用于放射治疗中多能量应用的原型平板探测器的蒙特卡罗模型。

Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts, USA.

Varian Imaging Laboratory, Baden-Dattwil, Switzerland.

Med Phys. 2023 Oct;50(10):5944-5955. doi: 10.1002/mp.16689. Epub 2023 Sep 4.

BACKGROUND

The incorporation of multi-energy capabilities into radiotherapy flat-panel detectors offers advantages including enhanced soft tissue visualization by reduction of signal from overlapping anatomy such as bone in 2D image projections; creation of virtual monoenergetic images for 3D contrast enhancement, metal artefact reduction and direct acquisition of relative electron density. A novel dual-layer on-board imager offering dual energy processing capabilities is being designed. As opposed to other dual-energy implementation techniques which require separate acquisition with two different x-ray spectra, the dual-layer detector design enables simultaneous acquisition of high and low energy images with a single exposure. A computational framework is required to optimize the design parameters and evaluate detector performance for specific clinical applications.

PURPOSE

In this study, we report on the development of a Monte Carlo (MC) model of the imager including model validation.

METHODS

The stack-up of the dual-layer imager (DLI) was implemented in GEANT4 Application for Tomographic Emission (GATE). The DLI model has an active area of 43×43 cm , with top and bottom Cesium Iodide (CsI) scintillators of 600 and 800 μm thickness, respectively. Measurement of spatial resolution and imaging of dedicated multi-material dual-energy (DE) phantoms were used to validate the model. The modulation transfer function (MTF) of the detector was calculated for a 120 kVp x-ray spectrum using a 0.5 mm thick tantalum edge rotated by 2.5 . For imaging validation, the DE phantom was imaged using a 140 kVp x-ray spectrum. For both validation simulations, corresponding measurements were done using an initial prototype of the imager. Agreement between simulations and measurement was assessed using normalized root mean square error (NRMSE) and 1D profile difference for the MTF and phantom images respectively. Further comparison between measurement and simulation was made using virtual monoenergetic images (VMIs) generated from basis material images derived using precomputed look-up tables.

RESULTS

The MTF of the bottom layer of the dual-layer model shows values decreasing more quickly with spatial frequency, compared to the top layer, due to the thicker bottom scintillator thickness and scatter from the top layer. A comparison with measurement shows NRMSE of 0.013 and 0.015 as well as identical MTF of 0.8 mm and 1.0 mm for the top and bottom layer respectively. For the DE imaging of the DE-phantom, although a maximum deviation of 3.3% is observed for the 10 mm aluminum and Teflon inserts at the top layer, the agreement for all other inserts is less than 2.2% of the measured value at both layers. Material decomposition of simulated scatter-free DE images gives an average accuracy in PMMA and aluminum composition of 4.9% and 10.3% for 11-30 mm PMMA and 1-10 mm aluminum objects respectively. A comparison of decomposed values using scatter containing measured and simulated DE images shows good agreement within statistical uncertainty.

CONCLUSION

Validation using both MTF and phantom imaging shows good agreement between simulation and measurements. With the present configuration of the digital prototype, the model can generate material decomposed images and virtual monoenergetic images.

背景

将多能量能力融入放射治疗平板探测器具有诸多优势，例如通过减少重叠解剖结构（如骨骼）的信号来增强软组织可视化；为 3D 对比度增强、减少金属伪影和直接获取相对电子密度创建虚拟单能量图像。正在设计一种具有双能量处理能力的新型双层板载成像仪。与需要使用两种不同 X 射线光谱进行单独采集的其他双能量实现技术不同，双层探测器设计允许使用单次曝光同时采集高低能图像。需要一个计算框架来优化设计参数，并评估特定临床应用的探测器性能。

目的

本研究报告了一种用于模拟的蒙特卡罗（MC）模型的开发，包括模型验证。

方法

采用 GEANT4 应用于发射层析成像（GATE）实现了双层成像仪（DLI）的堆叠。DLI 模型的有效面积为 43×43 厘米，顶部和底部碘化铯（CsI）闪烁体的厚度分别为 600 和 800 微米。使用专用多材料双能（DE）体模的空间分辨率测量和成像来验证模型。使用 0.5 毫米厚的钽边缘以 2.5 的角度旋转，计算了探测器的调制传递函数（MTF），该钽边缘用于 120 kVp X 射线光谱。为了成像验证，使用 140 kVp X 射线光谱对 DE 体模进行成像。对于这两个验证模拟，均使用成像仪的初始原型进行了相应的测量。使用归一化均方根误差（NRMSE）和 1D 轮廓差异分别对 MTF 和体模图像的测量和模拟之间的一致性进行了评估。进一步使用从预计算查找表得出的基础材料图像生成的虚拟单能量图像（VMIs）进行了测量和模拟之间的比较。

结果

由于底部闪烁体厚度较大和顶部散射，双层模型的底部层的 MTF 显示出随空间频率更快下降的趋势，与顶部层相比。与测量结果的比较表明，顶部和底部层的 MTF 分别为 0.8 毫米和 1.0 毫米，NRMSE 为 0.013 和 0.015。对于 DE 体模的 DE 成像，尽管在顶层观察到最大 3.3%的 10 毫米铝和特氟龙插入物的偏差，但在顶层和底层，所有其他插入物的一致性都小于测量值的 2.2%。对无散射 DE 图像的模拟材料分解给出了 PMMA 和铝组成的平均精度，对于 11-30 毫米 PMMA 和 1-10 毫米铝物体，分别为 4.9%和 10.3%。使用包含散射的测量和模拟 DE 图像的分解值进行比较表明，在统计不确定性内具有良好的一致性。