Application of automatic tube current modulation on image quality and radiation dose at abdominal CT

Abstract

Objective To investigate the effect of automatic tube current modulation (ATCM) on image quality and radiation dose for abdominal CT. Methods An abdomen-equivalent tissue model in the present study was developed,by referring to the CTP515 auxiliary performance testing model made by the internationally recognized Phantom Laboratory,in conjunction with the real data from 68 Chinese male subjects with BMI between 18 to 24 kg/cm2 (subjects with fatty liver,cirrhosis,intraperitoneal huge tumor,ascites or pleural effusion or who were under 18 years old were excluded).Catphan600 performance served as observation object,and simulating adult abdominal CT low and high contrast characteristics to research.Control groups were scanned with CT protocol at fixed 120 k V,450 m A and experimental group were scanned with the same tube voltage and variable current range from 10 to 450 m A (NI value:4 to 20),respectively.CT value was measured by homogeneous area in the center of Catphan600 to verify the phantom for X-ray attenuation of equivalence.CT dose index (CTDIvol),size-specific dose estimates (SSDE),dose length product (DLP),image noise (SD) and contrast to noise ratio (CNR) were recorded and measured.Fifteen mm diameter low-contrast detectability in CTP515 and high-contrast detectability in CTP528 at Catphan600 were evaluated independently by 3 doctors working in abdominal diagnosis for more than five years from three hospitals in a five-point scale.The correlation between NI value and objective,subjective evaluating indicator of image quality and radiation dose were analyzed,and SD,CNR and LCD,HCD scores in different NI group and those in control group were compared with parameter and non-parametric rank sum test.The differences of SD,CNR and CTDIvol scores between NI group and control group were compared with t test.Kruskal-Wallis test was used for intra-group comparison of LCD and HCD scores,Mann-Whiteney U Test for inter-group and Kappa analysis for consistency check of the subjective evaluation result of the 3 radiologists. Results No significant difference was found between the CT value of ROIin catphan 600 central area[ (50.8±1.0) HU] scanned under various NI and that of the normal liver tissue[ (52.8±9.1) HU]obtained from 68 Chinese male subjects (t=0.31,P>0.05),with a deviation degree of 4.9%.This phantom was equivalent to adult abdomen for X-ray attenuation.One unit increase in NI was associated with 1.23 mGy,2.03 mGy,13.20 mGy·cm,0.20 mSv decrease in CTDlvol,SSDE,DLP and ED respectively.NI and objective,subjective evaluating indicator of image quality and radiation dose was highly linear correlated (r=0.871 to 0.982,P 0.05).No significant differences were found in LCD scores between NI group 4 to 10 and the control group (Z=-1.637 to 0.000,P>0.05).And the HCD scores between NI group 4 to 13 and the control group (Z=-1.423 to 0.000,P>0.05) were not significantly different either.The consistency check result ofKappaanalysis of the 3 doctors' LCD,HCD evaluation was 0.743 and 0.795. Conclusion Application of the ATCM technique in abdomen CT can reduce the radiation dose while ensuring the image quality. Key words: Tomography,X-ray computed; Radiation dosage; Abdomen; Phantom