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Objective To evaluate the effect of KV Assist on radiation dose reduction and image quality for abdominal CT in different BMI groups.Methods 100 patients underwent abdominal CT on a new generation spectral CT scanner(Discovery CT,GE healthcare).The patients were divided into 2 groups for prospective analysis.Group A(n=50)used KV assist protocol,which automatically selects an optimal kVp based on the scout view.While the conventional 120kVp scan using auto mA protocol was performed on Group B(n = 50).The main parameters of protocol were setup with noise index of 10,auto mA ranges at 100-450mA and rotation time at 0.8s.Group A and B were both further divided into two subgroups according to BMI(BMI<24 kg/m2for group A1/B1 and BMI≥24 kg/m2 for groupA2/B2).CT values and SD values,CNR of CA,PV,liver,pancreas and image quality score in abdomen were measured and calculated.CTDIvol and DLP of each patient was recorded and compared.Comparison between group A and B was implemented as a representative of those for the subgroups.Image quality scores of the protocols were assessed and compared using Rank-sum test.Student T test was used to analyze other all data.Results Image noise of protocol A and B in dual-phase were(10.70±2.65),(8.83±2.38)HU and(11.27±3.82),(8.82±2.25)HU,respectively.The difference was statistically significant(p=0.00).However,CNR values obtained in group A vs in group B were comparable or higher in both LAP(19.99±9.98 vs 18.64±6.29 in CA,p>0.05; 1.13±1.13 vs 1.13±1.25 in liver,p>0.05; and 3.38±1.64 vs 2.64±1.42 in pancreas,p<0.05)and PVP(8.61±3.03 vs 7.60±2.88 in PV,p>0.05; 4.30±1.67 vs 3.92±1.65 in liver,p>0.05; and 2.91±1.46 vs 2.25±1.45 in pancreas,p<0.05).Besides,the difference of the subjective rating scores in protocol A and B were statistically insignificant(p=0.554).Effective dose in group A was significantly lower than that in group B(4.6±2.4mSv vs 6.6±3.1mSv),(t=2.315,p=0.02)and was decreased by 30.31%.Percentages of 80kVp,100kVp,120kVp and 140kVp scans using KV assist were 31%,58%,11%and 0%for group A1,while 0%,0%,35%,and 65%for group A2.The radiation dose reduction in group A1 and A2 were 30.18%and 22.71%compared with group B1 and B2.Conclusion Abdominal enhanced CT scans using KV assist can provide better image quality and 30.31%radiation dose reduction compared with 120kVp scans.Radiation dose reduction on patients with BMI < 24 kg/m2 was more than patients with BMI ≥ 24 kg/m2.