WE-C-217BCD-11：基于厚像素化闪烁晶体的兆伏级电子射野影像装置的耦合辐射与光学Geant4模拟

Constantin D, Sun M, Abel E, Star-Lack J, Fahrig R

Stanford University, Stanford, CA.

Varian Medical Systems, Palo Alto, CA.

Med Phys. 2012 Jun;39(6Part27):3951-3952. doi: 10.1118/1.4736127.

PURPOSE

One way to greatly reduce the incidence of metal artifacts produced in kilovoltage (kV) CT images is by using megavoltage (MV) photons that penetrate high-Z objects, thus providing a measurable signal. For do se-efficient imaging, a high detective quantum efficiency (DQE) MV detector is desired. This study validates the coupled radiation and optical Geant4 simulation results against experimental data from various prototype pixelated scintillator MV detectors and determines the essential optical parameters which control the detector performance.

METHODS

Experimental data obtained with a 6MV radiation source from 8 different detectors was considered. The detectors used CsI, CdW and BGO as scintillating crystals and polystyrene septal wall material. Accurate Geant4 models of the detectors were implemented and coupled radiation and optical simulations were performed. The unknown optical properties of the models were determined by minimizing the difference between the modulation transfer functions (MTF) of the simulated data obtained with the slanted slit technique and the experimental MTFs. With the set of optical properties fixed, further simulation validation was performed against the experimental normalized noise power spectrum (NNPS(f)) and the experimental DQE(f) curves for each detector. All the simulations were performed on a computer cluster deployed on the Amazon EC2 platform.

RESULTS

The optimal values for the free optical parameters are 10%, 95% and 90% for the top surface reflectivity, the crystal-sept a surface reflectivity, and the Lambertian component contribution to the reflected beam from the crystal-septa interface respectively. The absolute difference between experimental and simulated data was below 10% for all the data sets.

CONCLUSIONS

To our knowledge this study is the first to present a full optical and radiative DQE(f) model using Geant4 that shows an excellent match with experimental data. The model indicates that improved performance can be obtained using more specular septa which are optically opaque. Support: NIH-T32-CA09695, NIH-1R01CA138426 NIH T32-CA09695, NIH R01- CA138426, Several authors work for Varian Medical Systems.

目的

大幅降低千伏（kV）CT图像中产生的金属伪影发生率的一种方法是使用能穿透高原子序数（Z）物体的兆伏（MV）光子，从而提供可测量的信号。对于高效成像而言，需要高探测量子效率（DQE）的MV探测器。本研究针对来自各种原型像素化闪烁体MV探测器的实验数据验证了耦合辐射与光学的Geant4模拟结果，并确定了控制探测器性能的关键光学参数。

方法

考虑了使用6MV辐射源从8种不同探测器获得的实验数据。这些探测器使用碘化铯（CsI）、钨酸镉（CdW）和锗酸铋（BGO）作为闪烁晶体，并使用聚苯乙烯作为间隔壁材料。实现了探测器的精确Geant4模型，并进行了耦合辐射与光学模拟。通过最小化使用倾斜狭缝技术获得的模拟数据的调制传递函数（MTF）与实验MTF之间的差异，确定模型的未知光学特性。在固定光学特性集的情况下，针对每个探测器的实验归一化噪声功率谱（NNPS(f)）和实验DQE(f)曲线进行了进一步的模拟验证。所有模拟均在部署于亚马逊弹性计算云（EC2）平台的计算机集群上进行。

结果

对于自由光学参数，顶面反射率、晶体 - 间隔壁表面反射率以及朗伯分量对来自晶体 - 间隔壁界面反射光束的贡献的最佳值分别为10%、95%和90%。对于所有数据集，实验数据与模拟数据之间的绝对差异低于10%。

结论

据我们所知，本研究首次提出了使用Geant4的完整光学和辐射DQE(f)模型，该模型与实验数据显示出极佳的匹配度。该模型表明，使用光学不透明的更镜面的间隔壁可获得性能提升。资助：国立卫生研究院 - T32 - CA09695、国立卫生研究院 - 1R01CA138426 国立卫生研究院 - T32 - CA09695、国立卫生研究院 - R01 - CA138426，多位作者就职于瓦里安医疗系统公司。