Comparison on radiation effective dose and image quality of right coronary artery on prospective ECG‐gated method between 320 row CT and 2nd generation (128‐slice) dual source CT

Abstract This retrospective study was to compare the image quality of right coronary artery (RCA) and effective radiation dose on prospective ECG‐gated method between 320 row computed tomography (CT) and 2nd generation (128‐slice) dual source CT. A total of 215 candidates underwent CT coronary angiography using prospective ECG‐gated method, 120 patients enrolled in 320 row CT group, and 95 patients in dual source CT group. We divided RCA image quality scores as 1/2/3/4, which means excellent/good/adequate/not assessable and heart rates were considered, as well as the radiation dose. There is no statistically significant difference of RCA image quality of Score 1/2 between 320 row CT and 2nd generation dual source CT, but lower heart rate (<70/min) improved RCA image quality. Meanwhile, the 2nd generation dual source CT scan have significant lower radiation dose. For patients with high level heart rate variation, both prospective ECG‐gated method of 320 row CT scan (Toshiba) and 2nd generation dual source CT scan (Siemens) basically provided good image quality on RCA. There is an advantage of effective radiation dose reduction in prospective ECG‐gated method using the 2nd generation dual source CT scan. After the iodine contrast agent was injected into elbow vein, the threshold triggering method was used to carry out prospective gated scanning, and the acquired fault image was reconstructed by the standard post‐processing software of each manufacturer. The radiation dose value is obtained through the dose report automatically generated after each scan.


| INTRODUCTION
Coronary atherosclerotic heart disease is the leading cause of death worldwide. 1 Many trials to date have investigated the diagnostic accuracy of coronary computed tomography angiography (CCTA) when compared to the gold standard diagnostic test, invasive coronary angiography. 1 Until now, CCTA is one of the important achievements on CT technology in the past decade, which provides a convenient method for the diagnosis of coronary artery disease. 2 Coronary computed tomography angiography was proved to have a high negative predictive value. Coronary computed tomography angiography plays an important role as a less invasive investigation of coronary atherosclerotic heart (CAD) disease proved by its high negative predictive value, which also decreases the necessity of invasive digital subtraction angiography (DSA) for CAD diagnosis. 3 Nowadays, the diagnostic performance of coronary CT angiography has been significantly improved with the technological developments in multislice CT scanners from the early generation of 4-slice CT to the latest 320-slice CT scanners. 4 Different CT manufactures has continually announced and recommend their new models with technical advantages on higher image quality, faster scanning time, wider detector range, and lower effective radiation dose. With the increase in the width of the practical application of the detector, the width of the collimation in front of the spherical tube is correspondingly increased, and the eave effect and the invalid scanning radiation dose at the edge of the scanning range are also increased.
Despite its ongoing success and worldwide clinical implementation, it remains an often-challenging procedure in which image quality, and hence diagnostic value, is determined by both technical and patientrelated factors. 5 Two examples of scanning models are 320 row CT [320-detector row dynamic volume computed tomography, (DVCT)] (Toshiba) and 2nd generation (128-slice) dual source CT scan (Siemens), and prospective electrocardiographic (ECG)-gated method is what we selected to study. Subtraction CCTA using a second-generation 320-detector row CT showed improvement in diagnostic accuracy at segment base analysis in patients with severe calcifications. 6 Low-dose CT is highly effective and can reduce the potential risk of exposure to ionizing radiation. 7,8 Compared with retrospective ECG-gated method, prospective ECG-gated protocol has the advantage of low effective CT radiation dose exposure, and it is the method that uses forward-looking prediction of R wave timing, stepand-shoot axial acquisition with no table motion while imaging, and for single type cone beam reconstruction. 9 To the best of our knowledge, since prospective cardiac-gated CCTA was always performed with Toshiba 320-detector or Siemens 2nd generation 128slice dual-source scanners, 10 there is no comprehensive study comparing these two specific CT models of different manufactures, based on their image quality on right coronary artery (RCA) and effective radiation dose reduction. In this retrospective study, we share our experience comparing the corresponding parameters on these two mentioned CT devices, discuss their corresponding technologies and explore which device has better image quality and lower radiation dose.

