Reducing radiation dose in liver enhanced CT scan by setting mAs according to plain scan noise

Objective To investigate the feasibility of setting m As in liver enhanced CT scan according to plain scan noise with fixed mA CT scanner,in order to reduce the radiation dose. Methods One hundred continuous patients underwent liver enhanced CT scan (group A) prospectively.Two hundred and fifty m As was used in plain and enhanced CT scans.Noises of plain and venous phase CT images were measured,and the image quality was evaluated.The equation between m As of enhanced scan and noise of plain scan image was derived.Another 100 continuous patients underwent liver enhanced CT scan (group B).Enhanced scan m As was calculated from noise on plain scan by using the equation above.Noises on venous phase images were measured and the image quality was measured.Based on body mass index (BMI),patients in groups A and B were divided into three subgroups respectively: BMI<18.5 kg/m2,18.5 kg/m2≤BMI<25.0 kg/m2and BMI≥25.0 kg/ m2.Image quality score was compared with nonparametric rank sum test,CT dose index (CTDI) and effective dose (ED) were measured and compared between each subgroup with 2 independent samples t or t'test. Results The equation between enhanced scan m As (m As X) and plain scan noise (SDp) was as follows: m AsX=mAs1×[ (0.989×SDp+ 1.06) /SDx]2,m As1=250 mAs,SDx=13.In patients with BMI<18.5 kg/m2,ED of group A[ (6.86±0.38) mSv,n=12]was significantly higher than group B[ (2.66±0.46) mSv,n=10) ] (t=18.52,P<0.01).In patients with 18.5 kg/m2≤BMI <25.0 kg/m2,ED of group A[ (7.08±0.91) mSv,n=66] was significantly higher than group B[ (4.50±1.41) mSv,n=73] (t'=10.57,P<0.01).In patients with BMI≥25.0 kg/m2,there was no significant difference between EDs of group A (7.54±0.62 mSv,n=22) and group B[ (8.19±3.16) mSv,n=17] (t'=0.89,P=0.39).Image quality of 5 patients in group A and none in group B did not meet the diagnostic requirement. Conclusion Setting m As of enhanced scan according to plain scan noise could reduce the radiation dose with maintainence of image quality. Key words: Liver; Tomography,X-ray computed; Noise; Radiation dosage