Star-Lack Josh, Shedlock Daniel, Swahn Dennis, Humber Dave, Wang Adam, Hirsh Hayley, Zentai George, Sawkey Daren, Kruger Isaac, Sun Mingshan, Abel Eric, Virshup Gary, Shin Mihye, Fahrig Rebecca
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Agile Technologies, Inc., 10337 Yellow Pine Lane, Knoxville, Tennessee 37932.
Med Phys. 2015 Sep;42(9):5084-99. doi: 10.1118/1.4927786.
Electronic portal imagers (EPIDs) with high detective quantum efficiencies (DQEs) are sought to facilitate the use of the megavoltage (MV) radiotherapy treatment beam for image guidance. Potential advantages include high quality (treatment) beam's eye view imaging, and improved cone-beam computed tomography (CBCT) generating images with more accurate electron density maps with immunity to metal artifacts. One approach to increasing detector sensitivity is to couple a thick pixelated scintillator array to an active matrix flat panel imager (AMFPI) incorporating amorphous silicon thin film electronics. Cadmium tungstate (CWO) has many desirable scintillation properties including good light output, a high index of refraction, high optical transparency, and reasonable cost. However, due to the 0 1 0 cleave plane inherent in its crystalline structure, the difficulty of cutting and polishing CWO has, in part, limited its study relative to other scintillators such as cesium iodide and bismuth germanate (BGO). The goal of this work was to build and test a focused large-area pixelated "strip" CWO detector.
A 361 × 52 mm scintillator assembly that contained a total of 28 072 pixels was constructed. The assembly comprised seven subarrays, each 15 mm thick. Six of the subarrays were fabricated from CWO with a pixel pitch of 0.784 mm, while one array was constructed from BGO for comparison. Focusing was achieved by coupling the arrays to the Varian AS1000 AMFPI through a piecewise linear arc-shaped fiber optic plate. Simulation and experimental studies of modulation transfer function (MTF) and DQE were undertaken using a 6 MV beam, and comparisons were made between the performance of the pixelated strip assembly and the most common EPID configuration comprising a 1 mm-thick copper build-up plate attached to a 133 mg/cm(2) gadolinium oxysulfide scintillator screen (Cu-GOS). Projection radiographs and CBCT images of phantoms were acquired. The work also introduces the use of a lightweight edge phantom to generate MTF measurements at MV energies and shows its functional equivalence to the more cumbersome slit-based method.
Measured and simulated DQE(0)'s of the pixelated CWO detector were 22% and 26%, respectively. The average measured and simulated ratios of CWO DQE(f) to Cu-GOS DQE(f) across the frequency range of 0.0-0.62 mm(-1) were 23 and 29, respectively. 2D and 3D imaging studies confirmed the large dose efficiency improvement and that focus was maintained across the field of view. In the CWO CBCT images, the measured spatial resolution was 7 lp/cm. The contrast-to-noise ratio was dramatically improved reflecting a 22 × sensitivity increase relative to Cu-GOS. The CWO scintillator material showed significantly higher stability and light yield than the BGO material.
An efficient piecewise-focused pixelated strip scintillator for MV imaging is described that offers more than a 20-fold dose efficiency improvement over Cu-GOS.
寻求具有高探测量子效率（DQE）的电子射野影像装置（EPID），以促进兆伏（MV）放射治疗束用于图像引导。潜在优势包括高质量的（治疗）射束视野成像，以及改进的锥束计算机断层扫描（CBCT），可生成具有更精确电子密度图且不受金属伪影影响的图像。提高探测器灵敏度的一种方法是将厚像素化闪烁体阵列与包含非晶硅薄膜电子器件的有源矩阵平板成像器（AMFPI）耦合。钨酸镉（CWO）具有许多理想的闪烁特性，包括良好的光输出、高折射率、高光学透明度和合理的成本。然而，由于其晶体结构中固有的0 1 0解理面，CWO的切割和抛光困难在一定程度上限制了其相对于其他闪烁体（如碘化铯和锗酸铋（BGO））的研究。这项工作的目标是构建并测试一个聚焦的大面积像素化“条形”CWO探测器。
构建了一个包含总共28072个像素的361×52mm闪烁体组件。该组件由七个子阵列组成，每个子阵列厚15mm。其中六个子阵列由像素间距为0.784mm的CWO制成，而一个阵列由BGO制成用于比较。通过将阵列通过分段线性弧形光纤板耦合到瓦里安AS1000 AMFPI来实现聚焦。使用6MV射束进行调制传递函数（MTF）和DQE的模拟和实验研究，并对像素化条形组件与最常见的EPID配置（包括附着在133mg/cm²硫氧化钆闪烁体屏（Cu - GOS）上的1mm厚铜增感板）的性能进行比较性研究。采集了体模的投影射线照片和CBCT图像。这项工作还介绍了使用轻质边缘体模在MV能量下进行MTF测量，并表明其与更繁琐的基于狭缝的方法功能等效。
像素化CWO探测器的测量和模拟DQE(0)分别为22%和26%。在0.0 - 0.62mm⁻¹频率范围内，CWO DQE(f)与Cu - GOS DQE(f)的平均测量和模拟比值分别为23和29。二维和三维成像研究证实了剂量效率有大幅提高，并且在整个视野范围内保持了聚焦。在CWO CBCT图像中，测量的空间分辨率为7lp/cm。对比噪声比显著提高，表明相对于Cu - GOS灵敏度提高了22倍。CWO闪烁体材料显示出比BGO材料更高的稳定性和光产额。
描述了一种用于MV成像的高效分段聚焦像素化条形闪烁体，其剂量效率比Cu - GOS提高了20倍以上。
