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DOI:10.2214/AJR.07.3120
AJR 2008; 190:1561-1568
© American Roentgen Ray Society

Original Research

Optimal Cardiac Phase for Coronary Artery Calcium Scoring on Single-Source 64-MDCT Scanner: Least Interscan Variability and Least Motion Artifacts

Noriaki Matsuura1, Jun Horiguchi2, Hideya Yamamoto3, Nobuhiko Hirai2, Tetsuji Tonda4, Nobuoki Kohno3 and Katsuhide Ito1

1 Department of Radiology, Division of Medical Intelligence and Informatics, Programs for Applied Biomedicine, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
2 Department of Clinical Radiology, Hiroshima University Hospital, 1-2-3, Kasumi-cho, Minami-ku, Hiroshima 734-8551, Japan.
3 Department of Molecular and Internal Medicine, Division of Clinical Medical Science, Programs for Applied Biomedicine, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
4 Department of Environmetrics and Biometrics, Research Institute for Radiation Biology and Medicine, Hiroshima University, Hiroshima, Japan.

Received September 7, 2007; accepted after revision January 2, 2008.

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

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Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of our study was to investigate the cardiac phase with the least interscan variability and motion artifacts on coronary artery calcium studies using a 64-MDCT scanner.

SUBJECTS AND METHODS. Ninety-one patients with suspected coronary artery disease were scanned twice on retrospective ECG-gated helical scans. Images with 2.5-mm thickness and 1.25-mm interval at nine cardiac phases (center of cardiac phase: 40-80% in 5% increments) were reconstructed. The interscan variability of coronary artery scores (Agatston, volume, and mass) per patient and motion artifact scores per branch, subjectively assigned by motion artifact grading (1, none; 2, minor; and 3, major), were compared between cardiac phases for all patients, low (< 65 beats per minute [bpm]) and high (≥ 65 bpm) heart rate patient groups.

RESULTS. For all patients, two-factor factorial analysis of variance revealed that the interscan variability was different between cardiac cycles (p < 0.01); however, this was not statistically significant between scoring algorithms (p = 0.46). The least variability was obtained at 70% on Agatston (8%) and volume (7%) and at 75% on mass (7%). Adjacent categories logit model analysis revealed that the motion artifact score was the least at 75% (left anterior descending coronary artery, 1.3; left circumflex coronary artery, 1.4; and right coronary artery, 1.9 in all patients) and that a smaller difference in calcium scores between the scans led to a smaller motion artifact score (p < 0.05).

CONCLUSION. Middiastole reconstruction (center of cardiac phase: 70-75%), with the least interscan variability and the least motion artifacts, is recommended on 64-MDCT.

Keywords: calcification • calcium score • cardiac imaging • coronary artery • CT


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
Coronary artery calcium scoring using electron-beam CT, introduced by Agatston et al. [1], is used to estimate the risk of coronary artery disease and is suggested for the follow-up of patients under treatment such as lipid-lowering pharmacologic therapy [2-4]. Compared with the normal progression of the coronary artery calcium score per year, reported to be 14-27% (average, 24%) and 33-48% with significant coronary disease [5, 6], the interscan variability of the Agatston score, reported to range from 20% to 37% [7-10], is high. Other coronary artery calcium scoring algorithms, such as the volumetric approach [7] and mass scoring [8], have been devised to improve the reproducibility of coronary artery calcium measurement.

Among many factors influencing interscan variability in coronary artery calcium, such as the measuring method, motion artifact, signal-to-noise ratio, partial volume effects, triggering errors, point of image acquisition, and varying analysis tools, the main factor is suggested to be the motion artifact of the coronary artery on the image [11]. For images free of cardiac motion artifacts, acquisition times shorter than 19.1 milliseconds are necessary [12, 13]. By reducing motion artifacts in the optimization of ECG triggering, however, cardiac images with the least motion can be obtained with an acquisition time of 50-70 milliseconds for a patient baseline heart rate of 50-100 beats per minute (bpm) [13]. The e-Speed scanner (GE Healthcare) and dual-source CT scanners with high temporal resolution may permit freezing the heart in most patients; however, no single-source 64-MDCT scanner with half reconstruction has a temporal resolution high enough to achieve this.

