associated with variations in the mAs and kVp. Among those centres using PGM, the lowest median DLP (169 mGy cm) was noted for Centre 2. A possible explanation for this low DLP is that more than 54% of CCTAs were performed with a kVp value ≤100 and mAs 13% lower than the mean mAs for all PGM scans. In contrast, Centre 1 showed the highest DLP amongst centres using RGM with IR, it is value that was 28% higher than the mean DLP of the sample. This high DLP may be due to the centre using higher kVp (≥120 kVp). An IR algorithm is routinely used by the majority of participating CT centres, with 91% of included CCTAs employing this method. Whilst Centre 6 is the only centre that performed CCTA without IR, Centre 5 has the same sizes of MDCT detectors (64 mm), pitch (0.2 mm) and kVp values (120) as Centre 6. Thus, the influence of the implementation of IR in dose reduction and DRLs for CCTA can be assessed. The present study found that the mean CTDIvol in Centre 6 was 17% higher than that at Centre 5, showing that IR is a powerful dose reduction tool and confirming data reported elsewhere in the literature(3, 22). Scanning parameters for CCTA are usually adapted based on body weight and BMI(23, 24). The mean patient weight in the current study was 77 kg, with more than 65% of patients weighing between 60 and 90 kg. The average weight of patients in the study sample reflects the distribution of patient weights in the Saudi population(25). Furthermore, the population’s weight is within the standard patient size (70 ± 15 kg) that is commonly used to establish the DRLs for CT in European countries(26). However, weight may not accurately account for variances in body habitus and for gender differences(24). The scan length must be adjusted based on patient indication. The z-axis coverage for CCTA examination is usually from carina to cardiac apex(27). A literature search showed that the median scan length for most CCTAs included in recent NDRL studies was 14 ± 1.5 cm(15, 17, 19, 28), which is in line with the results in this study. A noticeable variation in scan length was noted among CCTA scanning protocols applied with a median value of 14.5 cm and range of 8.5–25.9 cm. Several studies reported that the radiation exposure of patients during CCTA is reduced significantly if an adjusted scan length is used, based on a prior CS sequence(29, 30). Zimmermann and Dewey(29) reported that the mean effective dose of the total CCTA, CS sequence and CCTA was significantly smaller than the effective dose measured from CCTA without CS exam (8.5 ± 4.7 vs. 9.1 ± 6.0 mSv, p = 0.006). Considering that CS is performed solely for routine coronary screening purposes in potentially healthy individuals(31), findings show that 83% of patients whose data were used for this study had a CS scan. In addition, 11% of the total DLP resulted from the CS test, with <3% for scout views and test bolus scans. This dose level underscores the need to record dose quantities from CS sequence in any CCTA dose survey and report a separate DRL for the CS test. This was reported by several other authors as well as the current study, and the CS phase was presented as a separate portion of the CCTA DRLs(15, 19, 28). The 75th percentile DLP for CS was lower than those of recent NDRLs in Italy. This reduction in dose may be attributed to the shorter scan length used in the centres surveyed in the current work; the median scan length was 12.4 cm compared to 17.5 cm in Italy. Hospitals recruited for this study came from different sectors and regions. However, it is important to note that nine of the facilities were public institutions; only two private facilities responded to the survey. The inconsistency in response rates between public and private institutions can be explained by the absence of research centres in most private facilities and their need to protect commercial interests. However, the analysis showed no significant difference in CTDIvol and DLP between public and private hospitals. During data collection, there was potential for confusion between recording effective mAs and mAs. Our survey did not clearly differentiate which of these was required. Therefore, we contacted departments to clarify which mAs they had reported. The reason for confusion is that some CT scanner manufacturers report effective mAs in the dose report while others report both effective mAs and reference mAs. For example, the mAs is not recorded on the dose report in the GE system, which only records mA, whereas the Siemens system records only effective mAs. Use of effective mAs, which are obtained by dividing the true mAs per rotation by the pitch value, makes it difficult to compare mAs settings from different MDCT scanners(32). Thus, future work should provide clear instructions about which mAs to record on a DRL survey. Continuing professional education and training relating to best radiation protection practice in CT facilities may remedy this issue. This study also recommends that national regulatory authorities and hospitals continuously update DRLs to help reduce medical radiation exposure and potential risk of radiation-induced disease from CCTA in Saudi Arabia. CONCLUSION The data show that CCTA DRLs in Saudi Arabia are comparable or slightly lower than those in European countries. 