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Low-dose CT Coronary Angiography Using Prospective ECG-Triggering

Rationale and Objectives

The aim of this study was to evaluate the effect of mean heart rate (HR) and HR variability on image quality in low-dose computed tomographic coronary angiography (CTCA) using prospective electrocardiographic (ECG) triggering.

Materials and Methods

One hundred thirty-six consecutive patients were scheduled for low-dose CTCA using prospective ECG triggering. The image quality of all coronary segments was rated on a 5-point scale by two independent readers (scores of 1–3 were considered diagnostic, and scores of 4 and 5 were considered nondiagnostic). Intravenous β blockers were administered targeting HR < 65 beats/min before scanning, but not if HR increased during scanning.

Results

After the exclusion of seven patients because of arrhythmia ( n = 4) or mean HRs > 65 beats/min despite using β blockers ( n = 3), 129 patients underwent computed tomographic scanning. The estimated mean effective radiation dose was 2.2 ± 0.7 mSv (range, 1.1–3.5). The mean HR during scanning was 58.4 ± 6.6 beats/min (range, 44.2–80.1), with a variability of 1.6 ± 1.0 beats/min (range, 0.2–5.3). Mean HR ( r = 0.49, P < .001) but not mean HR variability ( r = 0.14) was related to image quality. Nondiagnostic image quality on CTCA was found in 5% of the coronary segments in 21 of 129 patients. However, on receiver-operating characteristic analysis, a cutoff HR of 62 beats/min was determined, below which nondiagnostic segments were significantly less frequent (2% vs 14%, P < .001).

Conclusion

Prospective triggering allows low-dose CTCA but requires a low HR. Because a low HR offers a prolonged diastole, widening the optimal phase for scanning, HR variability seems to have a negligible impact on image quality.

The implementation of multidetector computed tomography (MDCT) for coronary angiography allows fast imaging as well as increased temporal and spatial resolution ( ). However, motion artifacts caused by a higher heart rate (HR) or HR variability remain an issue, because they affect image quality. In four-slice and 16-slice MDCT, a higher HR has shown to cause a relevant degradation of image quality ( ). With 64-slice MDCT, the influence of HR and HR variability on image quality remains controversial, with such an effect reported by some authors ( ) but not by others ( ).

Despite further technical refinements and the introduction of dual-source computed tomographic (CT) coronary angiography (CTCA), leading to better temporal resolution, there still appears to be a negative influence of HR variability ( ) or both HR and HR variability ( ) on image quality. To minimize motion artifacts resulting from high HR or HR variability, images are usually reconstructed at the phase of near quiescence in mid-diastole called diastasis ( ). Scanning protocols using a spiral acquisition mode offer the opportunity for image reconstruction at any given point throughout the cardiac cycle, allowing the retrospective selection of any phase in the RR interval ( ) and the use of multisegment reconstructions for improving temporal resolution. With the introduction of prospective electrocardiographic (ECG) triggering ( ), radiation is administered at only one predefined time point in the cardiac cycle. This substantially reduces the effective radiation dose ( ) but in turn provides only a single data set for image reconstruction, achieving diagnostic image quality.

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Material and methods

Patients

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CT Data Acquisition and Postprocessing

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CT Image Analysis

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Statistical Analysis

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Results

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Image Quality of Coronary Artery Segments

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Figure 1, Volume-rendered and curved multiplanar reconstructions of low-dose computed tomographic coronary angiography using electrocardiographic triggering in a 51-year-old patient with a heart rate of 63 beats/min, demonstrating the use of 5-point image quality scoring. (a) Coronary artery tree with right coronary artery (RCA), left anterior descending coronary artery (LAD), and circumflex coronary artery (CX). (b) Image of RCA shows a mild motion artifact (a score of 2) distally and a severe motion artifact (a score of 4) proximally. (c) Image of CX shows a nonevaluative artifact (a score of 5) proximally and a moderate motion artifact with blurring of the vessel outline (a score of 3) distally. (d) Image of the LAD with no motion artifact (a score of 1).

Figure 2, Frequency of image quality for overall coronary segments. Scores of 1 to 3 were considered diagnostic, and scores of 4 and 5 were considered nondiagnostic.

Table 1

Image Quality Scoring of All Segments

Coronary Artery Image Quality Score 1 2 3 4 5 ( n = 755) ( n = 635) ( n = 253) ( n = 56) ( n = 38) RCA Mean 176 173 107 23 11 Segment 1 50 50 16 9 3 Segment 2 22 32 55 12 7 Segment 3 51 44 27 2 1 Segment 4 53 47 9 0 0 LAD Mean 354 263 87 12 13 Segment 5 81 39 7 0 1 Segment 6 71 39 15 0 1 Segment 7 63 49 10 3 3 Segment 8 32 50 38 5 4 Segment 9 60 46 7 3 2 Segment 10 47 40 10 1 2 CX Mean 225 199 59 21 14 Segment 11 68 44 8 6 3 Segment 12 44 36 8 3 3 Segment 13 45 49 19 7 4 Segment 14 42 44 19 3 3 Segment 15 5 6 3 1 1 Segment 16 21 20 2 1 0

CX, circumflex coronary artery; LAD, left anterior descending coronary artery; RCA, right coronary artery.

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Impact of Mean HR on Image Quality

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Figure 3, Linear regression plot of mean image quality scores over all coronary segments per patient ( y axis) against mean heart rate during computed tomographic coronary angiography ( x axis). Dashed lines represent 95% individual prediction interval. Linear correlation indicates significant degradation of image quality with increasing heart rate (Spearman's correlation, r = 0.49, P < .001).

Table 2

Correlations

Image Score Mean HR Mean HR Variability_r__P__r__P_ All arteries 0.49 <.001 0.17 NS RCA 0.59 <.001 0.14 NS LAD 0.37 <.001 0.17 NS CX 0.37 <.001 0.10 NS

CX, circumflex coronary artery; HR, heart rate; LAD, left anterior descending coronary artery; NS, not significant; RCA, right coronary artery.

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Figure 4, Receiver-operating characteristic curve identifying the cutoff heart rate below which diagnostic image quality is achieved, at 62 beats/min. AUC, area under the curve.

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Impact of Mean HR Variability on Image Quality

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Figure 5, Linear regression plot of mean image quality scores over all coronary segments per patient ( y axis) against mean heart rate variability during computed tomographic coronary angiography ( x axis). There was no correlation between heart rate variability and image quality (Spearman's correlation, r = 0.17, not significant).

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Discussion

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