定位像扫描参数对胸部CT影像质量和辐射剂量影响的模体研究

The influence of scout scanning parameters on image quality and radiation dose of chest CT: a phantom study

目的探讨胸部CT定位像扫描参数(X射线管投照角度和管电压)的选择对图像质量和辐射剂量的影响规律,为临床实践中成像参数的选择提供指导。方法选择不同扫描参数对成人胸部拟人模体进行定位像采集,X射线管投照角度(0°、90°和180°)、管电压(70、80、100、120和140 kV)和管电流(25 mA)共15种组合。根据所得定位像进行胸部螺旋扫描,扫描条件为Assist kV、Smart mA(设备允许最大范围)、探测器宽度80 mm,螺距0.992∶1,旋转时间0.5 s,扫描长度330 mm,层厚5 mm,噪声指数(NI)10,迭代指数(ASiR-V)前置为30%,后置50%。记录螺旋扫描管电压、4个层面(肺尖、气管分叉、乳腺水平和横膈顶部)管电流量和容积CT剂量指数(CTDIvol)。用热释光剂量计(TLD)测量每次扫描时乳腺器官剂量。所得图像在气管分叉和横膈顶部层面选取感兴趣区(ROI),计算对比噪声比(CNR)。结果X射线管投照角度0°时,螺旋扫描管电压自动选择80 kV,定位像管电压改变对4个层面管电流量影响较小,变化范围0~2%(5/230)。X射线管投照角度90°和180°时,螺旋扫描管电压自动选择100 kV,定位像管电压改变对气管分叉层面管电流量影响较大,变化范围14%(29/210)~44%(93/210)。X射线管投照不同角度其CTDIvol差异有统计学意义(P<0.017);乳腺器官剂量、气管分叉层面CNR和横膈顶部层面CNR差异有统计学意义(F=13.027、24.727和10.630,P<0.05)。根据定位像管电压分组,CTDIvol、乳腺器官剂量和两个层面的CNR差异均无统计学意义(P>0.05)。结论胸部CT扫描中,定位像参数中X射线管投照角度较管电压对图像质量和辐射剂量影响更显著。

Objective To explore the effects of scout scanning parameters(projection angle and tube potential)on image quality and radiation dose in chest CT and to provide guidance for the selection of scanning parameters in clinical practice.Methods Different parameter combinations were selected to acquire the scout view of the anthropomorphic adult chest phantom.Fifteen scout views were obtained under parameter combinations of the projection angle(0/90/180 degrees),tube potential(70/80/100/120/140 kV)and tube current(25 mA).And then,chest spiral scans were performed according to the scout views.The other parameters were Assist kV,smart mA(maximum range),80 mm detector width,0.992:1 pitch,0.5 s rotation time,330 mm scanning length,5 mm thickness,NI(noise index)10.The radiation dose modulation index(ASiR-V)was 30%,and the iterative level was 50%.Volume CT dose index(CTDIvol)and tube current at 4 body levels(pulmonary apex,trachea bifurcation,mammary level and diaphragmatic dome)were recorded.The thermoluminescent dosimeters(TLDs)were used to measure the organ dose of the breast glandular.Regions of interest(ROIs)were selected at the trachea bifurcation and diaphragmatic dome to calculate the contrast noise ratio(CNR).Results The tube potential of the spiral scanning was automatically selected as 80 kV when the projection angle was 0 degree.The changing of the tube potential for the scout had little influence on the tube current at the four body levels,and the variation range was 0-2%(5/230).The tube potential in the spiral scanning was automatically selected as 100 kV when the projection angles were 90 and 180 degrees.The changing of the tube potential for the scout had a great influence on the tube current at the slice of trachea bifurcation and the variation range was from 14%(29/210)to 44%(93/210).According to the angle projection,there were statistical differences in CTDIvol(P<0.017),organ dose of the breast glandular,and CNR at the slices of trachea bifurcation and at diaphragmatic dome(F=13.027,24.727,10.630,P<0.05).According to the tube potential in the scout scanning,there were no significant differences in CTDIvol,organ dose of the breast glandular and CNR at both levels(P>0.05).Conclusions In chest CT,the influence of the projection angle in scout scanning on the image quality and radiation dose was more significant than that of the tube potential.