A line-source method for aligning on-board and other pinhole SPECT systems.
A line-source method for aligning on-board and other pinhole SPECT systems.
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DOI：
10.1118/1.4828776
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发表时间：
2013-12
期刊：
影响因子：
3.8
通讯作者：
Susu Yan;J. Bowsher;F. Yin
中科院分区：
文献类型：
--
作者：
Susu Yan;J. Bowsher;F. Yin
为了实现功能和分子成像的病人在位置的放射治疗，机器人多针孔SPECT系统正在开发中。SPECT系统与直线加速器（LINAC）坐标系和其他机载成像系统（如锥束CT（CBCT））的坐标系的对准对于目标定位和图像重建至关重要。提出并研究了一种利用线光源和单针孔投影的对准方法。该方法也可应用于其它针孔SPECT系统的标定。方法建立了一个由多个对准参数组成的对准模型，该模型将三维（3D）空间中的线源映射到其在SPECT探测器上的二维（2D）投影。在计算机模拟研究中，线源的3D坐标被定义在参考室坐标系中，例如LINAC坐标系。通过计算机模拟生成相应的2D线源投影，包括SPECT模糊和噪声效应。Radon变换用于检测线源投影的角度（α）和偏移（ρ）。然后根据α和ρ值以及对准模型，通过非线性最小二乘法估计对准参数。对齐性能进行了评估，作为一个函数的线源的数量，拉东变换精度，有限的线源宽度，固有的相机分辨率，泊松噪声，和采集几何形状。使用物理线源幻影和针孔准直伽马相机连接到机器人进行实验评价。结果在计算机模拟研究中，当确定测量投影的角度（α）和偏移（ρ）时没有误差时，使用三线源可以完美地估计6个对准参数（3个平移和3个旋转）。当角度（α）和偏移量（ρ）由Radon变换提供时，估计精度降低。估计误差与Radon变换的舍入误差、有限线源宽度、泊松噪声、线源数目、固有相机分辨率和探测器采集几何形状相关。从统计学上看，使用四个线源而不是三个线源和更细的线源投影（通过更好的固有检测器分辨率获得）显着提高了估计精度。对于5个线源，探测器平移的中位误差为0.2 mm，探测器旋转半径的中位误差为0.7 mm，探测器旋转、倾斜和扭曲的中位误差小于0.5°。在实验评估中，相对于不同的独立配准技术，探测器平移的平均误差约为1.8 mm，探测器旋转半径（ROR）为1.1 mm，探测器旋转和倾斜分别为0.5°和0.4°，探测器扭曲为1.2°。结论利用线源的单针孔投影可以估计准直参数。对准误差在很大程度上与Radon变换在确定线源投影的角度（α）和偏移（ρ）方面的有限精度有关。这种对准方法对于多针孔SPECT可能是重要的，其中相对针孔对准可能在旋转期间变化。对于针孔和多针孔SPECT成像机载放射治疗机，该方法可以提供SPECT坐标与CBCT和LINAC坐标的对准。
PURPOSE In order to achieve functional and molecular imaging as patients are in position for radiation therapy, a robotic multipinhole SPECT system is being developed. Alignment of the SPECT system-to the linear accelerator (LINAC) coordinate frame and to the coordinate frames of other on-board imaging systems such as cone-beam CT (CBCT)-is essential for target localization and image reconstruction. An alignment method that utilizes line sources and one pinhole projection is proposed and investigated to achieve this goal. Potentially, this method could also be applied to the calibration of the other pinhole SPECT systems. METHODS An alignment model consisting of multiple alignment parameters was developed which maps line sources in three-dimensional (3D) space to their two-dimensional (2D) projections on the SPECT detector. In a computer-simulation study, 3D coordinates of line-sources were defined in a reference room coordinate frame, such as the LINAC coordinate frame. Corresponding 2D line-source projections were generated by computer simulation that included SPECT blurring and noise effects. The Radon transform was utilized to detect angles (α) and offsets (ρ) of the line-source projections. Alignment parameters were then estimated by a nonlinear least squares method, based on the α and ρ values and the alignment model. Alignment performance was evaluated as a function of number of line sources, Radon transform accuracy, finite line-source width, intrinsic camera resolution, Poisson noise, and acquisition geometry. Experimental evaluations were performed using a physical line-source phantom and a pinhole-collimated gamma camera attached to a robot. RESULTS In computer-simulation studies, when there was no error in determining angles (α) and offsets (ρ) of the measured projections, six alignment parameters (three translational and three rotational) were estimated perfectly using three line sources. When angles (α) and offsets (ρ) were provided by the Radon transform, estimation accuracy was reduced. The estimation error was associated with rounding errors of Radon transform, finite line-source width, Poisson noise, number of line sources, intrinsic camera resolution, and detector acquisition geometry. Statistically, the estimation accuracy was significantly improved by using four line sources rather than three and by thinner line-source projections (obtained by better intrinsic detector resolution). With five line sources, median errors were 0.2 mm for the detector translations, 0.7 mm for the detector radius of rotation, and less than 0.5° for detector rotation, tilt, and twist. In experimental evaluations, average errors relative to a different, independent registration technique were about 1.8 mm for detector translations, 1.1 mm for the detector radius of rotation (ROR), 0.5° and 0.4° for detector rotation and tilt, respectively, and 1.2° for detector twist. CONCLUSIONS Alignment parameters can be estimated using one pinhole projection of line sources. Alignment errors are largely associated with limited accuracy of the Radon transform in determining angles (α) and offsets (ρ) of the line-source projections. This alignment method may be important for multipinhole SPECT, where relative pinhole alignment may vary during rotation. For pinhole and multipinhole SPECT imaging on-board radiation therapy machines, the method could provide alignment of SPECT coordinates with those of CBCT and the LINAC.