There have been many single-source MDCT studies [14-25] investigating the optimal cardiac window for coronary artery calcium scoring and coronary CT angiography (Table 1). Thus far, to the best of our knowledge, no 64-MDCT studies have been reported that document the optimal cardiac phase for coronary artery calcium scoring. In a study using 16-MDCT, Schlosser et al. [14] concluded that "To assess the accurate score for the follow-up examinations, it seems mandatory to perform reconstructions that cover the entire cardiac cycle. Further studies will be needed to show whether the mean values of diastolic or systolic measurements are better suited for follow-up examinations than are reconstructions performed at a fixed time point within the cardiac cycle." On coronary CT angiography using 64-MDCT with a gantry rotation time of 0.37 second, Leschka et al. [20] stated that the best image quality was obtained in systole at 80 bpm or higher. Wintersperger et al. [21], using 64-MDCT with a gantry rotation time of 0.33 second, concluded that the time point of best image quality shifts from middiastole to systole with increasing heart rate. In contrast to this, Leschka et al. [19], using 64-MDCT with a gantry rotation time of 0.33 second, stated that the best overall image quality was obtained in middiastole, although images were rated as nondiagnostic in a small number of patients.


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  		TABLE 1: Recommended Cardiac Cycle for Reconstruction of Coronary Arteries on MDCT

 

The main purpose of this study is to investigate the cardiac phase with the least interscan variability and motion artifacts on coronary artery calcium studies using single-source 0.35-second rotation speed 64-MDCT.


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Patients
The study was approved by our institutional review board and written informed consent was obtained from all patients. For 8 months, 91 consecutive patients (60 men and 31 women; mean age, 66 ± 10 years; age range, 42-83 years) who were suspected of having coronary artery disease by their primary physicians were enrolled in the study. Among the 81 patients who were also scheduled for coronary CT angiography, 59 had a β-blocker before coronary artery calcium scoring. Of the 22 who did not have the β-blocker before scanning, nine had their regular medication consisting of antiarrhythmic drugs such as β-blockers or calcium antagonists and another 19 patients had no medication. The remaining 10 patients, undergoing coronary artery calcium scoring only, did not have a β-blocker before scanning, although two of them had their regular β-blocker medication.

For patients with a positive coronary artery calcium measurement, the mean heart rate, the change in heart rate during scanning, and number of patients with the change in heart rate during scanning ≥ 5 bpm were obtained in all patients, both the low (< 65 bpm) and the high (≥ 65 bpm) heart rate groups. These values were compared between the low and high heart rate patient groups.


Figure 1
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  		Fig. 1A —Grading of intensity of motion artifacts from coronary artery calcium. Cardiac CT calcium scoring image in 63-year-old woman shows grade 1 (arrow): none, no motion artifacts from coronary artery calcium.

 


Figure 2
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  		Fig. 1B —Grading of intensity of motion artifacts from coronary artery calcium. Cardiac CT calcium scoring image in 68-year-old man shows grade 2 (arrow): minor, streaking artifacts from coronary artery calcium or blurred margin of coronary artery calcium.

 


Figure 3
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  		Fig. 1C —Grading of intensity of motion artifacts from coronary artery calcium. Cardiac CT calcium scoring image in 73-year-old man shows grade 3 (arrow): major, coronary artery calcium with star-shaped or doubling artifacts.

 
Retrospective ECG-Gated Helical CT Protocol
Two repeated scans with a table advancement of 1 mm during scanning were performed using a 64-MDCT scanner (LightSpeed VCT, GE Healthcare) with simultaneous ECG digitizing and recording. Volumetric scans were performed 4-5 seconds after breath-holding on mild inspiration to minimize change of heart rate during scanning [26]. Scanning parameters were as follows: 0.35-second gantry rotation speed, 1.25-mm collimation width x 32 detectors, and 120 kV. The tube current was controlled using the ECG modulation technique. The maximal current was set at 130 mA during the cardiac phase 30-90%, and was reduced in the other phase to a minimum of 30 mA. CT pitch factors varied from 0.18 to 0.24 by the heart rate, according to the manufacturer's recommendations for coronary CT angiography. Images of 2.5-mm thickness were retrospectively reconstructed with a 1.25-mm interval to reduce partial volume effect. Single-sector image reconstruction was used when the heart rate was < 75 bpm, and multisector recon struction was used when the heart rate was ≥ 75 bpm. The temporal resolution ranged from 90 to 175 milliseconds, according to the heart rate and the number of cardiac cycles used for image reconstruction. The matrix size was 512 x 512 pixels and the display field of view was 26 cm. The reconstruction kernel Standard was used. Nine cardiac phase image data sets were reconstructed by positioning the center of the temporal window at regular 5% increments from 40% to 80% of the R-R interval.

