小动物正电子断层成像仪性能测试

设计了一系列实验对Concorde MicroPET Rodent R4小动物正电子断层成像系统(该系统是Concorde Microsystems公司开发的小动物专用PET扫描仪，该扫描仪具有32层探测器环，动物入口孔径为120mm，横向视野为100mm，轴向深度为78mm，标称的视野中央处的空间分辨率可达1.8mm以下)的空间分辨率、灵敏度、散射率等性能参数进行了测试，测试结果显示该扫描仪视野中心的横向空间分辨率为1.9mm, 轴向空间分辨率为1.88mm, 在能窗设定为250—750keV时系统的绝对灵敏度为39.88 cps/kBq, 同样能窗下二维重建散射分数为50.6%,三维重建散射分数为32.3%. 关键词： 正电子断层成像仪 性能测试     

利用作用深度信息提高正电子断层成像仪分辨率一致性

获取作用深度信息是设计小动物正电子断层成像仪的关键技术之一.我们利用新设计的含有作用深度信息的探测器，从模拟与实验两个方面观测了作用深度信息对于分辨率一致性的影响.结果表明：在γ射线垂直入射时，深度编码探测器和一般的无深度检出机能的探测器，均获得了高分辨率，而γ射线斜入射时，深度编码探测器与一般的探测器相比能提供更好的空间分辨率. 关键词： 作用深度 空间分辨率 正电子断层成像仪     

正电子发射型计算机断层成像技术

 正电子发射型计算机断层成像(positronemissioncomputedtomograph简称PET),其原理是将含有发射正电子的放射性核素,如18F、11C、15O、13N等显像剂注入或吸入人体,通过探测正电子放射性核素衰变时产生的正电子与组织内电子湮灭产生两个能量相等、方向相反的γ光子,由计算机重建图像,显示人体代谢和生化等改变,被誉为活体的分了断层图像。因此PET能将人的思维、行为和脑化学联系起来,探讨、解释和定位人脑的功能活动,对于许多精神、情感、功能及运动障碍等功能性疾病,PET具有理论意义和实用价值。     

基于蒙特卡罗模拟的散射介质中后向光散射模型及分析应用

构建了内光源模型探讨散射介质中的光散射现象,利用蒙特卡罗方法研究了逃逸出组织的后向散射光子数随光子在组织内部发生的散射次数的分布关系,探讨了光源照明方式、辐射强度、接收方式、调制等参数的变化对后向散射的影响,结果表明后向散射光子的数量随散射次数的分布并非简单的单调递增或递减,而是一条先增大后减小出现峰值的曲线. 峰值位置、峰值大小及曲线形状与光源、探测方式、组织光学特性参数等有关. 关键词： 医用光学与生物技术 散射介质 后向散射 蒙特卡罗模拟     

偏振门用于对散射介质成像的蒙特卡罗模拟研究

通过蒙特卡罗模拟研究偏振光在散射介质中的传播,对通过散射介质的偏振光的强度分布进行模拟,分析了偏振门和空间滤波对成像的影响.模拟结果表明,对于Rayleigh散射体,偏振门与空间滤波可以减小散射光的影响,提高图像对比度,圆偏振光入射比线偏振光具有更高的对比度.对于Mie散射粒子,偏振门的作用有限,不同偏振光的结果差别不大.对双组分的散射介质也进行了模拟,偏振门在这种体系中仍能提高图像对比度.     

定量中子数字成像散射校正的蒙特卡罗模拟

 中子数字成像过程中,散射中子可降低像质致使提取样品的定量信息变得困难。针对该情况,分析了中子微光成像系统图像散射降质的原理,采用点扩展函数的叠加来表征散射中子引起图像降质的过程,借助于蒙特卡罗方法（MC）对点扩展函数进行模拟和建模,将点扩展函数描述为样品厚度以及样品到探测器距离为参量的解析函数,研究了点扩展函数的计算方法。研究结果表明,借助于MC方法构建系统的点扩展函数之解析函数,并利用该函数对中子图像进行散射校正,是一种行之有效的定量中子数字成像散射校正方法。     

激光雷达回波信号的蒙特卡罗模拟研究:I.均匀薄雾的多重散射强度分布(英文)

本文简要报道了激光雷达回波信号及多重散射强度分布的蒙特卡罗模拟计算结果。激光雷达系统采用共轴设计 ,计算中采用具有一定厚度、均匀分布的球形薄雾粒子来模拟大气环境 ,研究了粒子尺寸对回波信号的影响 ,获得了不同粒子半径、不同接受视场角条件下多重散射信号强度分布的经验公式。以上两套经验公式具有良好的校正系数 ,可以直接用于激光雷达系统设计中多重散射效应的研究 ,节省大量的模拟计算时间 ,提高设计效率     

Investigation of scatter from out of the field of view and multiple scatter in PET using Monte Carlo simulations

