Image Quality in Reduced-Dose Coronary CT Angiography

Rationale and Objectives

Concerns for patient’s risk of radiation-induced cancer have increased demand for reduced-dose coronary computed tomography angiography (CCTA). Previous comparisons of full and reduced-dose CCTA were not conclusive, because results were compared in different groups of patients. Presented here are results in patients examined by a widely used full dose CCTA protocol and a new low-dose alternative.

Materials and Methods

Standard full-dose and low-dose CCTA with tube voltages of 120/100 kV were applied on 70 patients with intermediate probability of coronary artery disease (CAD). Both protocols used prospective electrocardiogram-gated acquisition on a 320-detector row CT scanner, whereas at low-dose CCTA the phase window was increased from 10% to 75% of R-R interval.

Results

Despite a mean dose reduction of 80%, from 4.9 ± 0.98 to 0.98 ± 0.24 mSv, visual image quality was not significantly affected at the low-dose protocol. Contrast level, image noise, and CNR for both protocols were similar in the majority of coronary segments. CNR for standard and low-dose protocol were 23.7 ± 17.1 and 23.2 ± 26.8, P = NS. Correlation between visual image quality and heart rate variability was strong at low dose: r = −0.58, P = .01, and absent at full dose: r = −0.07, P = .77.

Conclusion

Image quality of blood vasculature is generally not affected by 80% CCTA dose reduction applied to standard prospective electrocardiogram-gated acquisition. The performance at the low-dose protocol owes to the increased phase window, enhancing image quality at the cost of sensitivity to heart rate variability as compared with standard CCTA.

During the past decade, there has been a dramatic increase in the number of computed tomography (CT) examinations performed worldwide. The principal concern for any patient undergoing a diagnostic CT examination is the risk of developing a radiation-induced cancer, which may be fatal or nonfatal . Reducing the radiation dose while maintaining highest image quality possible has therefore become an important research topic. Furthermore, with existing helical CT systems, the z -coverage is limited to maximum 4 cm data acquisition in a single rotation. Any future use of additional rotations to cover the whole heart by the helical acquisition method will increase the radiation dose, the amount of which depends on the pitch.

Low-dose coronary CT angiography (CCTA) has recently been used in several studies . However, direct comparison in terms of image quality assessment between low-dose and standard full-dose protocols on the same group of patients is still lacking. The purpose of this study is to present a quantitative comparison of image quality for two scanning protocols applied in the same patient examination: full-dose (standard CCTA with tube voltage 120 kV) and a new low-dose protocol with tube voltage 100kV. Both protocols use prospective electrocardiogram (ECG)-gated acquisition on a 320-detector row CT scanner. This way, acquisition of the whole heart is achieved during a single heart beat in nonhelical scan acquisition. At the resulting exposure and scan time of just 0.35 seconds, the radiation dose is considerably reduced compared with helical data acquisition. Prospective ECG-gating also presents an opportunity for widening cardiac phase window still within one heart beat, a technique called padding ( Fig 1 ). This technique, which allows for the reconstruction of more cardiac phases within a single heart beat, still without pitch value (pitch = 0), has been implemented in the new low-dose protocol by increasing the phase window from the standard setting of 10% to 75% of R-R interval.

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Figure 1

(a) Prospective electrocardiogram-gated acquisition method aiming at 70%–80% of R-R interval and (b) padding of for example 40%–80% of R-R interval. The height of the pulse represents the z -coverage.

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Materials and methods

Patient Population

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Scan Protocol

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Visual Image Quality Assessment

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Evaluation of the Effective Radiation Dose

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Results

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Quantitative Assessment of Diagnostic Performance

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Table 1

Quantitative Analysis Results of CCTA Image Quality Parameters in Various Regions Measured After Contrast Administration

Region Standard Protocol Attenuation (HU) Image Noise (HU) CNR Low-dose Protocol Attenuation (HU) Image Noise (HU) CNR_P_ Value Ascending aorta 408.7 ± 90.4 388.3 ± 117.9 NS 11.2 ± 6.7 12.7 ± 3.7 NS 62.4 ± 68.7 34.9 ± 21.4 .002 Left ventricle 374.7 ± 95.5 360.2 ± 96.3 NS 14.9 ± 8.5 16.0 ± 6.7 NS 36.0 ± 26.2 26.6 ± 14.8 .01 Coronary arteries: left main 352.7 ± 81.1 366.9 ± 94.5 NS 16.0 ± 10.1 14.4 ± 6.1 NS 28.9 ± 15.1 30.3 ± 16.1 NS Coronary arteries: LAD proximal 319.9 ± 91.7 330.6 ± 89.7 NS 20.4 ± 9.7 18.7 ± 6.5 NS 19.4 ± 10.2 21.4 ± 8.7 NS Coronary arteries: LAD middle 316.6 ± 66.7 325.3 ± 89.8 NS 20.6 ± 10.7 18.6 ± 6.6 NS 16.2 ± 10.7 19.6 ± 15.3 NS Coronary arteries: LAD distal 242.7 ± 58.4 230.2 ± 58.9 NS 25.5 ± 12.7 25.8 ± 8.7 NS 11.3 ± 5.1 9.9 ± 4.3 NS Coronary arteries: Cx proximal 299.4 ± 73.5 324.1 ± 102.2 NS 18.3 ± 6.8 19.3 ± 9.7 NS 19.2 ± 9.8 22.7 ± 17.3 NS Coronary arteries: Cx middle 300.8 ± 68.9 308.9 ± 83.8 NS 17.8 ± 8.2 19.0 ± 7.2 NS 19.9 ± 9.1 18.6 ± 7.9 NS Coronary arteries: Cx distal 238.0 ± 54.9 260.1 ± 83.1 NS 24.0 ± 10.1 22.0 ± 6.8 NS 14.2 ± 12.5 12.8 ± 4.7 NS Coronary arteries: RCA proximal 345.0 ± 85.6 316.3 ± 67.4 .02 15.0 ± 5.2 17.4 ± 8.7 NS 27.0 ± 14.9 26.3 ± 25.0 NS Coronary arteries: RCA middle 310.9 ± 74.6 270.7 ± 66.2 .001 20.4 ± 8.8 23.1 ± 11.4 NS 20.1 ± 13.9 14.1 ± 6.7 .001 Coronary arteries: RCA distal 268.4 ± 78.9 276.9 ± 74.8 NS 24.4 ± 13.4 22.5 ± 11.6 NS 14.7 ± 8.2 18.1 ± 16.4 NS

CCTA, coronary computed tomography angiography; Cx, circumflex; HU, Hounsfield units; LAD, left anterior descending artery; NS, not significant; RCA, right coronary artery.

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Diagnostic Image Quality: Visual Assessment

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Heart Rate Variability versus Image Quality

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Discussion

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Conclusion

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