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DOI:10.2214/AJR.07.2191
AJR 2008; 190:315-320
© American Roentgen Ray Society

Original Research

Radiation Dose, Image Quality, Stenosis Measurement, and CT Densitometry Using ECG-Triggered Coronary 64-MDCT Angiography: A Phantom Study

Jun Horiguchi1, Masao Kiguchi1, Chikako Fujioka1, Yun Shen2, Ryuichi Arie1, Kenichi Sunasaka3 and Katsuhide Ito4

1 Department of Clinical Radiology, Hiroshima University Hospital, 1-2-3, Kasumi-cho, Minami-ku, Hiroshima, 734-8551, Japan.
2 CT Lab of Great China, GE Healthcare, Mongkok, Kowloon, Hong Kong.
3 GE Yokogawa Medical Systems, Tokyo, Japan.
4 Department of Radiology, Division of Medical Intelligence and Informatics, Programs for Applied Biomedicine, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.

Received March 8, 2007; accepted after revision August 15, 2007.

 
Address correspondence to J. Horiguchi (horiguch{at}hiroshima-u.ac.jp).

J. Horiguchi, C. Fujioka, Y. Shen, R. Arie, K. Sunasaka, and K. Ito have a contracted research relationship with GE Healthcare in developing the software program, SnapShot Pulse, used in this study.


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to compare prospective ECG-triggered and retrospective ECG-gated coronary 64-MDCT angiography as to radiation dose, image quality, accuracy of stenosis measurement, and CT densitometry.

MATERIALS AND METHODS. Coronary artery models (n = 3) with different plaque densities ({approx} 50, {approx} 110, and {approx} 1,000 H) on a cardiac phantom were scanned in variable heart rate sequences (n = 14) with both prospective ECG-triggered and retrospective ECG-gated scanning. Radiation dose, image quality graded by motion and stairstep artifacts (grade 1, excellent, to grade 4, poor, with grades 1 and 2 defined as satisfactory), accuracy of stenosis measurement (area; 18%, 50%, and 82%), and CT densitometry of plaques (soft, {approx} 50; and intermediate, {approx} 110 H) were compared between the two protocols using the Mann-Whitney U test and repeated measures.

RESULTS. The radiation dose of prospective ECG-triggered CT angiography (CTA) (3.0 mSv) was lower than that of retrospective ECG-gated CTA (11.7–13.0 mSv) when the same tube current (mA) and voltage (kVp) were used in both methods. Prospective ECG-triggered CTA images were assigned a satisfactory quality rating in stable heart rate up to 75 beats per minute (bpm) when using the minimal X-ray exposure time. In this range, there were no significant differences in stenosis measurement (p = 0.17) and CT densitometry (p = 0.93) between the two protocols.

CONCLUSION. Prospective ECG-triggered coronary 64-MDCT has the potential to reduce radiation exposure while maintaining the diagnostic performance of retrospective ECG-gated coronary 64-MDCT.

Keywords: cardiac imaging • coronary artery • CT angiography • densitometry • radiation dose • stenosis measurement


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
The diagnostic capability of 64-MDCT for coronary artery disease has increasingly become accepted. For the detection of coronary artery stenosis in comparison with invasive coronary angiography, sensitivity ranging from 86% to 99% and specificity between 93% and 97% have been reported [1–6]. The high negative predictive value of 95–99% [1–6] suggests that coronary CT angiography (CTA) has the potential to rule out the presence of coronary stenosis in patients. In addition to luminography, coronary CTA is expected to play a roll in wall imaging, especially characterization of coronary artery plaque. The vulnerable plaque is prone to rupture and is known to be largely responsible for acute coronary syndrome. Plaque detection and characterization have been performed by invasive techniques such as intravascular sonography [7], optical coherence tomography [8], plaque thermography [9], and angioscopy [10]. Recently, plaque characterization using MDCT by calculating CT density has been reported [11–13]. Although the identification of calcified plaque is straightforward because of the higher CT density, differentiation of noncalcified plaque is vital and challenging.