2.A | Scanning method
Using double-syringe injector, 60 ml contrast agents and 30 ml saline were injected through cubital vein by 5.0 ml/s. The injection was triggered by corresponding threshold values of CT devices (threshold values of 320 row CT (Toshiba) and 2nd generation (128-slice) dual source CT (Siemens) were 150 and 120 HU, respectively), the regions of interest (ROI) were located in the aortic root. The 320 row CT (Toshiba) was only operated in prospective gated scan with the scan parameters: tube voltage at 100 kV with automated tube current modulation and slice thickness at 0.5 mm × 320. There is a range of 16 cm scanned in a single gantry rotation. Acquisition phase of cardiac cycle for heart rate (HR) below 70/min and the selection of R-R interval is at about 50%-80%. When HR is above 70/min and the selection of R-R interval, acquisition phase is at 30%-55%. The image reconstruction was using iterative reconstruction (IR) process.
The scanning of 2nd generation (128-slice) dual source CT (Siemens) is using step-and-shot method with setting of step-by-step distance at 38.4 mm. X ray tube voltage is at 80-100 kV with auto milliampere second. Acquisition phase of cardiac cycle for heart rate (HR) below 75/min and the selection of R-R interval is at about 30-50%. When HR is above 75/min and the selection of R-R interval, acquisition phase is at 50-80%. The corresponding image reconstruction is using iterative reconstruction (IR) method. 11 (scan parameters is shown in Table 1).  inadequate breath holding, which used retrospective ECG-gated method were excluded. All patients were treated with sublingual nitroglycerin 0.5 mg at 5 min before the CT examinations. For the candidates with HR > 90/min, they used 50 mg beta-blocker to stabilize the heart rate at 30-60 min before CT scan. After beta-blocker controlled, if the candidates still not able of achieving the desired heart rate (<90/min), still persistent arrhythmia with HR difference >10 beats/min or still atiral fibrillation, they underwent retrospective gated method by 2nd generation (128-slice) dual source CT scan (Siemens). These candidates were excluded from this study. Those could not hold breath were also excluded. Only the cases using prospective gated method of scanning on the two devices for CCTA, were included in this study. This study was conducted in accordance with the declaration of Helsinki. This study was conducted with approval from the Ethics Committee of Tongji University School of Medicine. Written informed consent was obtained from all participants.

2.E | Radiation dose assessment
Radiation dose reports on the two CT devices were generated automatically by the corresponding software from CT scanning equipment. By using radiation dose index (CTDI vol) and dose length product (DLP), multiplied by DLP and k conversion coefficient. (k), to estimate the effective dose (ED). The k value is based on adult chest for estimation, k = 0.014 mSv mGy −1 cm −1 . 13

2.F | Statistical analysis
Statistical work was performed using the SPSS17.0 software (SPSS Inc., Chicago, IL, USA). Those 215 cases of patients are divided into two groups according to the different heart rates (HR <70/min; HR70-90/min). In each of the two groups, further matching and analysis was done to prospective ECG-gated scan method between 320-row CT scan (Toshiba) and 2nd generation (128-slice) dual source CT scan (Siemens). Independent t-test on samples was performed to obtain mean and standard deviations for the right coronary image grading of quality and the effective radiation dose.
Statistically significant difference is defined if P < 0.05.

3.
A | Image quality score on RCA Excellent and good image quality of RCA scorings, are described on Score 1 and Score 2, respectively. To the images belongs to Score 1, To the HR group of 70 ≤ HR < 90/min, there is no statistical difference of case number proportions that belongs to Score 1 and Score 1 plus Score 2 using prospective ECG-gated method between 320-row CT (Toshiba) and 2nd generation dual source CT (Siemens) (P values are 0.508 and 0.094, respectively, P > 0.05). However, there is higher case number that belongs to Score 1 and Score 2 using 320-row CT (Toshiba) (Fig. 3).

