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Dual-Source CT Coronary Angiography: Image Quality, Mean Heart Rate, and Heart Rate Variability

¹ Institute of Diagnostic Radiology, University Hospital Zurich, Raemistrasse 100, 8091 Zurich, Switzerland.
² Siemens Medical Solutions, Forchheim, Germany.
³ Cardiovascular Center, University Hospital, Zurich, Switzerland.
⁴ Zurich Center for Integrative Human Physiology, Zurich, Switzerland.

Received December 18, 2006; accepted after revision February 28, 2007.

 
Address correspondence to H. Alkadhi.

Supported by the National Center of Competence in Research, Computer Aided and Image Guided Medical Interventions of the Swiss National Science Foundation.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. The purpose of this study was to evaluate the effect of mean heart rate and heart rate variability on the image quality of dual-source CT coronary angiography.

SUBJECTS AND METHODS. Eighty patients underwent dual-source CT coronary angiography. Thirteen data sets were reconstructed in 5% steps from 20-80% of the R-R interval. Heart rate variability was calculated as SD of mean heart rate. Two independent blinded reviewers assessed the image quality of each segment.

RESULTS. Mean heart rate was 65.3 ± 13.9 (SD) beats per minute (bpm) (range, 35-99 bpm) with a variability of 3.4 ± 4.1 bpm (range, 0.4-17.5 bpm). Image quality was sufficient for diagnosis for 97.8% (1,043/1,066) of arterial segments. No significant correlation was found between mean heart rate and image quality in any segment or any coronary artery. No significant correlation was found between heart rate variability and image quality in any segment, the right coronary artery, or the left anterior descending artery, but there was a significant (p < 0.05) correlation in the left circumflex artery. A significant correlation (p < 0.05) between overall image quality was found for mean and variability of heart rate as shared predictors, the latter having a greater contribution.

CONCLUSION. The overall image quality of dual-source CT coronary angiography is sufficient for diagnosis within a wide range of mean heart rates and variability of heart rates. Only heart rates that are both high and variable significantly deteriorate image quality, but the quality remains adequate for diagnosis.

Keywords: coronary angiography • dual-source CT • heart rate • image quality

Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Advances in MDCT technology have allowed the implementation of noninvasive CT coronary angiography in daily clinical routine [1]. In particular, 64-MDCT scanners have exhibited high diagnostic accuracy in the assessment of coronary artery disease [2-6]. The American College of Cardiology Foundation and the American College of Radiology have formulated recommendations for the use of CT coronary angiography as an alternative to conventional catheter angiography in the examination of patients who have symptoms but have inconclusive findings at stress ECG or are unable to exercise [7]. CT coronary angiography, however, remains sensitive to motion artifacts that cause a decline in diagnostic accuracy. These artifacts are especially pronounced at higher heart rates [2-6, 8-11]. With 4-MDCT, the best quality of images of coronary arteries has been found at heart rates less than 75 beats per minute (bpm) [10]. With 16-MDCT, motion-free depiction of most coronary segments has been possible in patients with heart rates less than 80 bpm, the best quality being obtained at less than 75 bpm [11]. Studies of 64-MDCT have shown that CT coronary angiography can be performed with image quality sufficient for diagnosis in a wide range of heart rates; however, image quality decreases with increasing heart rate [12]. Results of a study [13] of 64-MDCT coronary angiography reported in 2006 showed a nonsignificant tendency toward lower image quality with higher mean heart rate and a significant negative relation between image quality and heart rate variability.

Dual-source CT is characterized by two X-ray tubes and two corresponding detectors mounted on the gantry with an angular offset of 90° [14]. With a gantry rotation time of 330 milliseconds, this scanner has heart rate-independent temporal resolution of 83 milliseconds with use of a single-segment reconstruction mode. Results of preliminary studies [15, 16] have shown that the quality of images obtained with dual-source CT coronary angiography at a relatively high mean heart rate is sufficient for diagnosis. Moreover, an early experience [17] showed that compared with invasive coronary angiography, dual-source CT coronary angiography had high diagnostic accuracy in the diagnosis of substantial coronary artery stenosis in a patient population whose heart rates were not controlled. The purpose of our study was to systematically evaluate the effect of the relation between mean heart rate and variability of heart rate during scanning on the image quality of dual-source CT coronary angiography.

Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Patients
Between July 2006 and November 2006, we prospectively enrolled 80 patients (27 women, 53 men; mean age, 62.1 ± 13.5 [SD] years; range, 36-86 years) for CT coronary angiography. The mean body mass index (weight in kilograms divided by the square of height in meters) was 25.9 ± 4.3 (range, 16.3-40). Forty-four (55%) of the patients were taking ß-receptor blocking agents as part of their baseline medication at the time of CT. No additional ß-blockers were administered before CT. Demographic data on the patient population are listed in Table 1. The referral to CT was based on clinical indications only. Fifty-six patients were referred for CT coronary angiography with a low pretest probability of coronary artery disease, and 24 had known or suspected coronary artery disease (i.e., stable angina, stress test showing nonocclusive disease). Exclusion criteria for CT were allergy to iodinated contrast medium, nephropathy (creatinine level > 120 µmol/L), nonsinus rhythm, and hemodynamic instability. The study protocol was approved by the local ethics committee; written informed consent was waived.

Dual-Source CT Protocol
All CT examinations were performed with a dual-source CT scanner (Somatom Definition, Siemens Medical Solutions). Scanning was conducted in a craniocaudal direction covering the region from 1 cm caudal to the level of the tracheal bifurcation to the diaphragm. Scanning parameters were as follows: detector collimation, 2 x 32 x 0.6 mm; slice collimation, 2 x 64 x 0.6 mm by means of a z-flying focal spot; gantry rotation time, 330 milliseconds; heart rate-dependent pitch, 0.2-0.43; tube current-time product, 350 mAs per rotation; tube potential, 120 kV. The ECG-pulsing technique for radiation dose reduction was used in all patients with full tube current from 20% to 80% of the R-R interval. With this protocol, mean dose-length product was 703.4 ± 196.3, and mean CT dose index was 56.2 ± 16.9.

Two minutes before CT, all patients received a single sublingual 2.5-mg dose of isosorbide dinitrate (Isoket, Schwarz Pharma). Eighty milliliters of nonionic isoosmolar contrast material (iodixanol 320 mg I/mL, Visipaque 320, GE Healthcare) was injected at 5 mL/s and followed by a 30-mL saline flush. Bolus-tracking was performed in the ascending aorta, and image acquisition was started after attainment of a threshold of 100 H with a delay of 5 seconds.

CT Data Postprocessing and Analysis
All images were reconstructed with retrospective ECG gating. A monosegment reconstruction algorithm consisting of the data from a quarter rotation of both detectors was used for image reconstruction [14]. For each patient, 13 data sets were reconstructed in 5% steps from 20% to 80% of the R-R interval with a slice thickness of 0.75 mm, reconstruction increment of 0.5 mm, and medium soft-tissue convolution kernel (B26f). The field of view was manually adjusted to encompass the heart (mean, 156 ± 21 mm; range, 130-182 mm; image matrix, 512 x 512 pixels). All images were transferred to an external workstation (Leonardo, Siemens Medical Solutions) equipped with cardiac postprocessing software (Syngo Circulation, Siemens).

The coronary arteries were classified into 15 segments according to the scheme proposed by the American Heart Association [18]. The right coronary artery (RCA) included segments 1-4, the left main coronary artery and left anterior descending coronary artery (LAD) included segments 5-10, and the left circumflex coronary artery (LCX) included segments 11-15. If present, the intermediate artery was designated segment 16. Coronary artery analysis was performed on all vessels with at least 1-mm luminal diameter at the origin. Images were analyzed and graded by two independent reviewers blinded to mean heart rate and heart rate variability during scanning and using axial source images, multiplanar reformations, and thin-slab maximum intensity projections. The reviewers analyzed each coronary artery segment first at 60% and 70% of the R-R interval. If the images were not of sufficient quality for diagnosis at 60% and 70%, additional data sets were reconstructed in 5% steps within the time interval of full tube current from 20% to 80% to obtain images of diagnostic quality. After finding the reconstruction interval with the best image quality, the reviewers semiquantitatively assessed image quality and degree of motion artifacts on the 4-point Likert scale as follows: 1, excellent, no motion artifacts, clear delineation of the segment; 2, good, minor artifacts, mild blurring of the segment; 3, adequate, moderate artifacts, moderate blurring without structure discontinuity; and 4, not evaluative, doubling or discontinuity in the course of the segment preventing evaluation or vessel structures not differentiable (Figs. 1, 2, 3 and 4). Scores 1-3 were considered diagnostic. In case of disagreement between reviewers, consensus interpretation was appended.