The major drawback of retrospective ECG-gated coronary CTA is its high radiation dose because of overlapping data acquisition, acquiring data in cardiac phases that do not contribute to image reconstruction. The estimated effective dose reported with 16- or 64-MDCT using 120 kV and ECG-correlated tube current modulation (reduction of tube current in systole), which reduces radiation exposure by 30–50% [14, 15], was estimated to be between 6.4 and 14.7 mSv [15–17]. Prospective ECG-triggered coronary CTA has so far been performed using electron-beam CT. Wide coverage and reduced gantry rotation time on 64-MDCT and shortening of the interscan delay enable the acquisition of thin-slice data sets during one breath-hold [18]. The prospective ECG-triggered coronary 64-MDCT software minimizes overlapping X-ray exposure, thus enabling far greater reduction of radiation exposure. The purpose of this study was to compare prospective ECG-triggered and retrospective ECG-gated coronary 64-MDCT as to radiation dose, image quality, accuracy of stenosis measurement, and CT densitometry in a phantom study.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Cardiac Phantom
We used a prototype commercially available cardiac phantom (Alpha 2, Fuyo Corporation). The phantom consisted of five components: driver, control, support, rubber balloon, and ECG. A controller with an ECG-synchronizer drives the balloon. The construction of the phantom is described in detail elsewhere [19, 20]. The main characteristics of this phantom are programmable variable heart rate sequences and mimicking of natural heart movements. In this study, 14 types of heart rate sequences were programmed (Table 1).


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  		TABLE 1: Heart Rate Sequences Programmed

 

Coronary Artery Plaque Models
Four coronary artery cylinder models (Coronary Artery Vessel Phantom, Fuyo Corporation) with a diameter of 4 mm and different plaque densities were manufactured for this experiment. For plaque materials, acrylonitrile-butadiene-styrene resin ({approx} 50 H), acrylic ({approx} 110 H), and polytetrafluoroethylene (Teflon, DuPont) ({approx} 1,000 H) were used. The plaque CT values of 50, 110, and 1,000 H represented soft, intermediate, and calcified plaques, respectively. Each coronary artery model had three levels of plaque size (1, 2, and 3 mm in diameter), resulting in area stenoses of the coronary arteries of 18%, 50%, and 82% (Fig. 1). The coronary artery models were attached to the balloon phantom (mimicking the heart) with the long axis of the model corresponding to the z-axis and were surrounded by corn oil (–112 H) simulating epicardial fat (Fig. 2).


Figure 1
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  		Fig. 1 —Drawing of coronary arterial plaque models shows size, D-shape, and three levels of stenosis of coronary arterial plaque models. Plaque diameters of 1, 2, and 3 mm correspond to area stenosis of 18%, 50%, and 82%. Three coronary artery models of different plaque densities ({approx} 50, {approx} 110, and {approx} 1,000 H) were used.

 

Figure 2
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  		Fig. 2 —Diagram of cardiac balloon phantom shows balloon was filled with mixture of water and contrast medium (45 H) to simulate unenhanced blood and was submerged in corn oil (–112 H), simulating epicardial and pericardial fat. Coronary artery models (n = 4) were attached to balloon surface. ABS = acrylonitrile-butadiene-styrene.

