TY - JOUR
AU - Akhtari, Amin Shams
AB - Abstract Cardiac computed tomography angiography (CCTA) studies have risen concern of radiobiological effects over the patients. Therefore, estimating radiation doses absorbed during CCTA is important. In this study, we compared radiation dose and image quality by using three different retrospective electrocardiography (ECG) protocols. A total of 123 patients undergoing CCTA were divided in three different groups. We used full-dose modulation (CareDose4D) technique in group (1); fixed tube current 200 mAs for group (2); and in group (3), chest circumference was used to adapt tube current (180–200 mAs) and tube potential (100–120 kVp). For groups (1) and (2), tube potential adapted depends on body mass index (BMI) in which it was 100 kVp for BMI < 27 kg/m2, and 120 kVp for BMI ≥ 27 kg/m2. Quantitive assessment of image quality was calculated by measuring signal intensity (SI) and image noise (IN) in the proximal segments of aorta root on left and right coronary arteries. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were also calculated by using SI and IN. Two experienced radiologists using a 4-point scale assessed the subjective image quality. Our results show that in group (1), the mean effective dose was 4.46 mSv (range: 1.75–8.6 mSv) and for group (2), the mean effective radiation dose was 5.07 mSv (range: 2.57–9.74 mSv) and in group (3), the mean effective dose was 5.85 mSv (range: 3.36–12.17 mSv). Group (1) representing 12% and 23% decrease in radiation dose comparing by groups (2) and (3). In multivariate analysis, adjusting for BMI, radiation dose for patients with BMI < 27 kg/m2 was significantly different; 2.53 mSv for group (1) compared to 3.54 mSv in group (2) and 5.207 in group (3) (p < 0.0001). In addition, lowering tube potential from 120 to 100 kVp in 200 mAs fixed tube current, represents 27% decrease in radiation dose. The quantitative image quality (IN, SI, SNR and CNR) was not statistically significant among the groups. To sum up, Retrospective-ECG gating may reduce radiation dose by using automatic tube current modulation and 100kVp tube potential with preservation of image quality in patient’s whose BMI < 27 kg/m2. INTRODUCTION A total of 56 million deaths occurred worldwide in 2012. Of these, 38 million were due to noncommunicable diseases (NCDs), principally cardiovascular diseases, cancer and chronic respiratory diseases. The leading cause of NCD deaths in 2012 was 46.2% for cardiovascular diseases (17.5 million deaths)(1). Accordingly, a noticeable number of persons with suspected cardiovascular disease required clinical evaluation(2). Coronary computed tomography angiography (CCTA) has emerged as a gold standard diagnostic imaging modality for the noninvasive assessment of coronary artery disease with accepted clinical indications in selected patient groups(3, 4). Although computed tomography (CT) angiography has high diagnostic performance to detect acute coronary syndrome in the cardiac patients, there are concerns regarding radiobiological effects related to its use of radiation exposure over the patients(5). A multicenter study has reported that a mean effective radiation dose of CCTA is 12 mSv, and ranging from 5 to 30 mSv(6). Therefore many computed tomography angiography (CTA) studies used some techniques and strategies to reduce radiation dose including automated anatomy-based exposure control, different electrocardiography (ECG) controlled tube current modulation techniques in retrospective-gated computed tomography coronary artery (CTCA), and prospective ECG-triggering(7, 8). Body mass index (BMI)-adapted tube voltage and current protocols have been introduced in low-dose scanning protocols. However, BMI estimation does not represent human body shape reliably and which may perhaps potentially lead to overdosing in patients(8). In addition, low radiation dose has negative effect on image quality and temporal resolution in some patients(9, 10, 11). Due to fact that the importance of a diagnostic image quality for having accuracy on detection of heart disease cannot be ignored, using techniques that can reduce radiation dose without any changes in image quality are in consideration. The aim of this study was to evaluate