View larger version (87K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1 —55-year-old man with mean heart rate of 61 beats per minute (bpm) and variability of 1.8 bpm. Curved multiplanar reconstruction and volume-rendered (inset) dual-source CT are of excellent quality without motion artifacts (score 1) in all segments of right coronary artery.

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2 —47-year-old woman with mean heart rate of 99 beats per minute (bpm) and variability of 1.6 bpm. Curved multiplanar reconstruction and volume-rendered (inset) dual-source CT images are of good quality with mild vessel blurring (score 2) in middle segment of right coronary artery.

View larger version (114K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3 —60-year-old man with mean heart rate of 52 beats per minute (bpm) and variability of 2.7 bpm. Curved multiplanar reconstruction and volume-rendered (inset) dual-source CT images are of adequate quality with moderate motion artifacts (score 3) in proximal and middle segments of right coronary artery.

View larger version (92K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4 —68-year-old woman with mean heart rate of 68 beats per minute (bpm) and variability of 8.4 bpm. Curved multiplanar reconstruction and volume-rendered (inset) dual-source CT images are not evaluative, having severe motion artifacts (score 4) and vessel discontinuity in middle segment of right coronary artery.

Statistical Analysis
Quantitative variables were expressed as mean ± SD and categoric variables as frequencies or percentages. Interobserver agreement for image quality assessment was interpreted by use of kappa values. Pearson's correlation analysis was performed to compare image quality scores of all segments together and separately for the RCA, LAD, and LCX in each patient with mean heart rate and with the SD of the mean heart rate (i.e., heart rate variability) during scanning. To test for colinearity, Pearson's correlation analysis was performed between mean heart rate and heart rate variability. Multivariate regression analysis was performed to test for mutual effects of mean heart rate and heart rate variability on mean image quality score per patient. Individual contribution to image quality modification was assessed with standardized beta coefficients. Wilcoxon's two-sample test was performed to evaluate the effect of ß-blocker medication on heart rate variability and mean heart rate. Two-tailed p < 0.05 was considered statistically significant. All statistical analysis was performed with commercially available software (SPSS 12.0, SPSS).

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Image Quality of Coronary Artery Segments
A total of 1,066 coronary segments in 80 patients were evaluated. Initial reconstructions at 60% and 70% of the R-R interval provided the best image quality for 21% (224/1,066) and 68.7% (732/1,066) of the segments, respectively, and image quality sufficient for diagnosis (i.e., scores of 1-3) for 71.3% (760/1,066) and 86.5% (923/1,066) of the segments. For images of nondiagnostic quality on initial reconstructions, additional reconstructions were necessary to improve image quality in 22.4% (239/1,066) and to yield diagnostic image quality in 11.3% (120/1,066) of the cases. Image quality of additional reconstructions obtained at 20-60% and 70-80% of the R-R interval improved for 16.1% (172/1,066) and 6.3% of the segments (67/1,066) compared with the image quality on initial 60% and 70% reconstructions. Using the individual best reconstruction interval, we obtained motion artifact-free images (score 1) of 54.4% (580/1,066) of the coronary segments. Minor artifacts (score 2) were found on images of 34.5% (368/1,066) of the segments and moderate artifacts (score 3) on images of 8.9% (95/1,066) of the segments. Severe artifacts (score 4) rendering image quality nondiagnostic were present in 11 (13.8%) of the patients and involved 2.2% (23/1,066) of the segments. Thus diagnostic image quality (scores 1-3) was achieved for 97.8% of all coronary segments (1,043/1,066). Not-evaluative image quality was found in 95.7% (22/23) in distal segments (segment 3, n =4; segment 8, n = 1; segment 13, n = 1) and side branches (segment 4, n = 4; segment 10, n = 5; segment 12, n = 1; segment 14, n =3; segment 15, n = 3), and the middle RCA (segment 2) was affected in one patient. The 11 patients with not-evaluative segments had a mean heart rate of 65.5 ± 11.1 bpm (range, 48.9-83.7 bpm) and a mean heart rate variability of 3.1 ± 3.0 bpm (range, 0.7-9.1 bpm). The one not-evaluative middle RCA segment was found in a patient with a mean heart rate of 68.3 bpm and a heart rate variability of 8.4 bpm. Interobserver agreement for image quality rating was good ( = 0.72). Table 2 summarizes image quality scores for all segments and coronary arteries.