 
Retrospective ECG-Gated Coronary 64-MDCT Angiography Protocol
The 64-MDCT retrospective ECG-gated coronary CTA was performed using a LightSpeed VCT (GE Healthcare). Scanning covered the entire volume of the cardiac phantom (range, 14 cm) with simultaneous recording of the ECG trace. Detector collimation was 64 x 0.625 mm, gantry rotation speed was 350 milliseconds per rotation, and tube voltage was 120 kV. The maximum current was set to 650 mA during the cardiac phase 70–90% of the R-R cycle and was reduced in the other phase to a minimum of 130 mA. CT pitch factors varied from 0.20 to 0.24 depending on the heart rate in accordance with the manufacturer's recommendations for coronary CTA. The matrix size was 512 x 512 pixels and the display field of view was 12 cm. The reconstruction kernel was "Std," which is routinely used for abdominal imaging. For heart rates < 75 beats per minute (bpm), a half-scan algorithm was applied, and for heart rates ≥ 75 bpm, a two-sector reconstruction algorithm, offering an improved temporal resolution, was used. A half-scan reconstruction requires about a two-thirds rotation data acquisition compared with full rotation (360°) for a full-scan reconstruction. Temporal resolution in single-sector reconstruction is approximately equal to half the rotation time. A two-sector reconstruction provided a temporal resolution down to 87 milliseconds, depending on heart rates. For image reconstruction, a cardiac phase with the center of the temporal window corresponding to 80% (diastole of the phantom) of the R-R interval was used.

Prospective ECG-Triggered Coronary 64-MDCT Angiography Protocol
A prospective ECG-triggered software protocol (SnapShot Pulse, GE Healthcare) in which a 64-detector rows x 0.625 mm-collimation configuration (40 mm) is used and three or four radiographic exposures per examination with 5-mm overlapping that can cover 10.5 cm or 14 cm in the z-axis was available in our institution. This overlapping is used for minimizing cone-beam artifacts (Fig. 3). The tube current of 650 mA and other scanning parameters were the same as in the retrospective ECG-gated coronary CTA protocol. In this software, X-ray exposure time was changeable from 233 to 633 milli-seconds, maintaining the temporal resolution of 175 milliseconds. We set the exposure time to the minimum (233 milliseconds) in the current study. The prospective scanning was performed so that the center of the temporal window corresponded to 80% of the R-R interval.


Figure 3
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  		Fig. 3 —Prospective ECG-triggered coronary 64-MDCT software (SnapShot Pulse, GE Healthcare) shows X-ray beam of 40 mm (64 rows x 0.625 mm collimation) is exposed when table is stationary, and then table moves by 35 mm, allowing 5-mm overlapping to next location for another scan. Four exposures cover 14 cm in z-axis. Overlap of 5 mm is used for minimizing cone-beam artifacts.

 
Radiation Dose
The volume CT dose index (CTDIvol) and dose–length product (DLP) displayed on the dose report on the CT scanner were recorded on prospective ECG-triggered and retrospective ECG-gated scans. A reasonable approximation of the effective dose (E) can be obtained using the equation [21]

Formula
where E is the effective dose estimate and with k = 0.017 mSv x mGy–1 x cm–1. This value is applicable to chest scans and is averaged between male and female models.

Image Quality Graded by Motion and Stairstep Artifacts
In each of the 14 heart rate sequences, the image quality of three coronary artery models was assessed separately by three independent observers who had coronary CTA reading experience of 5, 3, and 3 years. All data sets were blinded with regard to the protocols, reconstruction technique, and heart rate, and they were assessed in a random order. Images were displayed with a fixed window setting (width, 800 H; level, 130 H) and evaluated on a separate workstation (Advantage Workstation version 4.2, GE Healthcare). We used a classification system that was modified from Herzog et al. [22], in which image quality was graded in a 4-step scale (grade 1, excellent; grade 2, good; grade 3, moderate; and grade 4, poor). Excellent image quality was attributed to vessels showing a continuous course, without stairstep artifacts on volume-rendered or reformation images and appearing on transverse CT scans as bright circular areas without motion artifacts and surrounded by low-attenuation fat tissue. Good image quality was assigned in the presence of discrete blurring of vessel margin, minor motion artifacts seen as a discrete tail or streak-emitting shadows on transverse images, and minimal stairstep artifact on volume-rendered or reformation images. Moderate image quality was assigned to noticeably blurred vessels or plaque margins, distinctly broader motion artifacts extending less than 5 mm from the vessel center, and stairstep artifacts of less than 25% of the vascular diameter. Poor image quality was defined as an inadequate delineation between the vessel and surrounding tissue, the presence of streak artifacts extending at least 5 mm from the center of the vessel, and stairstep artifacts of more than 25% of the vessel diameter. The grading was performed nine times (three coronary artery models, three readers) per heart rate sequence, and the grades were compared between prospective ECG-triggered and retrospective ECG-gated images using the Mann-Whitney U test.

