Estimation of CT cone-beam geometry using a novel method insensitive to phantom fabrication inaccuracy: Implications for isocenter localization accuracy
Estimation of CT cone-beam geometry using a novel method insensitive to phantom fabrication inaccuracy: Implications for isocenter localization accuracy
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DOI：
10.1118/1.3589130
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发表时间：
2011-06-01
期刊：
影响因子：
3.8
通讯作者：
Williamson, Jeffrey F.
中科院分区：
文献类型：
--
作者：
Ford, J. Chetley;Zheng, Dandan;Williamson, Jeffrey F.
目的：机载锥形束计算机断层扫描（CBCT）成像系统在机架旋转期间发生的机械不稳定性限制了图像引导放射治疗的功效。已经提出了用于校准CBCT几何形状和校正误差的各种方法，包括利用专用基准体模的一些方法。这项工作的目的是调查一个特定的CT锥束几何形状估计的准确性，并测试一种新的方法，以减轻错误的光束几何形状所产生的不完美的制造phantomies.Methods：作者实施了一个基准的幻影为基础的光束几何形状估计Cho [Med Phys 32（4），968-983（2005）]所描述的一个。该算法利用不同机架角度的体模投影图像作为输入，并提供源和探测器位置以及探测器方向与机架角度的完整九个参数射束几何特征。开发了一种方法，用于在原点位于源轨迹中心并与机架旋转轴对齐的坐标系中重新计算射束几何形状，从而使射束几何形状估计独立于体模的放置。将体模绕其长轴旋转180度的第二次CBCT扫描与第一次扫描取平均值，以减轻体模不精确的误差。进行计算机模拟，以评估由于图像离散化导致的2D基准标记位置误差对投影的影响，以及由于体模制造不精确导致的3D基准标记位置误差。获得了基准体模的实验CBCT图像，并使用该算法测量了带有机载CBCT的Varian Trilogy的射束几何形状。模拟和实验结果均显示，由于体模从CBCT等中心的位移以及与机架轴的未对准，计算的射束几何参数随机架角度发生大的正弦振荡，通过在源坐标系中重新计算射束几何形状来消除这些误差。模拟和实验还揭示了由于体模制造不精确而引起的基准标记位置误差引起的振荡的附加源，通过将结果与体模旋转的第二次CBCT扫描的结果进行平均来减轻。对于0.020 mm（0.001 in.）的典型基准标记位置误差，在模拟中发现源和检测器位置在真实值的250 μ m内，并且检测器和台架角度小于0.2度。探测器偏移量在已知值的100 μ m范围内。实验结果验证了第二次扫描在减轻射束几何误差以及由体模制造不精确引起的大的表观源/探测器等中心偏移方面的功效。作者开发并验证了一种新型基准体模-基于CBCT射束几何形状估计算法，不需要在机器等中心精确定位体模，并且对由于制造误差而导致的体模。该方法可以准确地定位源和探测器等中心，即使在使用不精确的体模，这是非常重要的治疗和车载成像系统的等中心重合的测量。(C)2011年美国医学物理学家协会。[DOI 10.1118/1.3589130]
Purpose: Mechanical instabilities that occur during gantry rotation of on-board cone-beam computed tomography (CBCT) imaging systems limit the efficacy of image-guided radiotherapy. Various methods for calibrating the CBCT geometry and correcting errors have been proposed, including some that utilize dedicated fiducial phantoms. The purpose of this work was to investigate the role of phantom fabrication imprecision on the accuracy of a particular CT cone-beam geometry estimate and to test a new method to mitigate errors in beam geometry arising from imperfectly fabricated phantoms.Methods: The authors implemented a fiducial phantom-based beam geometry estimation following the one described by Cho [Med Phys 32(4), 968-983 (2005)]. The algorithm utilizes as input projection images of the phantom at various gantry angles and provides a full nine parameter beam geometry characterization of the source and detector position and detector orientation versus gantry angle. A method was developed for recalculating the beam geometry in a coordinate system with origin at the source trajectory center and aligned with the axis of gantry rotation, thus making the beam geometry estimation independent of the placement of the phantom. A second CBCT scan with the phantom rotated 180 degrees about its long axis was averaged with the first scan to mitigate errors from phantom imprecision. Computer simulations were performed to assess the effect of 2D fiducial marker positional error on the projections due to image discretization, as well as 3D fiducial marker position error due to phantom fabrication imprecision. Experimental CBCT images of a fiducial phantom were obtained and the algorithm used to measure beam geometry for a Varian Trilogy with an on-board CBCT.Results: Both simulations and experimental results reveal large sinusoidal oscillations in the calculated beam geometry parameters with gantry angle due to displacement of the phantom from CBCT isocenter and misalignment with the gantry axis, which are eliminated by recalculating the beam geometry in the source coordinate system. Simulations and experiments also reveal an additional source of oscillations arising from fiducial marker position error due to phantom fabrication imprecision that are mitigated by averaging the results with those of a second CBCT scan with phantom rotated. With a typical fiducial marker position error of 0.020 mm (0.001 in.), source and detector position are found in simulations to be within 250 mu m of the true values, and detector and gantry angles less than 0.2 degrees. Detector offsets are within 100 mu m of the known value. Experimental results verify the efficacy of the second scan in mitigating beam geometry errors, as well as large apparent source/detector isocenter offsets arising from phantom fabrication imprecision.Conclusions: The authors have developed and validated a novel fiducial phantom-based CBCT beam geometry estimation algorithm that does not require precise positioning of the phantom at machine isocenter and is insensitive to positional imprecision of fiducial markers within the phantom due to fabrication errors. The method can accurately locate source and detector isocenters even when using an imprecise phantom, which is very important for measurement of isocenter coincidence of the therapy and on-board imaging systems. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3589130]