In fully three-dimensional (3D) positron emission tomography (PET) imaging, the scatter fraction (SF) is about 40%-60%, which may degrade the imaging quality severely. Scatter correction is important for high quality image reconstruction. Model-based scatter correction has been proved to be accurate and available in clinical PET. However, it does not correct the scatter from out of the field of view (OFOV) and multiple scatters. In this study, we demonstrate the radial and axial distribution of scatters from OFOV when the source is located in different radial positions. In order to apply the above conclusions to different PET systems, we characterize the scatters from OFOV as a function of the ratio of the scanner diameter to the length of the axial field of view (AFOV) by modeling several typical whole-body and micro PET systems. The proportions of true events (S_0-0), single scatter of one photon (S_1-0) , single scatter of both photons (S_1-1) , double scatter of one photon (S_2-0) and multiple scatter (S_m) are also calculated and compared. Here the 3D-PET Monte Carlo simulations are performed with the Geant4 Application for Tomography Emission (GATE). In summary, the scatters from OFOV tend to be recorded on the lines of response (LOR) far away from the source. They have a much more serious impact on whole-body PET than micro PET depending on the ratio of scanner diameter to the length of AFOV. In whole-body PET, twice scatters including single scatter of both photons (S_1-1) and double scatter of one photon (S_2-0) add up to about 12% so that twice scatter correction must be taken into account to acquire a high quality reconstruction image.     

蒙特卡罗模拟分析电子束发射度对照射量空间分布影响

加速器靶前正前方1 m处的照射量是衡量加速器光源辐射能力的重要物理参量。电子束轰击高原子序数靶产生的X射线空间分布具有很强的前冲性,照射量空间分布与电子束的发射度密切相关。采用高斯函数对不同电子束发射度条件下入射电子的空间分布和角度分布进行建模。应用蒙特卡罗方法对电子束打靶的轫致辐射过程进行模拟,分析电子束发射度对照射量的影响。同时还对整靶结构和叠靶结构下的轫致辐射光源照射量进行计算比较。结果表明,电子束的发散角是影响轫致辐射光源照射量的主要因素。与采用整靶结构相比,采用叠靶结构所获得的照射量空间分布基本一致,在正前方小角度范围内（0~4）的照射量有2%~3%的降低。     

蒙特卡罗模拟分析电子束发射度对照射量空间分布影响

加速器靶前正前方1 m处的照射量是衡量加速器光源辐射能力的重要物理参量。电子束轰击高原子序数靶产生的X射线空间分布具有很强的前冲性,照射量空间分布与电子束的发射度密切相关。采用高斯函数对不同电子束发射度条件下入射电子的空间分布和角度分布进行建模。应用蒙特卡罗方法对电子束打靶的轫致辐射过程进行模拟,分析电子束发射度对照射量的影响。同时还对整靶结构和叠靶结构下的轫致辐射光源照射量进行计算比较。结果表明,电子束的发散角是影响轫致辐射光源照射量的主要因素。与采用整靶结构相比,采用叠靶结构所获得的照射量空间分布基本一致,在正前方小角度范围内（0~4）的照射量有2%~3%的降低。     

An effective scatter correction method based on single scatter simulation for a 3D whole-body PET scanner

Hamamatsu SHR74000 is a newly designed full three-dimensional (3D) whole body positron emission tomography (PET) scanner with small crystal size and large field of view (FOV). With the improvement of sensitivity, the scatter events increase significantly at the same time, especially for large objects. Monte Carlo simulations help us to understand the scatter phenomena and provide good references for scatter correction. In this paper, we introduce an effective scatter correction method based on single scatter simulation for the new PET scanner, which accounts for the full 3D scatter correction. With the results from Monte Carlo simulations, we implement a new scale method with special concentration on scatter events from outside the axial FOV and multiple scatter events. The effects of scatter correction are investigated and evaluated by phantom experiments; the results show good improvements in quantitative accuracy and contrast of the images, even for large objects.     

用于HIRFL重离子治癌的PET成像实验研究与蒙特卡罗模拟

为了HIRFL重离子治癌的需要,使用两个位置灵敏闪烁体探测器组成了一个简单的PET成像系统.对PET成像进行了实验研究,实验测量得到了物体成像以及γ射线能谱,对于511keV全能峰处的能量分辨率为186%,峰总比为52.4%.采用GEANT3程序对该系统进行了模拟计算,并与实验进行比较,最后对该系统进行了优化设计     

材料中He深度分布演化的Monte Carlo模拟研究

基于He泡生长的迁移-合并机理,用Monte Carlo方法模拟了对材料进行等温退火过程中He深度分布的演化,探讨了不同参数对这一演化的影响.研究表明:材料中He泡的初始浓度和尺寸将影响He深度分布的变化,而退火温度则对演化速率起重要作用但对最终的He深度分布影响较小;随着反应的进行,整个系统的演化是逐渐趋缓的. 关键词： He 深度分布 Monte Carlo模拟     

