运动靶区CT三维重建对其体积与形状的影响

目的探讨CT扫描过程中运动伪影对放射治疗计划设计的影响。方法用步进电机和低密度泡沫材料设计能模拟肿瘤随呼吸而作往返周期运动的体模;在GE LightSpeed 16 CT上对体模进行10个不同序列的扫描,每个扫描序列的扫描螺距、扫描层厚或体模运动状态各不相同,扫描后用GE AdvantageSim6.0版CT模拟定位系统的体绘制(volume rendering)软件分别对各靶区进行三维重建和体积测量,并进行对比分析;再用WiM RT计划系统对静态和动态扫描的靶体进行三维适形放疗计划设计和对比分析。结果体模静止时,扫描层厚与螺距的改变不影响扫描后靶区的三维重建,重建靶区外观无明显失真,在不同扫描条件下同一个靶区重建体积差异甚微;当体模运动时,扫描层厚、螺距和体模运动状态的改变,都会显著影响靶区重建形状和重建体积,导致不同运动状态下同一靶区适形照射野的形状迥异,照射范围差异十分明显,重建靶体积的最大相对偏差约90%;同一运动状态下,各靶区重建体积变化随靶而异;外形较小的靶区重建体积相对偏差变化范围为-39.8%~89.5%,外形较大的靶区重建体积相对偏差变化范围是-18.4%~20.5%。结论运动伪影对三维适形放疗计划设计的影响极大,肿瘤靶区处于运动状态下扫描的图像会导致适形射野和剂量体积直方图严重失真。

Volume and Shape of Motion Target on CT 3D Reconstruction

Objective To study the effect of motion artifacts in CT scanning on radiotherapy planning design. Methods A phantom which can mimic the periodic motion of lung was constructed by step motor, small ball of polymethyl methacrylate and potato. Ten series with different scan pitches, slices, periods and initial phases were set and the phantom was scanned by GE LightSpeed16 CT scanner. The three-dimensional reconstruction of CT images of the phantom was completed by adopting the volume rendering technology of GE AdvantageSim 6.0 system, and the volumes of each target of different series were measured by the three-dimensional measuring tools. Thus, the relative deviations of reconstruction volumes between moving targets and the stationary ones were calculated. The three-dimensional conformal radiotherapy plans were created for the stationary and moving targets. Results There was no obvious difference between the three-dimensional reconstructions of stationary targets that were scanned with different pitches and slices. There were great different appearance and three-dimensional conformal fields between the three-dimensional reconstructions of moving and stationary targets. The maximal relative deviation of volume was nearly 90% for the same target scanned with different pitches, slices and periods. The relative deviation of reconstruction volume was variable among the different targets, about -39.8% to 89.5% for the smaller targets and -18.4% to 20.5% for the larger targets. Conclusion The motion artifacts have great effect on three-dimensional conformal radiotherapy planning design. The conformal radiation fields will be distorted greatly and the dose volume histogram (DVH) will be false if the CT images were acquired when the targets were not stationary.