The grade per coronary artery model was also determined by consensus of three readers or by a two- to-one decision. Images from heart rate sequences, in which all grades for coronary artery models (n = 4) on prospective ECG-triggered images were graded 1 or 2, were used for further investigations.

Measurement Error of Stenosis
Stenosis levels of soft, intermediate, and calcified plaques were measured on both prospective ECG-triggered and retrospective ECG-gated coronary CTA protocols using the workstation semiautomatic software Vessel Analysis (Advantage Workstation version 4.2, GE Healthcare). Using this software, one reader plotted four or five points at the center of the lumen, and then stenosis in the vessel was automatically calculated. The measurements were recorded in 12 slices per stenosis level. The measurement error of coronary artery stenosis was defined as abs (measured area of the stenosis – known stenosis area)/known stenosis area. The measurement errors of the three kinds of plaque were compared between prospective ECG-triggered and retrospective ECG-gated coronary CTA protocols using repeated measures analysis of variance.

CT Densitometry of Soft and Intermediate Plaque
CT densities of soft and intermediate plaques were measured on both prospective ECG-triggered and retrospective ECG-gated coronary CTA protocols. Regions of interest (ROIs) of 1 mm2 were set at the center of the 50% stenosis plaque (Fig. 4). CT densitometry measurements were performed in 10 slices per plaque by one reader. CT densitometry, from the heart rate sequences selected, was compared between prospective ECG-triggered and retrospective ECG-gated coronary CTA protocols.


Figure 4
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  		Fig. 4 —Drawing shows region of interest (ROI) placement in plaque. ROI of 1 mm2 is set at center of plaque to minimize influence of intracoronary artery enhancement.

 
Statistical Analysis
All statistical analyses were performed using a commercially available software package (Statcel2, OMS Publishing). For statistical analysis, the Student's t test, Mann-Whitney U test, and repeated measures analysis of variance were used to determine the differences. When statistical significance was observed by repeated measures analysis of variance, the results were made post hoc by the Scheffé test for multiple pairwise comparisons. Differences were considered statistically significant at p < 0.05.


Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The Student's t test revealed no statistical difference in scanning times (needed for a typical patient with a scanning range of 14 cm) between prospective ECG-triggered and retro-spective ECG-gated coronary CTA (5.5 ± 0.7 [SD] vs 5.2 ± 0.4 seconds, p = 0.27).

Radiation Dose
CTDIvol and DLP displayed on the dose report on the CT scanner were 12.6 mGy and 176 mGy x cm for the prospective ECG-triggered and 49.3–54.5 mGy and 690–763 mGy x cm (depending on the pitch selected) on the retro-spective ECG-gated coronary CTA. Effective doses, calculated for typical patient scanning ranges, were 3.0 mSv on prospective ECG-triggered and 11.7–13.0 mSv on retrospective ECG-gated coronary CTA.

Image Quality Graded by Motion and Stairstep Artifacts
Image quality grading results of both prospective ECG-triggered and retrospective ECG-gated coronary CTA on 14 heart rate sequences are shown in Table 2. Prospective ECG-triggered CTA on stable heart rate sequences up to 75 bpm had satisfactory image quality. In this range, the image quality of prospective ECG-triggered CTA was comparable or superior (at 70 bpm) to that of retro-spective ECG-gated CTA. In contrast, image quality of prospective ECG-triggered CTA was unsatisfactory on shift (unstable heart rate) and arrhythmia sequences. In accordance with coronary artery model analysis, there was a consensus decision in 52 (62%) and a two-to-one decision in 32 (38%) of 84 assignments. Split decision between the three readers was not observed. Image quality of prospective ECG-triggered CTA was graded 1 or 2 (satisfactory) on all images up to 75 bpm. Therefore, these images were used for further investigations.