Coronary Artery Calcium Scoring
The Agatston [1], volume [7], and mass [8] scores were determined by a radiologist (coronary artery calcium scoring experience of 1 year) on a commercially available external workstation (Advantage Windows version 4.2, GE Healthcare) and coronary artery calcium-scoring software (Smartscore version 3.5, GE Healthcare) according to the following equations:

Agatston score = slice increment/slice thickness x {sum} (area x cofactor)

Volume = {sum} (area x slice increment)

Mass = {sum} (area x slice increment x mean CT density) x calibration factor [27].

For patients with a positive coronary artery calcium measurement, each of the Agatston, volume, and mass scores was compared among cardiac phases and two repeated scans (repeated measures analysis of variance). This is because, in cases where coronary artery calcium scores in repeated scans are both negative, the variability value is not obtained. If this occurs in specific cardiac phases or coronary artery calcium measurement algorithms, cross comparison between cardiac phases and algorithms becomes impossible. The coronary artery calcium scores were transformed to logarithmic scale to reduce skewness.

For each coronary artery calcium score in each patient, the interscan variability for the first (scan1) and the second scan (scan2) was evaluated using the percentage difference:

Interscan variability = [absolute (scan1 - scan2)/0.5 x (scan1 + scan2)] x 100.

The interscan variability was compared between cardiac phases and scoring algorithms (two-factor factorial analysis of variance) for all patients, both the low-(< 65 bpm) and the high-(≥ 65 bpm) heart-rate patient groups. We performed monthly scanning of a calibration phantom (Anthropomorphic Cardio Phantom, Institute of Medical Physics and QRM GmbH).

Quantitative Assessment of Motion Artifacts
The motion artifact score was assigned for calcium-score-positive main coronary artery branches (left anterior descending, left circumflex, and right coronary arteries) using a 3-point grading scale (grade 1, none; grade 2, minor; and grade 3, major) (Figs. 1A, 1B and 1C) by two independent radiologists with 1 and 8 years of coronary artery calcium scoring experience, respectively. In cases in which two or more calcified plaques were present in the coronary artery branch, the score was assigned to the largest plaque. If the assigned grades were different between observers, the determination was achieved by consensus. The larger motion artifact scores between the scans—that is, the score assigned to the most severe motion artifact—was used for further investigation.

For the three heart rate groups and for the three coronary artery branches, the cardiac phase that provided the least motion artifact and the relationship between the degree of motion artifact and the difference in coronary artery calcium scores of two scans and cardiac phases were investigated by the adjacent categories logit model [28], which is a statistical model for describing the regression relationship between ordinal categoric response (e.g., motion artifact score) and explanatory variables (e.g., difference of coronary artery calcium score of two scans and cardiac phases). Furthermore, whether the motion artifact score was different between the low and high heart rate groups was tested using the Mann-Whitney U test.


Figure 4
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  		Fig. 2A —Graphs show interscan variability of coronary artery calcium scoring for three heart rate groups. Interscan variability of Agatston (dark gray), volume (medium gray), and mass (white) scores on nine cardiac phases are shown for all patients (A); low-heart-rate group (B) and high-heart-rate group (C) are also shown. Graphs show means (bars), mean plus SD (upper vertical line), and median (lower vertical line). Two-factor factorial analysis of variance revealed significant differences between cardiac phases (p < 0.01); however, there were no significant differences among scoring algorithms (all patients, p = 0.46; low-heart-rate group, p = 0.58; and high-heart-rate group, p = 0.75). Scheffé test, however, showed no statistical difference between cardiac phases.

 


Figure 5
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  		Fig. 2B —Graphs show interscan variability of coronary artery calcium scoring for three heart rate groups. Interscan variability of Agatston (dark gray), volume (medium gray), and mass (white) scores on nine cardiac phases are shown for all patients (A); low-heart-rate group (B) and high-heart-rate group (C) are also shown. Graphs show means (bars), mean plus SD (upper vertical line), and median (lower vertical line). Two-factor factorial analysis of variance revealed significant differences between cardiac phases (p < 0.01); however, there were no significant differences among scoring algorithms (all patients, p = 0.46; low-heart-rate group, p = 0.58; and high-heart-rate group, p = 0.75). Scheffé test, however, showed no statistical difference between cardiac phases.