radiation dose reduction and image quality in three different retrospective-ECG gating scans in CCTA. MATERIALS AND METHODS This study was approved by the Institutional Ethics Committee. Informed consent was obtained from all patients. Study population A total of 120 patients (from July 2016 to September 2016) who underwent cardiac Computed tomography angiography with 128-slice CT scanner (SOMATOM Definition AS, Siemens—Germany) were enrolled. Clinical exclusion criteria for CCTA included pregnancy, inability to achieve heart rate lower than 70 beat per minute (bpm), allergy to iodine-containing contrast material and respiratory system function (ability to hold a 20-s breath). None of the subjects had a history of chest surgery, traumatic deformity or breast augmentation. For patients with heart rate >70 bpm, under conditions of necessity, oral dose of 50 mg metoral was administered 30 min before CCTA examination. Body weight, height and chest circumference measurements were manually performed just before CCTA. Study protocol The patients were divided into three groups with different dose-saving strategies in retrospective-gated CCTA. In the first group, full dose modulation (CareDose4D) to determine mAs was applied (the quality reference mAs (QRM) was setting 176 and 160 mAs for the tube voltage 100 and 120 kVp, respectively) and in second group, fixed tube current (200 mAs) was used. In order to choose appropriate kVp, same methods based on the patients’ BMI were utilized. Accordingly, patients with BMI ≥27 kg/m2 were scanned at 120 kVp and those with <27 kg/m2 BMI were scanned using 100 kVp. In the third group, tube voltage and tube current were adapted to each patient’s chest circumference according to the protocol presented in Table 1. The procedure began with a topogram image of the patient reconstructed to rectangular grid, then calcium scoring was performed using retrospective ECG‑triggering over a single heartbeat with a gantry rotation time of 0.33 s, 1.2 mm slice collimation, peak tube voltage of 120 kV, tube current setting of 100 mA. After that, pre-monitoring scan was started (care bolus tracking) to determine the level of monitoring scans and then every patient was randomly assigned to a group and based on the selected group, tube voltage and tube current were adjusted. Finally coronary CTA was carried out for each patient. For all the three groups, retrospective-gated images, the best-phase mode was adopted for image reconstruction (during 40–70% of the RR interval). Table 1. Chest circumference-adapted scanning protocol for CCTA with retrospective ECG-triggering. Chest circumference (cm)  Tube voltage (kVp)  Tube current (mAs)  <85  100  180  85–90  100  200  90–95  120  180  ≥95  120  200  Chest circumference (cm)  Tube voltage (kVp)  Tube current (mAs)  <85  100  180  85–90  100  200  90–95  120  180  ≥95  120  200  Image analysis Subjective image quality was rated by two experienced CCTA radiologists who were blinded from the details of the CT acquisition. The image quality was subjectively assessed using the 16‑segment model proposed by the American Heart Association (AHA)(12). Images were analyzed in axial imaging using 4 scale ranging from 1 to 4, representing excellent, good, diagnostic and poor image quality, accordingly. Objective image quality was determined by measuring signal intensity (SI), image noise (IN), signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). The SI was derived by mean CT attenuation values (Hounsfield units) from two circular regions in the proximal segments of aorta root on left and right coronary arteries. The IN was defined by the average standard deviation (SD) of CT attenuation values in the same two regions of interest. The CNR was defined as the difference between the SI of the right and left coronary arteries and the mean density of the left lateral ventricular wall, which was divided by IN. The SNR was calculated as the SI of the right and left coronary arteries divided by the IN. Estimation of radiation dose The volumetric CT dose index (CTDIvol) and dose length product (DLP) were recorded from CT consul for each cardiac scan. The effective radiation dose was calculated by multiplied DLP to chest conversion factor k (0.014 