Abstract： Objective To assess the image quality (IQ) of an iterative reconstruction (IR) technique (iDose4) from prospective electrocardiography (ECG)-triggered coronary CTA on a 256 MSCT scanner and determine the optimal dose reduction using IR that can provide IQ comparable to filtered back projection (FBP).Methods Prospectively ECG gated CCTA were performed on 120 patients [76 men,44 women; age:(53 ± 10)y] using a 256-slice MSCT (Brilliance iCT,Philips Healthcare).The control group (Group A,n =30) were scanned using the conventional tube output (120 kVp,210 mAs) and reconstructed using FBP.The other 3 groups were scanned with the same kVp but successively reduced tube output as follows:B (n =30):105 mAs,C (n =30):84 mAs:D (n =30):65 mAs and reconstructed using IR levels of L4 to L6,respectively.All images were reconstructed using the same kernel (XCB).Two radiologists graded IQ in a blinded fashion on a 4-point scale (4-excellent,3-good,2-fair and 1-poor).Quantitative measurements of CT values,image noise,Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were obtained in each group.Analysis of variance (ANOVA) was used for comparisons of objective evaluation indices (noise,CNR) and radiation dose (CTDIvol,DLP,ED) between the four groups.The Kruskal-Wallis test was used for comparisons of demographic data and for detection of differences in subjective evaluation of IQ among groups.A level of P ＜ 0.05 was considered statistically significant.A ROC analysis was performed to determine a radiation reduction threshold up to which excellent IQ was maintained.Results There was no significant differences in objective noise among Groups A (37.4 ±7.9) HU,B(33.2±7.1) HU,C(35.7±9.8) HU,and D(36.0±6.8) HU (F=1.48,P=0.22).There was no significant differences in CNR among Groups A(15.0 ±2.3),B(16.5 ±3.6),C(16.3 ±3.5),and D(15.3±2.8) (F=1.70,P =0.17).Group B and C had good and excellent scores of the subjective IQ (≥3),and there was no significant differences in the scores of the subjective IQ between Group A,and Groups B,C (P =0.30-1.00).Significant differences in image sharpness and study acceptability were observed between groups A and D (P ＜ 0.01).Using the criterion of excellent IQ (score 4),the ROC curve of dose levels and IQ acceptability established a reduction of 60％ of tube output (Group C) as optimum cutoff point (AUC:0.76,95％ CI:0.65-0.87).The effective dose (ED) of Group C was 61％ lower than that of Group A,(1.2 ± 0.1) mSv vs.(3.1 ± 0.6) mSv.Conclusion Iterative reconstruction techniques can provide 61％ ED reduction in prospectively-triggered coronary CTA using 256-slice MSCT while maintaining excellent image quality.