不同体型特异性剂量估算值在儿童胸部CT的应用

Application of Different Size-Specific Dose Estimate in Children’s Chest CT

目的比较儿童胸部CT检查时有效直径(Effective Diameter,De)和水当量直径(Water Equivalent Diameter,Dw)的体型特异性剂量估算值(Size-Specific Dose Estimate,SSDE)间的差别并找出较为简便且能准确直观的反应儿童胸部CT检查辐射剂量的方法。方法前瞻性的连续收集行胸部CT检查的0~16岁患儿118例,依据患儿年龄进行分组,37例0~3岁患儿为A组,39例4~7岁患儿为B组,42例8~16岁患儿为C组。从工作站剂量报表获得患儿的平均CTDIVOL。在胸部最中间层面CT图像上测量患儿胸廓的前后径和左右径,并计算其De,依据De对应的转换因子fDe得到SSDEDe;在每一层胸部CT图像上画椭圆形感兴趣区,椭圆要覆盖横断面图像所有解剖结构,记录椭圆形面积及椭圆形区域的平均CT值,计算其Dw,依据Dw对应的转换因子fDw得到SSDEDw。然后采用方差分析对各组患儿组间De、Dw、fDe、fDw、SSDEDe、SSDEDw进行比较,采用配对t检验对各组患儿组内两种SSDE计算方法下的直径、转换因子和SSDE进行比较。结果三组患儿在De、Dw、fDe、fDw、SSDEDe、SSDEDw间的差异均有统计学意义,两组间对比差异均有统计学意义(P均小于0.05)。随着年龄的增大,患儿的De、Dw在增大,而fDe、fDw、SSDEDe、SSDEDw在减小。A组患儿De较Dw高10.5%,fDe较fDw低13.5%,SSDEDe较SSDEDw低13.6%,差异均有统计学意义(P<0.05);B组患儿De较Dw高10%,fDe较fDw低15.5%,SSDEDe较SSDEDw低16%,差异均有统计学意义(P<0.05);C组患儿De较Dw高13.2%,fDe较fDw低17.8%,SSDEDe较SSDEDw低18.1%,差异均有统计学意义(P<0.05)。A组患儿SSDEDe、SSDEDw与SSDEg呈正相关(r=0.90,0.91,P<0.05);B组患儿SSDEDe、SSDEDw与SSDEg呈正相关(r=0.89,0.92,P<0.05);C组患儿SSDEDe、SSDEDw与SSDEg呈正相关(r=0.88,0.90,P<0.05)。结论SSDEDw更能精确评估患儿的辐射剂量;胸部CT图像中间层面的SSDEDw是反映患儿辐射剂量的简单准确的代表参数。

Objective To compare the difference Size specific dose estimate(SSDE)between effective diameter(De)and water equivalent diameter(Dw)in children’s chest CT examination and find out a simple and intuitive method to reflect the radiation dose.Methods A prospective study of 118 children undergoing chest CT scans were enrolled and divided into group A(37 children aged 0-3 years),group B(39 children aged 4-7 years)and group C(42 children aged 8-16 years).CTDIVOL was obtained from the workstation dose report.The anteroposterior diameter and left and transverse diameter of thorax were measured on the middle CT image,and the De was calculated.SSDEDe was obtained according to the conversion factor fDe corresponding to De.The ellipse of interest was drawn on every CT image.The ellipse covered all anatomical structures of cross-sectional images.The average CT values of ellipse area and ellipse area were recorded,and the was calculated.According to the conversion factor fDw corresponding to Dw,SSDEDw was obtained.Then,variance analysis was used to compare De,Dw,fDe,fDw,SSDEDe and SSDEDw among groups,and the paired t test was used to compare the diameter,conversion factor and SSDE under two SSDE calculation methods in each group.Results There were significant differences in De,Dw,fDe,fDw,SSDEDe and SSDEDw among groups,and there were significant differences between every two groups(P<0.05).With the increase of age,the De and Dw increased,while the fDe,fDw,SSDEDe and SSDEDw decreased.In group A,De was 10.5%higher than Dw,fDe 13.5%lower than fDw,SSDEDe 13.6%lower than SSDEDw;in group B,De was 10%higher than Dw,fDe 15.5%lower than fDw,SSDEDe 16%lower than SSDEDw;in group C,De was 13.2%higher than Dw,fDe 17.8%lower than fDw,SSDEDe 18.1%lower than SSDEDw;all of these differences were statistically significant(P<0.05).SSDEDe and SSDEDw were positively correlated with SSDEg in group A(r=0.90,0.91,P<0.05);SSDEDe and SSDEDw were positively correlated with SSDEg in group B(r=0.89,0.92,P<0.05);SSDEDe and SSDEDw were positively correlated with SSDEg in group C(r=0.88,0.90,P<0.05).Conclusion SSDEDw can more accurately assess the radiation dose of children;SSDEDw at the central image is a simple and accurate representative parameter for reflecting radiation dose of children.