基于双边滤波的滑动运动补偿4D-CBCT仿真成像研究

Bilateral filtering based sliding motion compensated 4D-CBCT:a simulation study

目的将双边滤波引入基于可变形矢量场(DVF)的4D-CBCT重建,实现全自动滑动运动补偿4D-CBCT重建。方法首先利用所有相位投影,用改良的运动补偿瞬时代数重建技术(Modified Simultaneous Algebra Reconstruction Technique,mSART)生成高质量参考相位。初始4D-DVF通过0%相位和其他相位图像依次配准生成。之后通过配准目标相位测量投影和参考相位变形到目标相位后的正投来优化求解4D-DVF。优化过程中的损失函数平滑约束项中引入双边滤波。其包含3个子核:空间域Guassian核;图像强度域Guassian核;和DVF域Guassian核。选择合适的子核方差提取滑动运动,采用非线性共轭梯度算子优化,用B样条心脏躯干体模(NURBS-based Cardiac-Torso phantom,NCAT phantom)验证算法。采用量化评价指标:Root-Mean-Square-Error(RMSE)和最大误差(MaxE);重建图像提取的肺轮廓Dice系数和相对重建误差(RE)评价算法性能。结果NCAT模体的双边滤波重建运动轨迹的RMSE/MaxE为0.796/1.02 mm;原始重建方法的相应结果为2.704/4.08 mm。图像中的特定结构如肋骨位置,心脏边缘的定义,纤维结构通过双边过滤都得到了更好的纠正。结论开发了一种基于双边滤波的全自动滑动运动补偿4D-CBCT方案,数字模体研究证实了改进的运动估计和图像重建能力,其可被用作肺SBRT治疗的4D-CBCT图像引导工具。

Objective This study reconstructed 4D-CBCT for fully automatic compensated sliding motion by incorporating the bilateral filtering into the Deformable Vector Field(DVF).Methods First,a motion compensated simultaneous algebraic reconstruction technique(Modified Simultaneous Algebra Reconstruction Technique,mSART)was used to generate a high quality reference phase by using all phase projection stogether with the initial 4D-DVFs,which were generated via Demons registration between 0% phase and each other phaseimage.The 4D-DVF was optimized by matching the forward projection of the deformed 0% phase with the measured projection of the target phase.The loss function’s DVF smoothing constrain term contained bilateral filtering kernel that contained:1)an spatial domain Guassian kernel;2)animage intensity domain Guassian kernel;and 3)a DVF domain Guassian kernel.By choosing suitable kernel variances,the sliding motion can be extracted.A non-linear conjugate gradient optimizer wasused.We validated the algorithm on a Non-Uniform Rotational B-spline based Cardiac-Torso(NCAT)phantom.Quantification was evaluated by:1)the Root-Mean-Square-Error(RMSE)together with the Maximum-Error(MaxE);2)the Dice coefficient of the extracted lung contour from the final reconstructed images and 3)the relative reconstruction error(RE)to evaluate the algorithm's performance.Results The motion trajectory's RMSE/MaxEare 0.796/1.02 mm for bilateral filtering reconstruction;and 2.704/4.08 mm for original reconstruction.Image content such a stherib position,the hearted gedefinition,the fibrous structures all had been better corrected with bilateral filtering.Conclusion We developed a bilateral filtering based fully automatic sliding motion compensated 4D-CBCT scheme.Digital phantom study confirmed the improved motion estimation and image reconstruction ability.It can be used as a 4D-CBCT image guidance tool for lung SBRTtreatment.