Effect of Mean Heart Rate on Image Quality
The mean heart rate during scanning was 65.3 ± 13.9 bpm (range, 35-99 bpm). No significant correlation was found between mean heart rate and mean overall image quality of dual-source CT coronary angiography in any segment in any patient (r = 0.20, p =not significant [NS]) (Fig. 5). Similarly, no significant correlation was present between mean heart rate and quality of images of the RCA (r = 0.11, p = NS), LAD (r = 0.18, p =NS), or LCX (r = 0.19, p =NS).

View larger version (12K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5 —Linear regression plot shows mean image quality scores over all segments per patient (y-axis) against mean heart rate during CT (x-axis). Curves represent 95% CIs. Linear regression indicates no significant degradation of image quality with increasing mean heart rate (Pearson's correlation, r =0.20; p = 0.070, not significant).

View larger version (11K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6 —Linear regression plot shows mean image quality scores over all segments per patient (y-axis) against heart rate variability during CT (x-axis). Curves represent 95% CIs. Linear regression indicates no significant degradation of image quality with increasing heart rate variability (Pearson's correlation, r = 0.21; p = 0.059, not significant).

Effect of ß-Blockers on Mean Heart Rate, Heart Rate Variability, and Image Quality
There was a significant difference between the mean heart rate of patients not taking ß-blocker medication (69.0 ± 14.4 bpm) and that of patients taking ß-blockers (62.2 ± 12.9 bpm). The mean heart rate was significantly lower among patients taking baseline ß-blocker medication (p < 0.01). The variability of heart rate among patients taking ß-blocker medication (3.6 ± 4.2 bpm) was not significantly different (p = NS) from that of those not taking ß-blockers (3.3 ± 4.2 bpm). There was no significant difference in image quality between patients taking ß-blocker medication (1.54 ± 0.38) and those not taking ß-blockers (1.61 ± 0.40, p =NS).

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
With each scanner generation, motion arti-facts reappear as a major cause of image quality degradation during CT coronary angiography [8, 9, 11-13, 19]. The RCA and LCX are especially affected by motion artifacts because of their proximity to the right and left atria [10, 20-22]. Our results indicate that the image quality of dual-source CT is sufficient for diagnosis throughout the coronary artery tree at a wide range of heart rates and that the overall quality of the images does not correlate with mean heart rate or heart rate variability during scanning. Significant image quality degradation occurs only at heart rates that are both high and variable, the latter being the main contributor. Nevertheless, the image quality of dual-source CT coronary angiography is diagnostic within a wide range of mean and variable heart rates.

Mean Heart Rate and Image Quality
CT coronary angiographic studies of each coronary artery with 4-MDCT at a gantry rotation time of 500 milliseconds had significantly decreased image quality with increasing mean heart rates [10]. Using 16-MDCT at a gantry rotation time of 420 milliseconds, Hoffmann et al. [11] found a significant negative correlation between overall image quality and mean heart rate. Studies with 64-MDCT at gantry rotation times of 370 and 330 milliseconds have shown that CT coronary angiography can be performed with diagnostic image quality at a wide range of heart rates. Whereas one study [12] showed a significant negative relation between overall image quality and mean heart rate, another study [13] showed the relation to overall image quality as nonsignificant. The latter study showed a weak though significant negative correlation between mean heart rate and image quality only for the LCX. This effect was believed to be caused by the increased susceptibility of the LCX to shortening of diastole at higher heart rates and by its higher motion velocity [20]. Using dual-source CT with a gantry rotation time of 330 milliseconds, we found no significant correlation between mean heart rate and the overall image quality for any coronary segment or for any individual coronary artery. We found no motion artifacts in images of 54.4% of segments and minor to moderate motion artifacts in images of 43.4% of coronary segments. Severe artifacts leading to nondiagnostic image quality were present in images of only 2.2% of the segments, almost exclusively involving distal segments or side branches. Although direct comparison of our results with those from a previous 64-MDCT study [13] is not feasible, our data indicate that the increased temporal resolution of 83 milliseconds with dual-source CT allows imaging of the coronary arteries with diagnostic quality at heart rates up to 100 bpm.