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DDM2 Project Report Part 2: Diagnostic Reference Levels (DRLs) in Europe [Internet] ( 2014 ). Available on http://ddmed.eu/_media/news:ddm2_project_report_part_2_final_rb_16052014.pdf. 27 Hedgire , S. , Ghoshhajra , B. and Kalra , M. Dose optimization in cardiac CT . Physica Medica 41 , 97 – 103 ( 2017 ). Google Scholar Crossref Search ADS PubMed 28 Treier , R. , Aroua , A. , Verdun , F. , Samara , E. , Stuessi , A. and Trueb , P. R. Patient doses in CT examinations in Switzerland: implementation of national diagnostic reference levels . Radiat. Prot. Dosim. 142 ( 2–4 ), 244 – 254 ( 2010 ). Google Scholar Crossref Search ADS 29 Zimmermann , E. and Dewey , M. Whole-heart 320-row computed tomography: reduction of radiation dose via prior coronary calcium scanning . RöFo 183 ( 01 ), 54 – 59 ( 2011 ). Google Scholar PubMed 30 Gopal , A. and Budoff , M. J. A new method to reduce radiation exposure during multi-row detector cardiac computed tomographic angiography . Int. J. Cardiol. 132 ( 3 ), 435 – 436 ( 2009 ). Google Scholar Crossref Search ADS PubMed 31 Hong , C. , Bae , K. T. , Pilgram , T. K. , Suh , J. and Bradley , D. Coronary artery calcium measurement with multi–detector row CT: in vitro assessment of effect of radiation dose . Radiology 225 ( 3 ), 901 – 906 ( 2002 ). Google Scholar Crossref Search ADS PubMed 32 Nagel , H. D. CT Parameters That Influence the Radiation Dose ( New York, Berlin : Springer ) ( 2007 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

ESTABLISHING DIAGNOSTIC REFERENCE LEVELS FOR CARDIAC COMPUTED TOMOGRAPHY ANGIOGRAPHY IN SAUDI ARABIA

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Abstract

Abstract Cardiac computed tomography angiography (CCTA) is a commonly used diagnostic imaging tool for cardiovascular disease. Despite constant improvements to imaging technologies, the radiation dose to patients remains a concern when using this procedure. Diagnostic reference levels (DRLs) are used as a trigger to identify and alert individual facilities that are using high doses during CT. This study aims to assess patient radiation dose and establish new national DRLs (NDRL) associated with CCTA in Saudi clinical practices. A structured booklet survey was designed for recording patient and scanning protocols during CCTA procedures. The data were collected retrospectively from the participating centres. NDRLs for CCTA were defined as the 75th and 25th of volumetric CT dose index (CTDIvol) and dose length product (DLP). Specific DRLs based on two main ECG-gating modes were also proposed. Data sets related to 197 CCTAs with a mean weight of 77 kg were analysed in detail. The DRL values for CTDIvol and DLP for prospective gating mode and retrospective gating mode were 29 and 62 mGy and 393 and 1057 mGy cm, respectively. NDRLs for CCTA in Saudi Arabia are comparable or slightly lower than European DRLs due to the current use of dose-saving technology. There are major variations in patient doses during CCTA due to differences in CT scanners, scanning modes and departmental CCTA protocols. INTRODUCTION Cardiac computed tomography angiography (CCTA) has long played an important role in both the diagnosis and management of coronary artery disease (CAD). Current advances in CT scanners have allowed CCTA to directly visualise coronary anatomy, which has led to considerably improved diagnostic accuracy(1). Compared to conventional invasive coronary angiography (ICA), CCTA has high accuracy in the detection of coronary atherosclerosis through a non-invasive method, has shorter examination times and is more cost-effective than conventional coronary angiography(2). Therefore, the number of medical imaging centres that perform CCTA examinations is continuously increasing(3). One 2009 study that compared utilisation of ICA before and after introduction of CCTA in a large urban cardiology practice in the United