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  		TABLE 2: Image Quality Graded by Motion and Stairstep Artifacts

 


Figure 5
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  		Fig. 5 —Graph shows measurement error of stenosis of soft ({approx} 50 H), intermediate ({approx} 110 H) and calcified ({approx} 1,000 H) plaques on five selected heart rate sequences (55–75 beats per minute, satisfactory image quality) on retrospective ECG-gated CT angiography (CTA) (gray bars) and prospective ECG-triggered CTA (white bars). Bars and vertical lines indicate mean and SD, respectively.

 


Figure 6
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  		Fig. 6 —Graph shows measured CT density of soft ({approx} 50 H) and intermediate ({approx} 110 H) plaques on five selected heart rate sequences (55–75 beats per minute, satisfactory image quality) on retrospective ECG-gated CT angiography (CTA) (gray bars) and prospective ECG-triggered CTA (white bars). Bars and vertical lines indicate mean and SD, respectively.

 
Measurement Error of Stenosis
The result of measurement error of stenosis is shown in Figure 5. Repeated measures analysis of variance revealed that there was no statistical significance of stenosis error between retrospective ECG-gated and prospective ECG-triggered coronary CTA (p = 0.60); however, there was statistical significance between the three plaques (p < 0.01). The Scheffé test revealed statistical significance between soft and calcified plaques (p < 0.01) and between intermediate and calcified plaques (p < 0.01).

CT Densitometry of Soft and Intermediate Plaque
The results of CT densitometry are shown in Figure 6. Repeated measures analysis of variance revealed that there was no statistically significant difference of CT densitometry between retrospective ECG-gated and prospective ECG-triggered coronary CTA (p = 0.93); however, there was a significant difference between soft and intermediate plaques (p < 0.01). The Scheffé test revealed statistical significance between the plaques (p < 0.01).


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The additional lifetime risk of fatal cancer due to undergoing CT has been estimated as approximately 1 in 20,000 per mSv for the whole population by the International Commission on Radiological Protection [23]. In CT, the exposure to radiation needs to be kept as low as reasonably achievable (ALARA). In its increase in spatial and temporal resolution, coronary 64-MDCT is associated with an increased radiation dose compared with coronary 16-MDCT [15]. The effective dose is higher than that reported for diagnostic coronary angiography reported—that is, 2.1/2.5 mSv (male/female) [24], 3–10 mSv [21], 5.6 mSv [16], and 6/7 mSv (male/female) [25]. For reduction of radiation dose, the ECG-correlated tube current modulation or the use of low tube voltage such as 80 kV [17] or 100 kV [15] has been applied. The radiation dose varies with the square of the kilovoltage.

Furthermore, the decreased tube voltage leads to increased opacification of vascular structures during contrast-enhanced CTA owing to an increase in the photoelectric effect and a decrease in Compton scattering [26]. The radiation dose of the current prospective ECG-triggered coronary CTA is dramatically reduced because overlapping data acquisition is minimized. In addition, the technology principally acquires data only when it is necessary for image reconstruction. Thus the radiation dose of the prospective ECG-triggered CTA in the current study was lower than that of diagnostic coronary angiography, retrospective ECG-gated coronary 64-MDCT, and average annual background radiation in the United States (3.6 mSv) [21]. The low-dose prospective ECG-triggered coronary 64-MDCT CTA has the advantage of following up patients after stent implantation and coronary artery bypass grafts.

