Abstract: Objective: To compare image quality and radiation dose for coronary artery computed tomographic angiograms(CTA) with retrospectively gated helical(RGH) CT technique in two methods using 64-slice spiral CT.Methods: 80 patients with low heart rate enrolled in this study were randomly divided into two groups.The low dose RGH group was 40 patients with average body mass index(BMI) 27.06 using ECG modulation technique(full mA on 70%~80% of the R-R interval and the lowest mA on rest of phases).The traditional RGH group was 40 patients with BMI 25.66 using ECG modulation technique(full mA on 35%~80% of the R-R interval and the lowest mA on rest of phases).Two groups were evaluated for image quality(each coronary artery segment,5-point scale) and radiation dose and compared with image quality score and radiation dose by Mann-Whitney U-test and unpaired t-test.A p value below 0.05 was considered statistically significant.Results: The mean effective dose for the low dose RGH group was 13.76 mSv;this represents a 31.40% reduction as compared with that for the traditional RGH group(mean,20.06 mSv;t=-8.391,P=0.00).The mean image quality score was 4.05 for images obtained with low dose ECG modulation technique versus 4.19 for images obtained with traditional ECG modulation technique(Z=-4.78,P=0.23).Conclusion: RGH coronary CT angiography using ECG modulation technique(full mA on 70%~80% of the R-R interval and the lowest mA on rest of phases) could offer assessable image quality and substantially reduced effective radiation dose compared with traditional RGH coronary CT angiography at low heart rate.