基于厚分段 BGO 和 CsI:Tl 闪烁体的高 DQE EPID：极低剂量下的性能评估。
High-DQE EPIDs based on thick, segmented BGO and CsI:Tl scintillators: performance evaluation at extremely low dose.
机构信息
Department of Radiation Oncology, University of Michigan, Ann Arbor Michigan 48109, USA.
出版信息
Med Phys. 2009 Dec;36(12):5707-18. doi: 10.1118/1.3259721.
PURPOSE
Electronic portal imaging devices (EPIDs) based on active matrix, flat-panel imagers (AMFPIs) have become the gold standard for portal imaging and are currently being investigated for megavoltage cone-beam computed tomography (CBCT) and cone-beam digital tomosynthesis (CBDT). However, the practical realization of such volumetric imaging techniques is constrained by the relatively low detective quantum efficiency (DQE) of AMFPI-based EPIDs at radiotherapy energies, approximately 1% at 6 MV. In order to significantly improve DQE, the authors are investigating thick, segmented scintillators, consisting of 2D matrices of scintillating crystals separated by septal walls.
METHODS
A newly constructed segmented BGO scintillator (11.3 mm thick) and three segmented CsI:Tl scintillators (11.4, 25.6, and 40.0 mm thick) were evaluated using a 6 MV photon beam. X-ray sensitivity, modulation transfer function, noise power spectrum, DQE, and phantom images were obtained using prototype EPIDs based on the four scintillators.
RESULTS
The BGO and CsI:Tl prototypes were found to exhibit improvement in DQE ranging from approximately 12 to 25 times that of a conventional AMFPI-based EPID at zero spatial frequency. All four prototype EPIDs provide significantly improved contrast resolution at extremely low doses, extending down to a single beam pulse. In particular, the BGO prototype provides contrast resolution comparable to that of the conventional EPID, but at 20 times less dose, with spatial resolution sufficient for identifying the boundaries of low-contrast objects. For this prototype, however, the BGO scintillator exhibited an undesirable radiation-induced variation in x-ray sensitivity.
CONCLUSIONS
Prototype EPIDs based on thick, segmented BGO and CsI:T1 scintillators provide significantly improved portal imaging performance at extremely low dose (i.e., down to 1 beam pulse corresponding to approximately 0.022 cGy), creating the possibility of soft-tissue visualization using MV CBCT and CBDT at clinically practical dose.
目的
基于有源矩阵、平板成像器（AMFPIs）的电子门户成像设备（EPIDs）已成为门户成像的金标准，目前正在研究用于兆伏锥形束计算机断层扫描（CBCT）和锥形束数字断层合成（CBDT）的方法。然而，基于 AMFPI 的 EPIDs 在放射治疗能量下相对较低的探测量子效率（DQE）限制了这种容积成像技术的实际实现，在 6 MV 时约为 1%。为了显著提高 DQE，作者正在研究厚的、分段的闪烁体，这些闪烁体由通过隔板分开的二维闪烁晶体矩阵组成。
方法
使用 6 MV 光子束评估了新构造的分段 BGO 闪烁体（11.3 毫米厚）和三个分段 CsI:Tl 闪烁体（11.4、25.6 和 40.0 毫米厚）。使用基于这四种闪烁体的原型 EPIDs 获得了 X 射线灵敏度、调制传递函数、噪声功率谱、DQE 和体模图像。
结果
发现 BGO 和 CsI:Tl 原型在零空间频率下的 DQE 提高了约 12 到 25 倍，与传统的基于 AMFPI 的 EPID 相比。所有四个原型 EPID 都在极低剂量下显著提高了对比度分辨率，低至单个射束脉冲。特别是，BGO 原型提供了与传统 EPID 相当的对比度分辨率，但剂量低 20 倍，空间分辨率足以识别低对比度物体的边界。然而，对于该原型，BGO 闪烁体表现出不期望的 X 射线灵敏度的辐射诱导变化。
结论
基于厚的、分段的 BGO 和 CsI:T1 闪烁体的原型 EPID 在极低剂量下（即低至 1 个射束脉冲，对应于约 0.022 cGy）提供了显著改善的门户成像性能，为使用 MV CBCT 和 CBDT 在临床实用剂量下进行软组织可视化创造了可能性。
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