MLC quality assurance using EPID: A fitting technique with subpixel precision
MLC quality assurance using EPID: A fitting technique with subpixel precision
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DOI：
10.1118/1.2919560
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发表时间：
2008-06-01
期刊：
影响因子：
3.8
通讯作者：
Low, Daniel A.
中科院分区：
文献类型：
--
作者：
Mamalui-Hunter, Maria;Li, Harold;Low, Daniel A.
基于非晶硅的电子射野成像设备（EPID）已被证明是射线照相胶片的良好替代品，可用于多叶准直器（MLC）定位精度的常规质量保证（QA）。在这项工作中，我们提出了一种方法，获取EPID图像的传统条测试图像使用分析适合的interleaf和leaf abutment图像签名。在曝光之后，EPID图像像素值除以开放场图像以去除EPID响应和辐射场变化。在与叶片运动正交的方向上获得的轮廓表现出由叶间泄漏引起的小峰。高斯分布拟合到叶间泄漏峰值，其结果是，使用多目标优化，用于计算相对于准直器旋转轴的图像旋转角。相对角度用于旋转图像，以将MLC叶片行程与图像像素轴对齐。叶基台也呈现出由启发式函数拟合的峰值，在这种情况下是修改的洛伦兹函数。洛伦兹函数的参数用于参数化叶隙宽度和位置。通过对一组具有不同间隙的MLC场进行成像，形成对称和非对称基台，获得了关于相对峰高（RPH）与标称间隙宽度的校准曲线。基于该校准数据，计算各个叶片位置以与标称编程位置进行比较。结果表明，准直器旋转角度可以被确定为精确到0.01度。MLC间隙宽度变化0.2 mm导致RPH变化约10%。对于不对称产生的间隙，0.2 mm MLC叶间隙宽度变化导致0.2像素峰值位置变化。通过使用相对较大的邻接峰的参数化拟合来获得亚像素分辨率。相比之下，对于对称的间隙变化，峰值位置保持不变，标准差为0.05像素，或0.026 mm。36个测试图像的试运行，每个间隙宽度从0.4到1.4 mm不等，用于分析8640个基台。叶位置的变化检测精度为0.1 mm，在95%的置信水平，平均值为0.04 mm，标准差为0.03 mm。所提出的方法是强大的，并尽量减少图像噪声和像素大小的影响，并可能帮助物理学家建立可靠和合理的行动水平，在常规MLC QA。(C)2008年美国医学物理学家协会。
Amorphous silicon based electronic portal imaging devices (EPIDs) have been shown to be a good alternative to radiographic film for routine quality assurance (QA) of multileaf collimator (MLC) positioning accuracy. In this work, we present a method of acquiring an EPID image of a traditional strip-test image using analytical fits of the interleaf and leaf abutment image signatures. After exposure, the EPID image pixel values are divided by an open field image to remove EPID response and radiation field variations. Profiles acquired in the direction orthogonal to the leaf motion exhibit small peaks caused by interleaf leakage. Gaussian profiles are fitted to the interleaf leakage peaks, the results of which are, using multiobjective optimization, used to calculate the image rotational angle with respect to the collimator axis of rotation. The relative angle is used to rotate the image to align the MLC leaf travel to the image pixel axes. The leaf abutments also present peaks that are fitted by heuristic functions, in this case modified Lorentzian functions. The parameters of the Lorentzian functions are used to parameterize the leaf gap width and positions. By imaging a set of MLC fields with varying gaps forming symmetric and asymmetric abutments, calibration curves with regard to relative peak height (RPH) ver sus nominal gap width are obtained. Based on this calibration data, the individual leaf positions are calculated to compare with the nominal programmed positions. The results demonstrate that the collimator rotation angle can be determined as accurate as 0.01 degrees. A change in MLC gap width of 0.2 mm leads to a change in RPH of about 10%. For asymmetrically produced gaps, a 0.2 mm MLC leaf gap width change causes 0.2 pixel peak position change. Subpixel resolution is obtained by using a parameterized fit of the relatively large abutment peaks. By contrast, for symmetrical gap changes, the peak position remains unchanged with a standard deviation of 0.05 pixels, or 0.026 mm. A trial run of 36 test images, each with gap widths varying from 0.4 to 1.4 mm, were used to analyze 8640 abutments. The leaf position variations were detected with a precision of 0.1 mm at a 95% confidence level, with a mean of 0.04 mm and a standard deviation of 0.03 mm. The proposed method is robust and minimizes the effect of image noise and pixel size and may help physicists to establish reliable and reasonable action levels in routine MLC QA. (C) 2008 American Association of Physicists in Medicine.