 


Figure 6
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  		Fig. 2C —Graphs show interscan variability of coronary artery calcium scoring for three heart rate groups. Interscan variability of Agatston (dark gray), volume (medium gray), and mass (white) scores on nine cardiac phases are shown for all patients (A); low-heart-rate group (B) and high-heart-rate group (C) are also shown. Graphs show means (bars), mean plus SD (upper vertical line), and median (lower vertical line). Two-factor factorial analysis of variance revealed significant differences between cardiac phases (p < 0.01); however, there were no significant differences among scoring algorithms (all patients, p = 0.46; low-heart-rate group, p = 0.58; and high-heart-rate group, p = 0.75). Scheffé test, however, showed no statistical difference between cardiac phases.

 
Statistical Analysis
Continuous variables are expressed as mean ± 1 SD and categoric variables as percentages and frequencies. For statistical analysis, the Mann-Whitney U test, chi-square test, repeated measures analysis-of-variance test, and adjacent categories logit model were used to determine differences. When statistical significance was observed by analysis of variance, the results were made post hoc by the Scheffé test for multiple pairwise comparisons. A p value of < 0.05 was considered to identify significant difference. Statistical analyses were performed using a commercially available software package (Statcel2, OMS-Publishing) except for the adjacent categories logit model.


Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
All patients participated in the study. Of a total 91 patients, 18 patients with no coronary artery calcification, one patient with both positive and negative coronary artery calcium scores among cardiac phases, and one patient who failed in breath-holding during scanning were excluded from the study. We did not find any patient whose coronary artery calcium scores were all positive (i.e., for the nine phases and three coronary artery calcium measurement algorithms) in one scan and all negative in the other scan. For the remaining 71 patients with positive coronary artery calcium scores, scanning time was 6.4 ± 0.8 and 6.4 ± 0.7 seconds on scan 1 and scan 2, respectively. The heart rate results are summarized in Table 2. In the high-heart-rate patient group (n = 24), the change in heart rate was greater than that in the low heart rate patient group (n = 47) (Mann-Whitney U test, p < 0.05). The number of patients with a change in heart rate during the scan of ≥ 5 bpm tended to be greater compared with the low-heart-rate patient group; however, it did not reach the level of significant difference (chi-square test, scan 1: p = 0.42, and scan 2: p = 0.06).


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  		TABLE 2: Heart Rate Results by Patient Group

 

Coronary Artery Calcium Scores
The Agatston, volume, and mass scores on the nine cardiac phases in the two scans are summarized in Table 3. On volume and mass scores, repeated measures analysis of variance revealed that there were no statistical differences between the two scans (p = 0.99 and 0.98); however, there were statistically significant differences between cardiac phases (p < 0.01). On the Agatston score, repeated measures analysis of variance revealed that there were no statistical differences between the two scans (p = 0.99) or between cardiac phases (p = 0.1).


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  		TABLE 3: Agatston, Volume, and Mass Scores on the Nine Cardiac Phases in the Two Scans

 

Interscan Variability
The interscan variability in Agatston, volume, and mass scores on the nine cardiac phases are shown in Figures 2A, 2B and 2C. For the analysis of all patients, two-factor factorial analysis of variance revealed that there were significant differences between cardiac phases (p < 0.01); however, there were none between scoring algorithms (p = 0.46). For the all-heart-rate patient group, the least variability was obtained in diastole—that is, at 70% on Agatston (8% ± 8%) and volume (7% ± 10%) and at 75% on mass (7% ± 8%). For the low-heart-rate patient group, images in diastole showed low variability; however, images in end-systole, especially at 45%, showed high variability. In contrast to this, for the high-heart-rate patient group, images at 45% showed the lowest vari ability, although images in diastole showed comparable results to those in end-systole (variability {approx} of 10%).

Quantitative Assessment of Motion Artifacts
Of a total of 2,574 assignments of motion artifact score (143 coronary arteries x 9 cardiac cycles x 2 scans), the same grading was observed in 2,446 (95%). The remaining 128 (5%) assignments differed between observers 1 and 2, and this was settled by consensus. Motion artifact scores (larger scores between the scans) per branch in the three heart rate groups are shown in Table 4. The differences between cardiac phases were much greater than the interobserver variability of 5%. The adjacent categories logit model revealed that in the all-heart-rate groups the least motion artifacts were shown at 75-80% on the right coronary artery, 70-75% on the left anterior descending coronary artery, and 75-80% on the left circumflex coronary artery (p < 0.05).