Heart Rate Variability and Image Quality
With interheartbeat variability of the heart rate during scanning, the ECG-gated image reconstruction technique at a fixed interval does not generate images in the same cardiac phases. This problem is caused by nonproportional shortening and prolongation of the cardiac phases at different heart rates [23]. For images reconstructed with monosegment techniques, data obtained from slightly different cardiac phases result in a time shift from image to image, giving reformats a stair-step appearance. Multisegment algorithms for data reconstruction merge data from two or more consecutive heartbeats to increase the temporal resolution [24]. Use of these algorithms in CT examinations with variable heart rates leads to merging of data that do not exactly match. Although this technique may reduce the appearance of stair-step artifacts, blurring of images increases [12, 13, 24].

The issue of heart rate variability with regard to CT coronary angiography was addressed in a 64-MDCT study by Leschka et al. [13]. By using a two-segment reconstruction algorithm at heart rates greater than 65 bpm, the authors found significant negative correlation between heart rate variability and image quality for the whole coronary artery tree and each coronary artery. This finding is in contrast to our findings with dual-source CT. We found no significant effect of interheartbeat variation on overall image quality. Because the increased temporal resolution of dual-source CT is not likely to be beneficial for improving image quality in scans with variable heart rates, the most probable cause of our findings is the use of a monosegment reconstruction algorithm that does not merge data from adjacent heartbeats in different cardiac phases. Nonetheless, we found a significant coeffect of both heart rate variability and mean heart rate on the image quality of dual-source CT coronary angiography. The contribution of heart rate variability was greater than that of mean heart rate. This finding indicates that the combination of high and irregular heart rates negatively affects the image quality of noninvasive coronary angiography, even with dual-source CT. On the other hand, most of the not-evaluative segments were distal segments or side branches in 11 patients who had a mean heart rate of 66 ± 11 bpm and a heart rate variability of 3 ± 3 bpm, suggesting vessel size but not heart rate is the main limiting factor for image quality. A more proximal segment (mid RCA) was considered not evaluative in only one patient, who had a mean heart rate of 68 bpm but a high heart rate variability of 8.4 bpm during scanning.

ß-Receptor Blockers and Image Quality
Reduction of heart rate with ß-blocker medication has been reported to improve the image quality of CT coronary angiography [13, 22, 25] and has been proposed in most 64-MDCT coronary angiography studies [2-4, 6, 26]. The main mechanism of ß-blockers in improving image quality is its ability to reduce heart rate variability [13]. We found no significant difference between the heart rate variability of patients taking ß-blocker medication and that of patients not taking the medication. This finding is in contrast to the findings of Leschka et al. [13] and may be explained by different indications for ß-blocker treatment, individual response to ß-blocker medication [27], and by our cohort's being too small to reveal statistically significant differences between subgroups. Nevertheless, because overall image quality for all patients was not related to either mean heart rate or variability of heart rate, our results suggest that heart rate control with ß-blockers is not necessary before dual-source CT coronary angiography. This suggestion is supported by results of a study [17] of the performance of dual-source CT coronary angiography in the diagnosis of coronary artery stenosis. That study showed high diagnostic accuracy in patients not receiving heart rate control. On the other hand, our results indicate that administration of ß-blockers may be beneficial in improving the image quality of dual-source CT of patients whose heart rate is both high and variable.

Study Limitations
We acknowledge the following study limitations. First, image quality scoring might have been influenced by a subjectivity bias. Second, we did not analyze the influence of heart rate values on the diagnostic accuracy of coronary angiography. Twenty-four patients admitted for suspected coronary artery disease declined invasive coronary angiography because of negative findings on CT. Therefore, we did not include coronary artery stenosis detection in our study, which might have resulted in bias due to an incomplete data set. Finally, we did not investigate the effect of heart rate values on the image quality of CT coronary angiography with the absolute ECG-gating technique. This technique has been suggested to be advantageous in patients with variable heart rates [28], but there is no difference between relative and absolute ECG gating in patients with regular sinus rhythm.

Conclusion
In evaluation of the coronary arteries, dual-source CT has diagnostic image quality within a wide range of mean and variable heart rates. Neither mean heart rate nor variability of heart rate has a negative effect on the overall image quality of dual-source CT coronary angiography. Only heart rates that are both high and variable deteriorate image quality, but the quality remains sufficient for diagnosis.

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