Abstract： Objective To evaluate the effect of low-dose colonic CT scanning in image quality and lesion revealing with different noise idex.Methods Model group:in order to find the optimal scanning parameter range,small simulated polyps in pig colon was prepared and mA was modulated by regulating noise index.Images were reprocessed with CT colonography and Raysum reconstructions.Image quality and the lesion revealing situation were evaluated and scored based on both reconstructive and axial images.The study of patient group was prospective and continuous.All positive cases were comfirmed by biopsy or surgery pathology.Two groups of patients were enrolled.First group(39 cases from January 2007 to June 2007):patients were divided into three sub-groups randomly(15 cages with NI=10,13 cases with NI:13,11 cases with NI=16),while NI was not changed with position.Second group(32 cases from Jury 2007 to January 2008):patients were scanned and NI was changed with different position(NI=10,20).The average X-ray radiation values of two patient groups were compared with standard values.All data were processed with rank sum tesL Results First group:when NI=10,the average scores of axial and constructive images were 4.2 and 2.4 respectively,when NI=13,those were 3.2 and 2.5 respectively,When NI=16.those were 2.9 and 2.7 respectively.and the average CTDlw were 17.51,12.90 and 5.94 mGy respectively.When NI increased,the average scores of axial decreased(H=20.01,P＜0.01),the average scores of constructive images did not changed(H=0.81,P＞0.05).Second group:when NI=10,the average scores of axial and constructive images were respectively 3.6 and 2.3,when NI=20,those were 2.2 and 2.3 respectively,and the average CTDIw was 11.63 mGy.When NI increased from 10 to 20,the average scores of axial decreased(H=20.84,P＜0.01),the average scores of constructive images did not decreased(H=0.29,P＞0.05).In the first group,the average CTDIw wag reduced by 35.0%,52.1%and 77.9%respectively when NI was 10,13 and 16.In the second group,the average CTDIw Was reduced by 56.8%.Conclusions Auto-mA low-dose CT scanning could reduce X-ray radiation obviously,while not affecting image quality.