Abstract： Objective To establish and clinically evaluate a method to adjust tube current(mA)individually for obtaining images with consistent noise and optimized radiation dose in 64-slice spiral helical CT coronary angiography.Methods (1)80 patients,as a control group,were underwent CT coronary angiography(CTCA)using 64-muhislice spiral helical CT with fixed mA(650 mA).Two radiologists first assessed image quality of the fixed mA group by scoring from 1 to 5.A minimal dose requirement for meeting clinical diagnosis with the desired noise level(standard deviation SD0)were decided.By analyzing the images of the control group,we also established correlation between the image noises of timing bolus(TB)scans and cardiac(CA)scans and set up a formula to predict the required mA value for obtaining the desired cardiac image noise for individual patient.(2)Subsequently,another group of 80 patients(study group)wero scanned with individually adjusted mA using the established formula and the practicability of this method was then evaluated.The independent sample t test was used to compare the values of the two groups.Results (1)The noise of TB and CTCA scans was linearly correlated with r2=0.994. (2)A cardiac image noise level of 28 HU was found to be adequate for clinical diagnostic purpose based on the image quality analysis.(3)The average noise for the acquired cardiac images was found to be 28.97 HU for the study group with individually adapted mA using the established formula.The standard deviation of the image noisein study group was 1.93,which was much smaller than 5.14 in the fixed mA group,but the average image scores for the study group(4.27±0.68)and fixed group mA(4.03 4-0.93)were not statistically different.(4)In the study group,63 out of 80 patients required tube currents below the standard 650 mA,yielding an average CTDIvol of58.32 mGy,which was about 32.1%dose reduction than that of fixed mA(85.94 mGy).The lowest CTDIvol was 19.27 mGy with 160 mA which was 22.4%of the average CTDIvol in control group.Conclusion Adjusting mA individually based on timing bolus scan noise analysis is an accurate and practical method to obtain consistent image quality for different cardiac pailents and to minimize the radiation dose for individuals compared to the fixed mA.