States showed increased utilisation of CCTA, which was associated with a 45% decrease in ICA procedures(4). Several studies, however, have reported large variability in patient doses from CCTA, with diagnostic reference levels (DRLs) ranging from 671 to 1510 mGy cm(5). Those variations have been found to be due to differences in both scanning techniques and dose-reduction technologies available in multi-detector CT (MDCT) scanners, which can be customised to individual patient characteristics. DRLs represent a valuable tool for the continuous improvement of dose optimisation in medical imaging practice(6). They help identify and alert individual facilities using high doses during routine radiological procedures so that they can take corrective action regarding dose optimisation(7). They also provide a standard comparison that allows CT centres to compare the quantities of their local CT doses with national DRLs (NDRLs) for most CT examinations(6). Thus, it is important to establish DRLs for CCTA to enable doses from different scanning techniques to be assessed and compared. DRLs are based on determining the 75th percentile values of the distribution of reported median facility volume CT dose index (CTDIvol) and dose–length product (DLP) for routine CT examinations(8, 9). A literature review found noticeable variations in the sampling methodology used to identify patients and scanning protocols for setting CCTA NDRLs worldwide(5). In addition, there are no published studies on either doses or technologies used for CCTA procedures in Saudi Arabia. The purpose of this study was to assess CT dose levels in Saudi Arabia and establish DRLs for CCTA. The findings can provide a baseline for optimisation of CCTA examinations in Saudi Arabia. MATERIALS AND METHODS Ethical approval was granted by the University of Sydney HREC (2015/605) and by the Institutional Review Boards (IRB) of the individual hospitals recruited for the study. The Saudi health statistics annual report and a geographic information system for the Saudi Ministry of Health portal were reviewed to identify centres that provide a high proportion of cardiac imaging services to the whole population(10). Based on that information, 18 public and private hospitals and CT centres across Saudi Arabia were invited, on a voluntary basis, to participate in this study. Data sets for at least 20 most recent CCTA patients were collected retrospectively from the Radiology Information System (RIS) and Picture Archiving and Communication System (PACS). These included data on recent CCTAs performed for the assessment of CAD in adult patients. All CCTA examinations for the evaluation of bypass graft patency were excluded as they require a longer scan length than a standard CCTA, which would skew the results. No exclusion criteria based on patient weight were applied. Copies of the annual quality assurance (QA) reports were also collected from participating CT centres to confirm that all the CT scanners had a QA programme. Baseline information related to CCTA scanning parameters and patient characteristics were collected from each centre, for each patient scan included in the study data. This consisted of scan mode, tube potential (kVp), tube current–time product (mAs), rotation time, pitch, slice thickness, type and level of iterative reconstruction (IR). Patient characteristics, namely, age, sex, weight, height and cross-sectional area (CSA), were also recorded. The CT dose descriptor, DLP and CTDIvol were recorded from the dose report, including the total DLP from the scanogram, calcium score (CS), test bolus scan and angiographic phases. The scan length for a complete CCTA examination was calculated as shown in the following equation. Scanlength(cm)=DLP/CTDIvol. (1) A booklet was designed to record both the patient and scanner parameters from each centre. This booklet was adapted from that used for the United Kingdom NDRL surveys in 2011(11). Feedback from four registered radiographers was incorporated into a revised format. Instructions to assist radiographers to complete the survey successfully were also provided. Data sets were analysed using IBM SPSS Statistics v22.0 (IBM Corp. Armonk, NY). The median CTDIvol and DLP for CCTA and CS examinations were calculated and NDRL was reported as the 75th, median and 25th percentile. Those percentiles are most commonly and recently used as an indicator of the DRL(5). RESULTS Eleven of 18 hospitals (seven public, two university and two private) completed and returned the survey data booklets. The CT scanners ranged from 64 to 128 slices, and data sets from 11 