mSv/mGy−1/cm−1)(13). Statistical data analysis The statistical software package used for data analysis was SPSS software (16.0, SPSS Inc., Chicago, IL, USA). We used the mean and standard deviation for continuous variable such as age and frequency and percentage for categorical data like gender. Agreement of image quality scores was assessed by using Cohen kappa coefficient. Comparison of patient data among groups used one-way ANOVA test. We used linear regression analysis including tube current and tube voltage as variables influencing image quality and radiation dose due to differences in groups’ protocols. RESULTS Patient characteristics The demographic data of 120 patients, 40 patients in each retrospective gating groups is shown in Table 2. Comparing heart rate, BMI and age, there are not any remarkable differences between the groups (p > 0.05). Although scan length and scan time were significantly different among groups (p = 0.03 and 0.04, respectively). Table 2. Demographic data.   Group (1)a mean ± SD (range)  Group (2)b mean ± SD (range)  Group (3)c mean ± SD (range)  p-Value  BMI (kg/m2)  27.53 ± 4.59 (18.94–38.02)  27.19 ± 3.26 (20.58–37.20)  28.03 ± 4.24 (19.57–37.95)  0.65  Age (y)  55.12 ± 10.67 (37–77)  56.42 ± 8.26 (27–76)  56.97 ± 8.06 (44–80)  0.64  Heart rate (bpm)  57.2 ± 5.61 (50–70)  58.67 ± 5.67 (50–70)  60.37 ± 7.17 (45–75)  0.07  Scan time (s)  6.71 ± 0.62 (5.58–7.74)  6.88 ± 0.88 (5.59–9.04)  6.44 ± 0.79 (5.26–8.44)  0.04  Scan length (mm)  182.72 ± 12.05 (161–202)  189.35 ± 12.78 (160–215)  182.87 ± 12.65 (153–204)  0.03    Group (1)a mean ± SD (range)  Group (2)b mean ± SD (range)  Group (3)c mean ± SD (range)  p-Value  BMI (kg/m2)  27.53 ± 4.59 (18.94–38.02)  27.19 ± 3.26 (20.58–37.20)  28.03 ± 4.24 (19.57–37.95)  0.65  Age (y)  55.12 ± 10.67 (37–77)  56.42 ± 8.26 (27–76)  56.97 ± 8.06 (44–80)  0.64  Heart rate (bpm)  57.2 ± 5.61 (50–70)  58.67 ± 5.67 (50–70)  60.37 ± 7.17 (45–75)  0.07  Scan time (s)  6.71 ± 0.62 (5.58–7.74)  6.88 ± 0.88 (5.59–9.04)  6.44 ± 0.79 (5.26–8.44)  0.04  Scan length (mm)  182.72 ± 12.05 (161–202)  189.35 ± 12.78 (160–215)  182.87 ± 12.65 (153–204)  0.03  aCareDose4D (full dose modulation). bFix tube current 200 mAs. cChest circumference adapted mAs. Radiation dose The mean effective dose was remarkably different in the three groups (Table 3). For the group (1), the radiation dose was 4.46 ± 2.1 mSv, however, for groups (2) and (3), the radiation dose was 5.07 ± 1.91 mSv and 5.85 ± 1.74 mSv, respectively. The mean CT dose index and DLP was lower for group (1) in comparison with group (2) and group (3) (p-values were 0.02 and 0.006, respectively). The result shows that for patient with BMI ≥ 27 kg/m2, the mean radiation dose was not different among the three group (p = 0.897). The scan time and scan length in three groups were different slightly. The resulted radiation dose of scan length of 182 mm was not different noticeably from scan length of 189 mm in three groups of patients (<0.1 mSv), therefore, it would be ignorable. The application of retrospective gating with dose modulation technique could reduce effective radiation dose up to 24% (p = 0.006). In addition, lowering tube voltage from 120 to 100 kVp in fix tube current (200 mAs) reduced radiation dose ~27%. Table 3. Radiation dose.   Group (1)  Group (2)  Group (3)  p-Value  Patient BMI < 27 kg/m2   EDa (mSv)  2.532 ± 0.393 (1.75–3.16)  3.542 ± 0.937 (2.58–5.78)  5.207 ± 1.37 (3.37–7.11)  < 0.00001   DLPb (mGy cm)  180.88 ± 28.132 (125.25–225.86)  253.01 ± 66.963 (186–413.04)  371.96 ± 97.86 (240.37–508.01)  < 0.00001  Patient BMI≥27 kg/m2   ED (mSv)  6.392 ± 1.061 (3.55–8.61)  6.605 ± 1.317 (4.79–9.74)  6.509 ± 1.859 (4.16–12.17)  0.897   DLP (mGy cm)  456.57 ± 75.849 (253.79–614.68)  471.81 ± 94.12 (342.19–696.16)  464.98 ± 132.83 (297.22–869.64)  0.897    Group (1)  Group (2)  Group (3)  p-Value  Patient BMI < 27 kg/m2   EDa (mSv)  2.532 ± 0.393 (1.75–3.16)  3.542 ± 0.937 (2.58–5.78)  5.207 ± 1.37 (3.37–7.11)  < 0.00001   DLPb (mGy cm)  180.88 ± 28.132 (125.25–225.86)  253.01 ± 66.963 (186–413.04)  371.96 ± 97.86 (240.37–508.01)  < 0.00001  Patient BMI≥27 kg/m2   ED (mSv)  6.392 ± 1.061 (3.55–8.61)  6.605 ± 1.317 (4.79–9.74)  6.509 ± 1.859 (4.16–12.17)  0.897   DLP (mGy cm)  456.57 ± 75.849 (253.79–614.68)  471.81 ± 94.12 (342.19–696.16)  464.98 ± 132.83 (297.22–869.64)  0.897  aEffective radiation dose. bDose length product. Image quality There were no significant differences between objective image quality scores except for SI which was reduced in group (3) (Table 4). The agreement between subjective images’ quality scoring by observers was good (k = 0.91). A total of 1920 segments were evaluated and there were no nondiagnostic coronary artery segments in each group (Table 5). Among 1203 segments scored excellent, 668 segments with good quality and 49 segments had diagnostic quality. There were significant differences between subjective images’ quality (p = 0.011). Table 4. Objective image quality data. Objective image quality  Group (1) mean ± SD (range)  Group (2) mean ± SD (range)  Group (3) mean ± SD (range)  p-Value  Signal intensity (HU)  434.48 ± 91.85 (241–668.5)  425.89 ± 95.93 (260–668)  383.14 ± 81.23 (237–607)  0.025  Image noise (HU)  28.7 ± 12.89 (10–81)  28.63 ± 13.22 (10–59)  24.68 ± 10.74 (12.50–59.5)  0.253  Signal-to noise-ratio  17.91 ± 8.84 (5.59–44.62)  17.77 ± 8.26 (5.01–47)  17.72 ± 6.92 (6.67–33.45)  0.994  Contrast-to noise ratio  14.72 ± 6.86 (3.63–35)  14.57 ± 7.07 (4.48–40.14)  14.07 ± 5.93 (5.07–27.86)  0.900  Objective image quality  Group (1) mean ± SD (range)  Group (2) mean ± SD (range)  Group (3) mean ± SD (range)  p-Value  Signal intensity (HU)  434.48 ± 91.85 (241–668.5)  425.89 ± 95.93 (260–668)  383.14 ± 81.23 (237–607)  0.025  Image noise (HU)  28.7 ± 12.89 (10–81)  28.63 ± 13.22 (10–59)  24.68 ± 10.74 (12.50–59.5)  0.253  Signal-to noise-ratio  17.91 ± 8.84 (5.59–44.62)  17.77 ± 8.26 (5.01–47)  17.72 ± 6.92 (6.67–33.45)  0.994  Contrast-to noise ratio  14.72 ± 6.86 (3.63–35)  14.57 ± 7.07 (4.48–40.14)  14.07 ± 5.93 (5.07–27.86)  0.900  Table 5. Image quality scores. Image quality score  Group (1) (%)  Group (2) (%)  Group (3) (%)  Excellent  69.2 (443/640)  61.2 (392/640)  57.5 (368/640)  Good  29.8 (191/640)  37 (237/640)  37.5 (240/640)  Diagnostic  0.9 (6/640)  1.7 (11/640)  5 (32/640)  Nondiagnostic  0 (0/640)  0 (0/640)  0 (0/640)  Image quality score  Group (1) (%)  Group (2) (%)  Group (3) (%)  Excellent  69.2 (443/640)  61.2 (392/640)  57.5 (368/640)  Good  29.8 (191/640)  37 (237/640)  37.5 (240/640)  Diagnostic  0.9 (6/640)  1.7 (11/640)  5 (32/640)  Nondiagnostic  0 (0/640)  0 (0/640)  0 (0/640)  DISCUSSION During last decades, radiation dose and concerns about the risk of cancer have emerged as argumentative subjects in clinical cardiology. To reduce radiation dose, many techniques such as ECG-controlled tube current modulation, lowering tube voltage, automatic exposure control, prospective ECG and high pitch scanning have been used, taking into account the dose-saving technique and possible effect on the image quality(9, 16–27). Many studies reported that the radiation dose was reduced by modulating tube voltage or current based on BMI values in CCTA(14, 15). It has been reported that decreasing tube voltage has reduced radiation dose and the image quality can remain constant, though(16, 17). In the present study, the patients in group (1) with modulated tube current protocol were compared with the patients in group (2) with the fixed tube current protocol. The patients in these two groups were matched for age, heart rate and gender and clinical indication. The tube potential was set at 100 kVp for BMI < 27 kg/m2 and 120 kVp for BMI ≥ 27 kg/m2. The mean radiation absorb dose of group (1) was 12% lower than group (2). The lower radiation dose and excellent image quality was the consequence of using modulated tube current technique in retrospective-ECG gating. In comparison with Feng et al.'s report, retrospective-ECG scanning with fixed tube potential 100 kVp versus the same protocol and tube potential 100–120 kVp in our current study, the mean radiation dose was roughly the same(18). In patients with a normal and regular heart rate, the use of ECG-controlled tube current modulation with diagnostic image quality was suggested by many authors(9, 19, 20). It was reported that the modulated tube current for retrospective ECG gating decreased the radiation dose up to 50% depending on heart rate(21–23). In our study, in