Objective To assess the effect of iDose4 in image quality and radiation dosage in CT angiography of craniocervical arteries.Methods Imaging data of 50 patients with 256-slice spiral CT angiography of craniocervical arteries were randomly divided into two groups:(A) 120 kV,250 mAs (25 patients),(B) iDose4 100 kV,180 mAs (25 patients).Dose length product (DLP) of the two groups were compared.Objective image noise was compared by two blinded readers,t test and nonparametric tests were used for statistical analysis.Results The overall image quality score of group A (3 points =1 patient,4 points =6 patients and 5 points =18 patients) and group B (4 points =5 patients and 5 points =20 patients) were shown.The image noise of group A (1 point =1 patient,2 points =5 patients and3 points =19 patients) and group B (2 points =4 patients,3 points =21 patients) were also evaluated.There was no statistical difference in revealing rate of image quality scores and objective noise between the two groups (Z =-0.334,-0.218,P ＞ 0.05).The estimated effective radiation dose was reduced by 56％ in group B than in group A.DLP was significantly lower in group B [(642.4 ±36.6) mGy · cm]than in group A [(1462.1 ± 64.3) mGy · cm].Conclusion Low dose contrast medium of CT angiography can display craniocervical arteries and the intracranial arteries very well with high image quality.