Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW, 2006, Australia.
Ingham Institute for Applied Medical Research, Sydney, NSW, 2170, Australia.
Med Phys. 2018 May;45(5):2154-2168. doi: 10.1002/mp.12882. Epub 2018 Apr 15.
First measurements of the imaging performance of a novel prototype water-equivalent electronic portal imaging device (EPID) designed for simultaneous imaging and dose verification in radiotherapy and previously characterized by our group for dosimetry are reported. Experiments were conducted to characterize the prototype's imaging performance relative to a standard commercial EPID and Monte Carlo (MC) simulations were performed to quantify the impact of several detector parameters on image quality and to inform the design of a proposed next-generation prototype.
The prototype EPID utilizes an array of 3 cm long plastic-scintillating fibers in place of the metal plate/phosphor screen in standard EPIDs. Using a clinical 6 MV photon beam, the prototype's modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were measured and compared to measurements taken using a standard commercial EPID. A sensitivity analysis was then performed using the MC model by quantifying these metrics while varying the values of several geometrical and optical transport parameters that were unspecified by the prototype manufacturer. Finally, the MC model was used to quantify the imaging performance of a proposed next-generation prototype incorporating 1.5 cm long fibers that is better suited for integration with clinical portal imaging and dosimetry systems.
The prototype EPID's zero spatial frequency DQE exceeded 3%, more than doubling that measured with the standard EPID (1.25%). This increased DQE was a consequence of using a prototype array detector with a greater equivalent thickness than the combined copper plate and phosphor screen in a standard EPID. The increased thickness of our prototype decreased spatial resolution relative to the standard EPID; however, the prototype EPID NPS was also lower than that measured with the standard EPID across all spatial frequencies. The sensitivity analysis demonstrated that the NPS was strongly affected by the roughness of the boundaries between fiber core and cladding regions. By comparison, the MTF was most sensitive to beam divergence and the presence of air between the fiber array and underlying photodiode panel. Simulations demonstrated that by optimizing these parameters, DQE(0) >4% may be achievable with the proposed next-generation prototype design.
The first measurements characterizing the imaging performance of a novel water-equivalent EPID for imaging and dosimetry in radiotherapy demonstrated a DQE(0) more than double that of a standard EPID. MC simulations further demonstrated the potential for developing a next-generation prototype better suited for clinical translation with even higher DQE.
报告一种新型水等效电子射野影像装置（EPID）原型的成像性能的首次测量结果，该原型专为放射治疗中的成像和剂量验证而设计，并且我们小组之前已经对其进行了剂量学特性描述。进行了实验以相对标准商业 EPID 来对原型的成像性能进行特性描述，并且进行了蒙特卡罗（MC）模拟以量化几个探测器参数对图像质量的影响，并为提出的下一代原型的设计提供信息。
原型 EPID 利用在标准 EPID 中的金属板/磷光屏中用 3 厘米长的塑料闪烁光纤阵列取代。使用临床 6MV 光子束，对原型的调制传递函数（MTF）、噪声功率谱（NPS）和探测量子效率（DQE）进行了测量，并与使用标准商业 EPID 进行的测量进行了比较。然后通过量化这些指标来进行 MC 模型的灵敏度分析，同时改变原型制造商未指定的几个几何和光学传输参数的值。最后，使用 MC 模型量化了一种改进的下一代原型的成像性能，该原型使用 1.5 厘米长的光纤，更适合与临床射野影像和剂量学系统集成。
原型 EPID 的零空间频率 DQE 超过 3%，比标准 EPID（1.25%）的测量值高出一倍多。这种增加的 DQE 是由于使用了原型阵列探测器，其等效厚度大于标准 EPID 中的铜片和磷光屏的总和。原型的厚度增加会使空间分辨率相对标准 EPID 降低；但是，原型 EPID 的 NPS 在所有空间频率上也低于标准 EPID 的测量值。灵敏度分析表明，NPS 强烈受到光纤芯和包层区域之间边界粗糙度的影响。相比之下，MTF 对光束发散度和光纤阵列与底层光电二极管面板之间的空气存在最为敏感。模拟表明，通过优化这些参数，下一代原型的设计可能实现 DQE(0)>4%。
首次对一种新型水等效 EPID 的成像性能进行了测量，用于放射治疗中的成像和剂量验证，其 DQE(0)比标准 EPID 高出一倍以上。MC 模拟进一步证明了开发更适合临床转化的下一代原型的潜力，其 DQE 更高。
