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Purpose To evaluate the image quality and radiation dose of 100 kV with 1000 mAs retrospective electrocardiography (ECG)-gated CCTA, compared to 120 kV with 800 mAs retrospective ECG-gated CCTA. Materials& Methods This prospective type of study is approved by our local hospital Institutional Review Board. We evaluated the CCTA images of 70 patients with normal body mass index (BMI more than 18.5 kg/m2 and less than 25 kg/m2). We divided 70 patients into two groups. Thirty-five patients (18 Males, 17 Females ; Mean age 56.94 ± 11.51 years) were performed 100-kV with 1000 mAs retrospective ECG-gated CCTA, and other thirty-five patients (21 Males, 14 Females ; Mean age 54.03 ± 9.81 years) were performed 120-kV with 800 mAs retrospective ECG-gated CCTA. Then two blinded observers analyzed the image quality of the coronary arteries independently. They performed subjective image quality and objective image quality. The subjective image quality was according to the 5 point scoring scale. The objective image quality was measured as signal-to-noise ratio [SNR] and contrast-to-noise ratio [CNR]. Thereafter, the radiation dose was also measured in both groups. The effective radiation dose [ED] was calculated using CT dose volume index [CTDIvol.], dose-length product [DLP] and conversion coefficient for chest (conversion factor k=0.014 mSv mGy-1 cm-1). Results Although the objective image quality of the 100-kV with 1000 mAs protocol images was significantly poorer than the other protocol images of 120-kV with 800 mAs (mean SNR, 17.23 ± 4.02 vs. 19.76 ± 3.15, P 0.05), there was no significant variation in the subjective image quality between these two groups of normal body mass index (mean image score, 4.54 ± 0.37 vs. 4.56 ± 0.25 for radiologist 1, P = 0.781; 4.52 ± 0.25 vs. 4.56 ± 0.25 for radiologist 2, P = 0.486). The radiation dose was found to be reduced by 28 % with the 100-kV/1000mAs retrospective ECG-gated CCTA than with the 120-kV/800mAs retrospective ECG-gated CCTA protocol (7.87 ± 0.59 vs. 10.95 ± 1.67 mSv, P 0.0001). Conclusions The protocol of low tube voltage CCTA using 100 kV/1000mAs retrospective ECG-gated shows marked reduction of the radiation dose without disturbing the subjective image quality of CCTA.