胸部低剂量CT应用大螺距及高机架转速行扫描对图像质量的影响

Study of impact low dose chest CT with big pitch and high rotation time on image quality

目的探讨在胸部低剂量CT扫描时应用大螺距及高机架转速降低扫描对图像质量的影响。方法应用GE Optima CT680 CT机对胸部仿真体模进行四组扫描。其中A组螺距比0.984:1,机架旋转时间1.0 s;B组螺距比1.531:1,机架旋转时间1.0 s;C组螺距比0.984:1,机架旋转时间0.4 s;D组螺距比1.531:1,机架旋转时间0.4s。将获取的图像利用0.625 mm层厚的图像进行多平面重建(MPR)层厚5 mm,记录每组图像的扫描时间,容积CT剂量指数(CTDIvol)和剂量长度乘积(DLP)。测量并比较轴位5 mm层厚图像的客观噪声。由两名有经验的放射科医生采用双盲法对5 mm层厚轴位图像和MPR图像磨玻璃密度结节(GGN)进行主观评分。结果四组图像,客观噪声及X线剂量基本一致。扫描时间明显缩短,分别为9.29 s、6.39 s、3.42 s和2.56 s。各组5 mm层厚轴位图像在对GGN主观评分无统计学差异(P>0.05),且两名观察者间的一致性中等或良好(k>0.4)。多平面重建图像在对GGN主观评分具有统计学差异(P<0.05),且两名观察者间的一致性中等或良好(k>0.4)。结论胸部低剂量CT检查时应用自动管电流调制技术结合大螺距、高机架旋转时间,可有效地降低扫描时间,防止呼吸伪影的产生。MPR图像空间分辨率略有下降,但轴位图像质量不会产生明显影响。

Objective The purpose of this study was to evaluate the consequences of different pitch and rotation time on radiation dose and image quality with an automatic tube current modulation(ATCM)system on CT scanning of chest.Methods The chest phantom was scanned on GE Optima CT680scanner.Images were obtained at pitch of0.984∶1and rotation time of1.0s,pitch of1.531∶1and rotation time of1.0s,pitch of0.984∶1and rotation time of0.4s,pitch of1.531∶1and rotation time of0.4s,respectively and named as group A,B,C and D with an ATCM system.Multiplanar reconstruction(MPR)images of slice thickness of5mm were reconstructed by images of slice thickness of0.625mm.Scan time,volumetric CT dose index(CTDIvol)and dose-length product(DLP)were recorded in the CT scanner.The objective noise of images were measured.Two experienced radiologists blinded to the CT parameters to assess the identification of GGN.Results There was no significant difference in objective noise and radiation dose but reduce of scan time.The scan time were9.29s,6.39s,3.42s and2.56s respectively.There was no significant difference in the assessment of GGN among the4groups of axial images while there was significant difference for MPR images.Both Kappa value of the identification of the GGNs was substantial or almost perfect.Conclusion There is a reduce of scan time on chest CT by combination of big pitch and high rotation time on low-dose chest CT examination,which can prevent breathing artifact.Space resolution of MPR images declines slightly while the axial image quality was not influenced.