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  		TABLE 4: Distribution of Motion Artifact Score Grading for Three Different Patient Groups

 

In the adjacent categories logit model, the reason the cardiac phase with the least motion artifact included two neighboring cardiac phases (e.g., 75-80%) was binding of cardiac phases without any statistical difference in adjacent categories. The test also revealed that a smaller difference of calcium mass scores between the scans was associated with a smaller motion artifact score (p < 0.01). Motion artifact scores for the low-heart-rate group in the three coronary artery branches were lower than those for the high-heart-rate group on images at 75% (Mann-Whitney U test, p < 0.01), whereas there was no significant difference on images at 40-60% (p = 0.09-0.93). Motion artifact scores in the low-heart-rate group were lower than those in the high-heart-rate group on images at 65%, 70%, and 80%, with partially statistical differences depending on the branch—that is, at 65%, left anterior descending coronary artery: p < 0.01; left circumflex coronary artery: p = 0.32; and right coronary artery: p = 0.09.


Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Interscan Variability
To the best of our knowledge, the present study is the first to report the optimal cardiac phase for coronary artery calcium scoring using a single-source 0.35-second rotation speed 64-MDCT scanner. The results suggest cardiac reconstruction at middiastole gives images with the least interscan variability of coronary artery calcium and the least motion artifacts as reported by previous MDCT studies [19-22]. For a representative scoring algorithm of mass, the variability of 7% in the current study was lower than 28.4% with electron-beam CT [8] or 30.3% with 4-MDCT [15] and 15% with 16-MDCT [29]. Such a low level of variability is thought to be achieved by improved temporal resolution, shorter scanning time, and a scanning delay of 4-5 seconds after breath-holding for minimizing change of heart rate during scanning.

Coronary Artery Calcium Score
The volume and mass scores were significantly different between cardiac phases; however, they were not significantly different between the two scans. The Agatston scores tended to be different between cardiac phases, although not reaching statistical significance (p = 0.1). The results are in line with previous studies showing the difference of Agatston and volume scores between cardiac phases on 4-MDCT [17, 30] and 16-MDCT [14]. The facts indicate that cardiac images on 64-MDCT, despite accelerated gantry rotation speed, are profoundly under the influence of cardiac motion. Therefore, reconstruction of the cardiac phase still needs optimization. In contrast to this, the mass scores were not significantly different between cardiac phases or between the two scans. The reason for this is not clear; however, it might be that the mass is the most reproducible algorithm in coronary artery calcium scoring.

Optimal Cardiac Phase for Coronary Artery Calcium Scoring
Although there is likely to be a consensus that cardiac images with the least motion artifacts can be obtained in diastole on a low-heart-rate patient, the optimal cardiac phase on a high-heart-rate patient is still under debate. We have found no MDCT studies investigating the optimal cardiac phase for coronary artery calcium scoring on high-heart-rate patients. Different conclusions for the optimal cardiac phase were found in 64-MDCT angiography studies with 0.33-second and 0.37-second gantry rotation speeds. In the current study, the least variability on high-heart-rate patients was observed at 45%; however, diastolic images also showed favorable results. This indicates that reconstruction in middiastole in all patients (or reconstruction at diastole in low-heart-rate patients while switching to end-systole in high-heart-rate patients) is reasonable.

The adjacent categories logit model revealed that a smaller difference of calcium scores between the scans was associated with fewer motion artifacts. Motion artifacts in all coronary artery branches were mostly reduced at 75%, which corresponded to the phase with the least variability in the mass algorithm. These facts indicate that motion artifacts are a dominant factor increasing variability and that the improvement of temporal resolution by acceleration of gantry rotation speed will contribute to further reducing variability.

Study Limitations
This study has limitations. We used retrospective ECG-gated CT to search for the optimal cardiac phase for 64-MDCT. To reduce partial volume effect increasing the variability, we used overlapping reconstruction. We have shown low variability values in the current study; however, these levels of variability may not be preserved for prospective ECC-triggered coronary artery calcium scoring. Next, we used multisector reconstruction for patients with a heart rate ≥ 75 bpm. In a CT angiography study on 0.33-second rotation 64-MDCT, Wintersperger et al. [21] stated that multisector reconstruction did not significantly improve image quality in any heart rate group (≤ 65, > 65 to ≤ 75, and > 75 bpm). We, however, are not able to fully understand the effect of multisector reconstruction. Ideally, cross-comparisons between two groups with single-sector and multisector reconstructions should have been performed for evaluating the effect of multisector reconstruction on variability and motion artifacts.

In conclusion, middiastole reconstruction (center of cardiac phase, 70-75%), with the least interscan variability and the least motion artifacts, is recommended on a 64-MDCT scanner.


Acknowledgments
 
We thank Megu Ohtaki for statistical analysis using the adjacent categories logit model.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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