different CT scanners were collected. Two types of MDCT systems by four different CT equipment manufacturers were included: 64 slice systems by General Electric (GE Healthcare, Milwaukee, USA), Siemens (Siemens Healthcare, Erlangen, Germany) and Philips (Philips Healthcare, Best, Netherlands); and a 128 slice dual source system by Siemens (Siemens Healthcare, Erlangen, Germany). Ten hospitals submitted complete CCTA dose information for at least 20 patients, and one hospital submitted data for only 12 patients due to the recent installation of a new CCTA scanner and PACS. After standardising for weight, this resulted in 197 CCTA data sets with the average hospital submitting of 18 ± 4 data sets. To standardise weight for the sampled population, which is necessary to establish CCTA DRLs, the survey data were filtered to match commonly accepted requirements(12). Thus, data sets for patients with weight above 105 kg were removed, resulting in standard weights for the sample of 70 ± 10 kg. This resulted in a sample of 197 CCTAs with mean weight of 77 kg. Of these, 108 CCTAs were performed using prospective ECG-gating mode (PGM) and 89 using retrospective ECG-gating mode (RGM). Some 63.5% of patients were male, reflecting the higher rates of cardiovascular disease and mortality in males compared to females in Saudi Arabia(13, 14). The characteristics of patients and CCTA protocols and equipment are shown in Table 1. Table 1. Patient and CCTA scanning characteristics (n = 197). Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) AP width, anterior posterior diameter; CS, calcium score; DLP, dose–length product; IQR, interquartile range; MDCT, multi-detector computed tomography; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 1. Patient and CCTA scanning characteristics (n = 197). Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) Characteristics No. of patients or mean (% or interquartile range) Gender  Male 125 (63.5%)  Female 72 (36.5%) Scan mode  PGM 108 (55%)  RGM 89 (45%) Patient age (years) 48 (IQR 39–56) Patient height (cm) 164 (IQR 158–170) Patient weight (kg) 77 (IQR 68–87) Body mass index (kg/m2) 30 (IQR 25–32) Transverse width (mm) 394 (IQR 361–420) AP width (mm) 260 (IQR 238–283) Cross-sectional area (cm2) 803 (IQR 674–894) MDCT model (slice)  GE (64) 141 (72%)  Siemens (64) 20 (10%)  Philips (64) 16 (8%)  Siemens (128) 20 (10%) Tube voltage (kV) 120 Median (IQR 80–140) Tube current–time product (mAs) 207 (IQR 160–245) Total DLP (mGy cm) 674 (IQR 379–936) Scan length (cm) 16 (IQR 14–18) Contrast media (mL) 82 (IQR 75–90) AP width, anterior posterior diameter; CS, calcium score; DLP, dose–length product; IQR, interquartile range; MDCT, multi-detector computed tomography; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Saudi national DRLs The distribution of CT dose quantities for each centre is presented in Figures 1 and 2. From these data, combined DRL values and DRL values for each gating mode were established. The 75th percentile, median and 25th percentile for CTDIvol and DLP related to CCTA and CS for the current study are summarised in Table 2. Comparison of the results with DRLs reported from other countries is shown in Table 3. Table 2. DRLs for CCTA and Calcium score test in Saudi Arabia. Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; mixed modes, all scanning modes were inclusive; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 2. DRLs for CCTA and Calcium score test in Saudi Arabia. Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 Scan type CTDIvol (mGy) DLP (mGy cm) 75th Median 25th 75th Median 25th Mixed modes 43 37 27 808 554.5 359 PGM 29 24 19 393 343 313 RGM 62 46 40 1057 808 605 CS test 5.8 4 3.7 69 58 46 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; mixed modes, all scanning modes were inclusive; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 3. CCTA DRLs in Saudi Arabia compared with other international DRLs. Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Table 3. CCTA DRLs in Saudi Arabia compared with other international DRLs. Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 Study CTDIvol (mGy) DLP (mGy cm) CS (mGy cm) Saudi Arabia 43 808 69 Switzerland (2010) 50 1000 150 Japan (2012) — 1510 — Netherlands (2013) — 671 — Italy (2014) 61 1208 131 France (2014) (PGM) 26 370 — (RGM) 44 870 — Iran (2016) 66.5 1073 187 CS, calcium score; CTDIvol, volume CT dose index; DLP, dose–length product; PGM, prospective ECG-gating mode; RGM, retrospective ECG-gating mode. Figure 1. