Objective To investigate the aortic CTA at reduced tube voltage and ultra-low contrast medium dose while maintaining or improving image quality through a knowledge-based Iterative Model Reconstruction(IMR)compared with conventional 120kVp scan with 70ml contrast medium.Methods 62 patients underwent aorta CTA examination on 256 MDCT(iCT,Philips Healthcare)were divided into 2 groups.31patients scanned using 120kvp with contrast medium(CM)volume of 70 ml and 5 ml/s injection rate were reviewed as control group A.31patients scanned using ultra-low does mode(low-tube-voltage at 80kvp and 100mAs/slice)with CM of 0.4ml/kg and injection rate calculated as volume/(delay time+exposure time)were the study group.Paired image sets were created using 3 types of reconstruction: filtered back projection(FBP)as group B,a hybrid type of iterative reconstruction(iDose4)as group C,and IMR as group D.Objective evaluation [CT values,image noise and contrast-to-noise ratio(CNR)of aorta and its branches]and subjective rating score(image noise score,overall image quality score and sharpness score)among the four groups were obtained and compared by One-way ANOVA and Kruskal-Wallis H test.Results The mean image noise in group A was lower than group B and C,and higher than group D,but showed no differences from group D(p>0.05).There were no significant differences in aorta CT attenuation among the 4 groups(p>0.05).The CNR of group D was better than other groups in all evaluated structures.There were significant differences among the four groups(all p<0.05).The images of group B and C were unacceptable for subjective image quality.The best subjective image quality for aorta were obtained in group D and A,and scores of these two groups were all greater than 3 and met clinical diagnostic level,except in group B and C.The amount of contrast agent for each patient in group A was 70 ml,while the amount of contrast agent in study group was(24.7±3.6)ml.There were significant differences of the contrast medium volume among the two groups(F = 537.091,P = 0.000).The radiation dose was significantly different between the study groups and the control group[CTDIvol,(2.58±0.94),(9.7±3.1)mGy,respectively].Conclusion IMR in aortic CTA with ultra-low volume contrast medium and dose reduction was feasible and considerably improved both objective and subjective image quality parameters compared with conventional 120kvp scan.Contrast volume and radiation dose can be reduced by 60%and 74.2%,respectively.