CT扫描/重建参数对三维治疗计划系统影像的影响

[目的]研究CT模拟定位中,CT扫描/重建参数对三维治疗计划重建的三维假体的几何精确度的影响.[方法]在西门子CT模拟机（Somatom plus 4）上扫描自制模体,扫描所得图像登记到ADAC三维治疗计划系统重建成三维假体,测量假体的相关坐标数据并与模体的实际数据相比较;对Catphan 412模体扫描并测量各组图像的实际层厚,讨论实际层厚的变化对计划系统中登记影像的几何精度影响.[结果]（1）CT扫描所采用的不同扫描/重建参数对三维计划系统中重建的三维假体的左右及上下方向的几何精度影响不大,但对重建假体的前后方向（即模体扫描的步进方向）的几何精度有一定的影响.（2）CT扫描所采用的螺距及重建模式会对层厚敏感度曲线（SSP）半高宽值产生影响,该变化对重建假体的前后方向几何精度同样有一定的影响.[结论]重建CT图像的前后方向的几何误差是随着扫描层厚增加而增加,主要是由于CT扫描的部分容积效应影响.单纯增加螺距或使用360度线性内插（Wide）重建模式,都会引起CT图像实际层厚的增加,引起更大的容积效应影响.同时部分容积效应也会导致三维治疗计划系统中数字重建影像（DRR）分辨率的降低.

Influence of CT Scan & Reconstruction Parameters on Imaging in 3-dimensional Treatment Planning System

[Purpose]To evaluate the influence of CT scan & reconstruction parameters on the accuracy of image reconstruction in 3D treatment planning system(TPS). [Methods] The standard calibration phantom was scanned by Siemens Somatom Plus 4 CT simulator under different CT scan & reconstruction parameters. The acquired images were registrated by ADAC pinnacle 3 TPS. The reconstructed phantom by TPS was then compared with real phantom. The Catphan phantom No.412 was also scanned to measure the CT scan slice thickness for each CT scan. The influence of variation of CT scan slice thickness on the accuracy of image reconstruction in 3D TPS was thus investigated. [Results] The variation of CT scan & reconstruction parameters had little effect on image reconstruction in both anteriorposterior and rightleft directions. However, it could affect the accuracy of image reconstruction in superiorinferior direction (i.e., along the axis that CT scan indexing). Further study demonstrated that CT scan slice thickness and reconstruction method may affect the full width at half maximum(FWHM) of slice sensitivity profiles (SSP), which in turn could impact on the accuracy of axial image reconstruction. [Conclusion] The geometric error of axial image reconstruction increases with the increase of CT scan slice thickness, due to CT partial volume effect. Large helical pitch and wide reconstruction mode are subject to partial volume effect, resulting from the increase of scan slice thickness. Also, partial volume effect can worsen the resolution of digitally reconstructed radiograph(DRR) in 3D TPS.