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using prospective ECG-gating mode (PGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for PGM. Centres 3, 5, 6, 10 and 11 did not provide data sets for CCTAs using PGM. Figure 1. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using prospective ECG-gating mode (PGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for PGM. Centres 3, 5, 6, 10 and 11 did not provide data sets for CCTAs using PGM. Figure 2. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using retrospective ECG-gating mode (RGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for RGM. Centres 2, 7 and 8 did not provide data sets for CCTAs using RGM. Figure 2. View largeDownload slide Distribution of volume computed tomography dose index (CTDIvol) (A), and the dose–length product (DLP) (B), per centre using retrospective ECG-gating mode (RGM): the lower and upper edges of the boxes indicate the 25th and 75th percentiles, respectively. The lines inside the boxes indicate the median and the whiskers indicate the smallest and largest values. The dotted lines indicate the corresponding diagnostic reference levels (DRLs) for RGM. Centres 2, 7 and 8 did not provide data sets for CCTAs using RGM. DISCUSSION Compared to other recently reported NDRL values, relatively low values were observed in this study (Table 3). The 75th percentile of the DLP value is 40% lower than the NDRL values from Italy(15) and Japan(16), but comparable to those from the Netherlands(17) and France(18). This low value can be attributed to the use of current dose saving technologies. For example, a PGM protocol was used for 55% of CCTAs in Saudi Arabia; this protocol was either not the routine scanning protocol employed by radiographers or was used in <23% of cases in Iran and Italy(15, 19). PGM has been shown to be the most effective recently developed dose-saving technology used in CCTA examinations(20, 21). The current study reports that PGM allowed a significant dose reduction with CTDIvol 54% lower than that for RGM. These wide dose variations between the ECG-gating modes emphasise the need to analyse the CTDIvol and DLP individually and, therefore, establish DRLs for each mode. The current work shows wide dose variations across the hospitals surveyed for CCTA; the highest was a 5-fold difference in median DLP between centres (169–984 mGy cm). These differences were associated with variations in the mAs and kVp. Among those centres using PGM, the lowest median DLP (169 mGy cm) was noted for Centre 2. A possible explanation for this low DLP is that more than 54% of CCTAs were performed with a kVp value ≤100 and mAs 13% lower than the mean mAs for all PGM scans. In contrast, Centre 1 showed the highest DLP amongst centres using RGM with IR, it is value that was 28% higher than the mean DLP of the sample. This high DLP may be due to the centre using higher kVp (≥120 kVp). An IR algorithm is routinely used by the majority of participating CT centres, with 91% of included CCTAs employing this method. Whilst Centre 6 is the only centre that performed CCTA without IR, Centre 5 has the same sizes of MDCT detectors (64 mm), pitch (0.2 mm) and kVp values (120) as Centre 6. Thus, the influence of the implementation of IR in dose reduction and DRLs for CCTA can be assessed. The present study found that the mean CTDIvol in Centre 6 was 17% higher than that at Centre 5, showing that IR is a powerful dose reduction tool and confirming data reported elsewhere in the literature(3, 22). Scanning parameters for CCTA are usually adapted based on body weight and BMI(23, 24). The mean patient weight in the current study was 77 kg, with more than 65% of patients weighing between 60 and 90 kg. The average weight of patients in the study sample reflects the distribution of patient weights in the Saudi population(25). Furthermore, the population’s weight is within the standard patient size (70 ± 15 kg) that is commonly used to establish the DRLs for CT in European countries(26). However, weight may not accurately account for variances in body habitus and for gender differences(24). The scan length must be adjusted based on patient indication. The z-axis coverage for CCTA examination is usually from carina to cardiac apex(27). A literature search showed that the median scan length for most CCTAs included in recent NDRL studies was 14 ± 1.5 cm(15, 17, 19, 28), which is in line with the results in this study. A noticeable variation in scan length was noted among CCTA scanning protocols applied with a median value of 14.5 cm and range of 8.5–25.9 cm. Several studies reported that the radiation exposure of patients during CCTA is reduced significantly if an adjusted scan length is