To keep the radiation dose to a minimum, we set the X-ray exposure time at 233 milliseconds (two thirds of the gantry rotation speed). The optimal cardiac cycle for reconstruction is known to differ between individuals and coronary arteries. In a study of coronary 16-MDCT with a rotation time of 420 milliseconds, the best image quality was obtained with end-systolic and early diastolic intervals in patients with high heart rates [22]. In a more recent study, however, using 64-MDCT with a rotation time of 330 milliseconds, the best image quality was provided in middiastole in most patients, although it was nondiagnostic at any reconstruction interval in some patients [27]. With the current software, we are able to set the exposure time up to a maximum of 633 milliseconds in an interval of 2 milliseconds. Using the "Dynamic Padding" function, at the expense of increased radiation exposure, other cardiac phases can be retrospectively reconstructed while a temporal resolution of 175 milliseconds is maintained. This function, a trade-off between increased radiation dose—however, far less than that of retrospective-ECG-gated CTA—and reconstruction availability, should be validated in clinical studies.

Limited temporal and spatial resolutions compared with catheter angiography and calcium deposition are major causes of reduced diagnostic performance of coronary CTA. Bulky calcification causes blooming effect and volume averaging. Therefore, the accuracy for detection of coronary stenosis is reported to be lower in the presence of severe calcification [27–29]. The results of measurement errors of stenosis on soft and intermediate plaques are acceptable in consideration of the spatial resolution of the CT scanner and the plaque sizes. We, however, must regretfully acknowledge that the stenosis measurement for calcified plaque is difficult on coronary CTA.

With the introduction of the previously mentioned software, we believe that our strategy for single-source coronary 64-MDCT will be as follows: If the patient's heart rate is up to 75 bpm and stable, prospective ECG-triggered CTA with minimal X-ray exposure is recommended. The robustness of prospective ECG-triggered CTA with the padding function against heart rates up to 75 bpm with a small variation needs investigation. If the patient's heart rate is higher than 75 bpm and stable, the use of retrospective ECG-gated CTA with ECG-correlated tube current modulation and multisector reconstruction is recommended. If the patient's heart rate is mildly irregular, retrospective ECG-gated CTA without ECG-correlated tube current modulation followed by ECG editing [30]—that is, by arbitrarily modifying the position of the temporal windows within the cardiac cycle to correct and compensate for part or all of the artifacts produced by mild heart rhythm irregularities—will be necessary.

This study has several limitations. We reconstructed only one cardiac phase (the center of the temporal window corresponding to 80% of the R-R cycle) on the prospective ECG-triggered CTA. Therefore, we did not validate the Dynamic Padding function. We did not investigate low-voltage scanning as another method of decreasing radiation exposure. We think that these issues should be verified in clinical studies.

In conclusion, SnapShot Pulse software has the potential to reduce the radiation dose of 64-MDCT. When using the minimal X-ray exposure time, this software seems to be comparable to retrospective ECG-gated coronary 64-MDCT on stable heart rates up to 75 bpm.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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C. Fujioka, J. Horiguchi, M. Kiguchi, H. Yamamoto, T. Kitagawa, and K. Ito
Survey of Aorta and Coronary Arteries With Prospective ECG-Triggered 100-kV 64-MDCT Angiography
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K. M. Takakuwa, E. J. Halpern, E. L. Gingold, D. C. Levin, and F. S. Shofer
Radiation Dose in a "Triple Rule-Out" Coronary CT Angiography Protocol of Emergency Department Patients Using 64-MDCT: The Impact of ECG-Based Tube Current Modulation on Age, Sex, and Body Mass Index
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P. Stolzmann, H. Scheffel, S. Leschka, A. Plass, S. Baumuller, B. Marincek, and H. Alkadhi
Influence of Calcifications on Diagnostic Accuracy of Coronary CT Angiography Using Prospective ECG Triggering
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E J HALL and D J BRENNER
Cancer risks from diagnostic radiology
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