Abstract: Objective:To evaluate the effect of relative temporal resolution on image quality in clinical 64-MSCT coronary angiography and establish the criteria of heart rate selection in coronary CTA.Methods:496 patients who underwent 64-MSCT coronary angiography with approximately normal cardiac function were included in the study.Images from 30% to 80% of cardiac cycle were generated with single sector reconstruction.The images at the vertical portion of the right coronary arteries in single cardiac cycle were used for selection of the cardiac phase with minimal motion artifact, and the heart rate at the cardiac cycle was recorded for calculation of relative temporal resolution.Then the images of the middle portion of right coronary arteries at the best cardiac phase were used for scoring of motion artifacts.We used a term named effective reliability(ER) to describe effective temporal resolution.The relationship between ER and image quality was analyzed to carry out the threshold of ER for different image quality requirements.Groups with acceptable ER and unacceptable ER were compared for the cardiac phase of reconstruction for optimal image quality.Results:Among the 496 patients, heart rates ranged from 43 bpm to 103 bpm.Using the score of 3 as the acceptable image quality, an ER of 81 was considered as the lower limit for successful examination.Using score 4 as the acceptable image quality, an ER of 84 was considered the lower limit.There is significant difference in phase selection between the groups with acceptable ER and unacceptable ER, with more reconstruction in end systole in the group with unacceptable ER.Conclusion:Relative temporal resolution is a critical point for coronary CT.A patient with an ER under the threshold is not a proper candidate for coronary CT imaging.