used, based on a prior CS sequence(29, 30). Zimmermann and Dewey(29) reported that the mean effective dose of the total CCTA, CS sequence and CCTA was significantly smaller than the effective dose measured from CCTA without CS exam (8.5 ± 4.7 vs. 9.1 ± 6.0 mSv, p = 0.006). Considering that CS is performed solely for routine coronary screening purposes in potentially healthy individuals(31), findings show that 83% of patients whose data were used for this study had a CS scan. In addition, 11% of the total DLP resulted from the CS test, with <3% for scout views and test bolus scans. This dose level underscores the need to record dose quantities from CS sequence in any CCTA dose survey and report a separate DRL for the CS test. This was reported by several other authors as well as the current study, and the CS phase was presented as a separate portion of the CCTA DRLs(15, 19, 28). The 75th percentile DLP for CS was lower than those of recent NDRLs in Italy. This reduction in dose may be attributed to the shorter scan length used in the centres surveyed in the current work; the median scan length was 12.4 cm compared to 17.5 cm in Italy. Hospitals recruited for this study came from different sectors and regions. However, it is important to note that nine of the facilities were public institutions; only two private facilities responded to the survey. The inconsistency in response rates between public and private institutions can be explained by the absence of research centres in most private facilities and their need to protect commercial interests. However, the analysis showed no significant difference in CTDIvol and DLP between public and private hospitals. During data collection, there was potential for confusion between recording effective mAs and mAs. Our survey did not clearly differentiate which of these was required. Therefore, we contacted departments to clarify which mAs they had reported. The reason for confusion is that some CT scanner manufacturers report effective mAs in the dose report while others report both effective mAs and reference mAs. For example, the mAs is not recorded on the dose report in the GE system, which only records mA, whereas the Siemens system records only effective mAs. Use of effective mAs, which are obtained by dividing the true mAs per rotation by the pitch value, makes it difficult to compare mAs settings from different MDCT scanners(32). Thus, future work should provide clear instructions about which mAs to record on a DRL survey. Continuing professional education and training relating to best radiation protection practice in CT facilities may remedy this issue. This study also recommends that national regulatory authorities and hospitals continuously update DRLs to help reduce medical radiation exposure and potential risk of radiation-induced disease from CCTA in Saudi Arabia. CONCLUSION The data show that CCTA DRLs in Saudi Arabia are comparable or slightly lower than those in European countries. There was variability in local DRLs between CT centres which could be attributed to differences in MDCT scanners, scanning modes and departmental CCTA protocols. PGM and IR are the current dose-reduction techniques used for most CCTA procedures in Saudi Arabia. Findings suggest that local DRL results should be communicated back to each CT centre to encourage optimisation of scan parameters and comparison with NDRL and other anonymised CT centres. ACKNOWLEDGEMENTS The authors sincerely thank the radiology management and CT radiographers in the CT centres that participated in this survey for their contribution to this study. REFERENCES 1 Pelgrim , G. , Oudkerk , M. and Vliegenthart , R. Computed Tomography Imaging of the Coronary Arteries ( Rijeka : InTech ) ( 2013 ). 2 Bastarrika , G. , Lee , Y. S. , Huda , W. , Ruzsics , B. , Costello , P. and Schoepf , U. J. CT of coronary artery disease . Radiology 253 ( 2 ), 317 – 338 ( 2009 ). 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Cardiol. 132 ( 3 ), 435 – 436 ( 2009 ). Google Scholar Crossref Search ADS PubMed 31 Hong , C. , Bae , K. T. , Pilgram , T. K. , Suh , J. and Bradley , D. Coronary artery calcium measurement with multi–detector row CT: in vitro assessment of effect of radiation dose . Radiology 225 ( 3 ), 901 – 906 ( 2002 ). Google Scholar Crossref Search ADS PubMed 32 Nagel , H. D. CT Parameters That Influence the Radiation Dose ( New York, Berlin : Springer ) ( 2007 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Journal

Radiation Protection DosimetryOxford University Press

Published: Oct 1, 2018

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