自动管电流调制技术对腹部CT图像质量及辐射剂量影响的体模研究

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

目的探讨应用自动管电流调制技术（ATCM）对腹部CT图像质量及辐射剂量的影响。方法通过实体测量因病情需要行腹部cT检查的68例体质量指数（BMI）为18—24kg／m2的成年男性腹部CT扫描的腹部径向数据，借鉴CTP515辅助测试模体，制作符合国人尺寸的腹部体环，嵌套Catphan600性能测试模型为观察对象，模拟成人腹部CT低、高对比特点用于实验研究。以管电压120kV，固定管电流450mA组为对照组；相同管电压条件下，自动管电流技术组为实验组，其中噪声指数（NI）值设置为4～20，共14组，管电流范围10—450mA。对模型进行扫描。测量Catphan600模体中心均质区域CT值，验证模型对x线衰减的等效性。记录每组扫描方案的辐射剂量指标容积CT剂量指数（CTDIvol）、个体化剂量估计值（SSDE）、剂量长度积（DLP）、有效剂量（ED）及图像质量客观评价指标图像噪声值（SD）、对比噪声比（CNR）。同时由3名分别来自不同医院有5年以上工作经验的腹部诊断医师对Catphan600模体中的CTP515低对比目标显示程度（LCD）模块及CTP528高对比目标显示程度（HCD）模块进行独立主观评价。对NI值与SDb、CNR、CTDIvol、DLP间的相关性采用Pearson相关分析，与LCD模块评分间相关性采用Spearman秩相关分析。不同NI组间SD、CNR及CTDIvol值与对照组的比较采用t检验。LCD和HCD评分的组内比较采用Kruskal—Wallis秩检验，组间差异采用Mann—WhitneyUTest秩检验。3名放射科医师主观评价结果的一致性检验采用Kappa分析。结果实测人体正常肝脏组织CT值为（52．8±9．1）HU，腹部模体中心区域平均CT值为（50．8±1．0）HU，两者差异无统计学意义（t=0．31，P〉0．05），偏差程度为4．9％。此模型与成人腹部对x线衰减具有较好的等效性。所有辐射剂量指标均随NI值增大而降低，NI值增加1个单位，辐射剂量指标CTDlvol、SSDE、DLP和ED值分别平均下降1．23mGy、2．03mGy、13．20mGy·cm、0．20mSv。NI值与图像质量主、客观评价指标及辐射剂量指标间呈高度线性相关（r=0．871—0．982，P值均〈0．05）。实验组NI=4—7组与对照组间SD、CNR差异均无统计学意义（t值分别为-1．63～-0．17，0．03—1．12，P值均〉0．05）。实验组NI=4—10组与对照组间LCD评分差异均无统计学意义（Z值为-1．637—0．000，P值均〉0．05）。实验组NI=4—13组与对照组间HCD得分差异均无统计学意义（z值为-1．423～0．000，P值均〉0．05）。3名医师对LCD、HCD评分的Kappa一致性检验结果分别为0．743、0．795。结论在腹部CT扫描中应用自动管电流调制技术可确保图像质量，同时降低辐射剂量。

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 CTPS15 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/cma （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 kV,450 mA and experimental group were scanned with the same tube voltage and variable current range from 10 to 450 mA （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 CTPS15 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 close 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 ROI in 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 ）. There were no significant differences in SD, CNR betweenNIgroup4 to 7 and the control group（t = - 1.63 to -0.17,0.03 to 1.12,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 of Kappa analysis 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.