Department of Radiation Oncology, Dana-Farber/Brigham and Women's Cancer Center and Harvard Medical School, Boston, MA, 02115, USA.
Varian Medical Systems Ginzton Technology Center, Palo Alto, CA, 94304-1030, USA.
Med Phys. 2017 Aug;44(8):4213-4222. doi: 10.1002/mp.12382. Epub 2017 Jun 28.
A novel Megavoltage (MV) multilayer imager (MLI) design featuring higher detective quantum efficiency and lower noise than current conventional MV imagers in clinical use has been recently reported. Optimization of the MLI design for multiple applications including tumor tracking, MV-CBCT and portal dosimetry requires a computational model that will provide insight into the physics processes that affect the overall and individual components' performance. The purpose of the current work was to develop and validate a comprehensive computational model that can be used for MLI optimization.
The MLI model was built using the Geant4 Application for Tomographic Emission (GATE) application. The model includes x-ray and charged-particle interactions as well as the optical transfer within the phosphor. A first prototype MLI device featuring a stack of four detection layers was used for model validation. Each layer of the prototype contains a copper buildup plate, a phosphor screen and photodiode array. The model was validated against measured data of Modulation Transfer Function (MTF), Noise-Power Spectrum (NPS), and Detective Quantum Efficiency (DQE). MTF was computed using a slanted slit with 2.3 angle and 0.1 mm width. NPS was obtained using the autocorrelation function technique. DQE was calculated from MTF and NPS data. The comparison metrics between simulated and measured data were the Pearson's correlation coefficient (r) and the normalized root-mean-square error (NRMSE).
Good agreement between measured and simulated MTF and NPS values was observed. Pearson's correlation coefficient for the combined signal from all layers of the MLI was equal to 0.9991 for MTF and 0.9992 for NPS; NRMSE was 0.0121 for MTF and 0.0194 for NPS. Similarly, the DQE correlation coefficient for the combined signal was 0.9888 and the NRMSE was 0.0686.
A comprehensive model of the novel MLI design was developed using the GATE toolkit and validated against measured MTF, NPS, and DQE data acquired with a prototype device featuring four layers. This model will be used for further optimization of the imager components and configuration for clinical radiotherapy applications.
最近报道了一种新型兆伏（MV）多层成像仪（MLI）设计，与目前临床使用的传统 MV 成像仪相比，具有更高的探测量子效率和更低的噪声。为了优化 MLI 设计，使其适用于肿瘤跟踪、MV-CBCT 和门户剂量测定等多种应用，需要一个能够深入了解影响整体和各个组件性能的物理过程的计算模型。本研究的目的是开发和验证一个可用于 MLI 优化的综合计算模型。
使用 Geant4 应用于断层发射（GATE）应用程序构建 MLI 模型。该模型包括 X 射线和带电粒子相互作用以及荧光体内的光传输。使用具有四层检测层的第一代原型 MLI 设备进行模型验证。原型的每一层都包含一个铜堆积板、一个荧光屏和一个光电二极管阵列。使用具有 2.3 度和 0.1 毫米宽度的倾斜狭缝计算调制传递函数（MTF），使用自相关函数技术获得噪声功率谱（NPS），并从 MTF 和 NPS 数据计算探测量子效率（DQE）。将模拟和测量数据之间的比较指标定义为皮尔逊相关系数（r）和归一化均方根误差（NRMSE）。
观察到测量和模拟的 MTF 和 NPS 值之间具有良好的一致性。MLI 所有层的组合信号的 Pearson 相关系数对于 MTF 等于 0.9991，对于 NPS 等于 0.9992；MTF 的 NRMSE 为 0.0121，NPS 的 NRMSE 为 0.0194。同样，组合信号的 DQE 相关系数为 0.9888，NRMSE 为 0.0686。
使用 GATE 工具包开发了一种新型 MLI 设计的综合模型，并使用具有四层的原型设备获取的测量 MTF、NPS 和 DQE 数据进行了验证。该模型将用于进一步优化成像仪组件和配置，以满足临床放射治疗应用的需求。