multivariate analysis, adjusting for gender group, the mean radiation doses were different for females and males. In comparison with Ketelsen et al.'s study with the same protocol (the effective radiation dose was higher for both gender groups), the present study shows that the mean radiation dose is significantly deceased for females(24). The reason may be because of the difference in heart rate which was lower in our study (mean heart was 54 versus 60 bpm in Ketelsen study in the female group). By advanced techniques in recent CT scanners, such as increased x-ray tube power, larger volume coverage, faster gantry rotation time; these devices are able to capture cardiac images with higher heart rate and even with single heartbeat(9, 25, 26). Law w et al. study reported a higher mean radiation dose for patients with heart rate more than 65 bpm by using retrospective helical scanning protocol in 320-detector CT scanner in comparison with our study with 128-slice CT scanner(25). The unlikeness between the radiation doses may be caused by the effect of pitch factor and tube current on radiation dose reduction. BMI as the most routine method of estimating the body fat does not truly represent human body shape and size, especially in female cardiac patients(8). Some recent studies have proposed anthropometrics measurements adaption tube current and tube voltage to reduce radiation in cardiac CTA(27, 28). However, based on this research, the use of chest circumference measurement to determine tube voltage and tube current settings have not been published for retrospective ECG-gating protocol in cardiac CT imaging. We expected to diminish the radiation dose by using CC measurements as an alternative method for adapting tube current and tube voltage in CCTA. However, our results showed that, for patients with BMI≥27 kg/m2 using CC measurements (in group (3)) did not reduce the mean radiation dose in comparison with using BMI adaption (group (2)). We found a significant correlation between chest circumference and BMI in patients who have BMI≥27 kg/m2. In sub analysis, of patients with BMI < 27 kg/m2 radiation exposure was increased due to the use of CC measurement adaption. This might be caused by the selection of tube voltage according to patients’ chest circumferences. In comparison with Goshhajra et al.'s study, dose length product was lower for present study(28). This dissimilarity may probably be caused by the difference between the actual patients and the anthropomorphic phantom size. LIMITATION Our study has a few limitations. Firstly, it is difficult to compare our data to those of many prior reports most of which used prospective ECG protocols. Secondly, the mean radiation dose is different possibly due in part to the fact that we evaluated actual patients who tend to have different body size than phantom size. Lastly, some differences in the methodologies used in these studies were observed. We used care monitoring (pre-monitoring) CTA instead of test bolus tracking technique (the mean scan time for pre-monitoring was ranged 3.89–4.63 s). CONCLUSION The study showed that decreasing tube current and tube voltage are the feasible approaches to reducing radiation dose in CCTA. Furthermore, for patients with BMI≥27 kg/m2, chest circumference measurements have good correlation with BMI; as our data shows there is no different whether we use BMI adaption or chest circumference measurement to adjust tube current and tube voltage. Whereas using chest circumference adaption for patient with BMI < 27 kg/m2 leads to an increase in the effective radiation dose. Our study suggest that selecting appropriate tube voltage based on BMI and using CareDose4D technique provide optimum balanced between the radiation dose in cardiac patients and imaging quality. 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TI - EVALUATION OF RADIATION ABSORBED DOSE AND IMAGE QUALITY IN DIFFERENT RETROSPECTIVE-ECG GATING ACQUISITION METHODS OF CARDIAC CT ANGIOGRAPHY
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncx111
DA - 2018-02-01
UR - https://www.deepdyve.com/lp/oxford-university-press/evaluation-of-radiation-absorbed-dose-and-image-quality-in-different-N6C0bgwHww
SP - 304
EP - 309
VL - 178
IS - 3
DP - DeepDyve
ER -