3.B | Effective CT radiation dose
To the groups of HR < 70/min and 70 ≤ HR ≤ 90/min, the effective radiation dose of prospective ECG-gated method between 320 row CT (Toshiba) and 2nd generation (128-slice) row dual source CT are shown in Table 3. In Table 4, the effective radiation dose between prospective and retrospective ECG-gated method of 2nd generation (128-slice) row dual source CT are shown.
The result indicates effective radiation dose of 2nd generation (128-slice) row dual source CT on both HR groups are lower than 320 row CT (Toshiba) and effective radiation dose of prospective ECGgated method on both HR groups are lower than retrospective ECGgated method of 2nd generation (128-slice) row dual source CT. There are significant statistical differences (P < 0.05, P < 0.01) between the two CT devices and two methods in these two HR groups.
Once the rapid and instable HR is beyond the limitation, the combination of non-synchronicity and displacement of coronary artery, as well as out of phase cross-sectional scanning module, may cause step artifacts [ Fig. 4(a)]. Furthermore, it progresses to stairstep artifacts [ Fig. 4(b)].

4.A | Effect of heat rate on the image quality of RCA
As the progression of CT technology on short scan time and higher spatial resolution, there is lesser degree of correlation between the image quality of coronary artery to heart rate. [14][15][16] Our result shows no statistically significant difference between the two different HR groups on excellent and good image quality of RCA, as shown in In the future, we may consider to have an alternative method to replace this bolus tracing, so as to reduce the additional effective radiation dose on 320 row CT scanner (Toshiba). In practical, the criteria of patients selected for prospective-gated method for 2nd generation (128-slice) dual source CT (Siemens) is rather stringent because of the high failure rate on patients with high level HR variation or even atrial fibrillation. Instead, CCTA on these kinds of candidates were then completed using retrospective ECG-gated method.
Thus, prospective ECG gated method using 2nd generation (128slice) dual source CT (Siemens) cannot be completely adopted, although it has the advantage of low effective radiation dose.
On the contrary, the application of prospective ECG-gated method on 320 row CT (Toshiba) is not limited on these factors with high successful rate. Clinically, we had never performed retrospective ECG-gated method for CCTA using 320 row CT (Toshiba).
This study has some limitations, that include: (a) image reconstruction on RCA image quality is manually selected and subjectively optimized the best phase (%) of cardiac cycle; (b) although most of the candidates with heart rate over 90/min were successful adjusted by 25-50 mg beta-blocker, 22 there were still some cases excluded in (a) (b) Step artifact caused by stepand-shoot acquisition, shown by the black and white discrimination. (b) Heart rate variation that producing the stair-step artifacts (white arrow) that mimicking stenosis, and white bars highlight true stenosis.
this study, due to uncontrolled high level and variable HR; (c) to the same reason, some cases were excluded when they were assigned to use retrospective ECG-gated method on 2nd generation dual source CT (Siemens). And thus, this study is not in true randomized sampling; (d) some cases were excluded due to serious calcified plaques depositions and too small of coronary diameter, it may produce

| CONCLUSION
For the patients with high level heart rate variation, both prospective ECG-gated method of 320 row CT scan (Toshiba) and 2nd generation dual source CT scan (Siemens) were basically satisfactory for clinical diagnosis on the image quality of RCA. There is lower effective radiation dose on prospective ECG-gated method using 2nd generation dual source CT scan (Siemens). There was low failure rate on the prospective ECG-gated method using 320 row CT scanner (Toshiba) even on poor control heart rate candidates, on the contrary, using 2nd generation dual source CT scanner (Siemens) may need to adopt the retrospective ECG-gated method when prospective ECG-gated method is failure on rapid and instable HR group, while retrospective method is known to provide higher effective radiation dose. In our opinion, if patients have variable, irregular or fast heart rate, CT could lead to higher successful rate and more safety in radiation dose exposure.

CONF LICT OF I NTEREST
None.