Direct and reverse energy transport in systems of monomers and imperfect traps: Monte Carlo simulations

A Monte Carlo simulation of the concentration dependence of the fluorescence quantum yield _M and emission anisotropyr _M of a system containing dye molecules in the form of monomers M and clusters T (statistical pairs and trimers) playing the role of the imperfect traps for nonradiative excitation energy transfer (NET) has been carried out. The simulation has been made for determined values of Förster critical distancesR ₀ ^MM andR ₀ ^MT and for several values ofR ₀ ^TM andR ₀ ^TT , assuming that the energy may be transferred from M^* to T as well as from T^* to M (reverse nonradiative energy transfer, RNET). It was shown that the RNET process in the range of high concentrations may strongly change the values ofr _M as well as those of _M. For emission anisotropyr _M an effect of repolarization was observed which decreases rapidly with increasingR ₀ ^TM andR ₀ ^TT . A very good agreement between the simulation results of _M and the theoretical model with no adjustable parameters was found. In the model, the RNET process and influence of correlation between active molecules on energy migration among monomers were taken into account.     

Thermodynamic properties of ternary alloys from Monte Carlo simulations

Methods are presented for obtaining both the integral and partial molar energies, entropies and free energies of the components in rigid ternary substitutional alloy systems from the one Monte Carlo simulation. Tests of the methods for some model systems are presented and discussed.     

光学相干层析信号的模拟分析与计算

对传统的模拟介质散射响应函数的蒙特卡罗算法的计算过程进行了改进，提出光子不可分割的假设，这样可以避免人为选定光子被吸收阈值的不确定性；同时采用双曲线法模拟入射高斯光束，解决了对表面为非平面介质的传输信号进行模拟的问题.通过对盖玻片（平面）与眼角膜（曲面）的光学相干层析探测信号及层析图的数值模拟，并与相应实验结果进行比较，证实了该方法与假设的可行性. 关键词： 光学相干层析 蒙特卡罗法模拟 散射     

气候变化中高温破纪录事件的蒙特卡罗模拟研究

采用蒙特卡罗方法,分析了南京市1961年—2000年40?a日观测温度资料中的高温破纪录事件的统计规律,并比较了20世纪全球变暖和南京市近40年来区域变暖背景对其统计规律的影响. 理论分析和蒙特卡罗模拟结果均表明：近40年来南京市第ｋ个高温破纪录事件的最概然发生强度与k（k=1,2,3,\:）呈线性增长,而年发生高温破纪录事件的频率随时间t呈1/（t+1）的衰减趋势,且平均温度高的年份,发生高温破纪录事件的概率较大,反之,概率较小. 结果还表明：20世纪的全球变暖速率（ｖ=0.006?℃/a）和南京市区域的变暖速率（ｖ=0.017?℃/a）在短期内还不至于引起高温破纪录事件的发生强度和发生频率有明显变化,但持续变暖最终将会使年发生高温破纪录事件的频率渐渐地收敛于一个常数,近似等于变暖速率的值. 此外,还研究了日温度之间的自相关和方差变化对高温破纪录事件的影响,研究发现异方差和弱的自相关对高温破纪录事件的发生强度和概率的影响基本可以忽略. 关键词： 高温破纪录事件 蒙特卡罗模拟 全球变暖     

Monte Carlo方法模拟非直视紫外光散射覆盖范围

针对大气中紫外光散射通信的特点, 用Monte Carlo方法对紫外光非直视(NLOS) 通信三种工作方式的覆盖范围进行分析, 建立了基于Monte Carlo方法的NLOS紫外光传输模型.利用Monte Carlo模拟方法对三种NLOS散射方式的单次和多次散射路径损耗及覆盖范围进行模拟研究, 结果表明, 多次散射和单次散射的路径损耗基本一致, NLOS(a) 类全向发送全向接收通信方式覆盖范围最小但全方位性好, NLOS(b) 类定向发送全向接收通信方式的覆盖范围较大但有一定方向性, NLOS(c) 类定向发送定向接收通信方式的覆盖范围最大但有很强的方向性.     

TLD measurements and Monte Carlo simulations for glandular dose and scatter fraction assessment in mammography: A comparative study

The main purpose of this study was to validate and compare Mean Glandular Dose (MGD) values obtained using Monte Carlo simulations with experimental values obtained from Entrance Surface Dose (ESD) and depth dose measurements performed in a Hospital mammography unit. ESD and depth dose were measured using ThermoLuminescent Dosimeters (TLDs), and a tissue equivalent mammography phantom recommended by the American College of Radiology (ACR). Measurements and Monte Carlo simulations were also compared with the MGD calculated using the Automatic Exposure Control (AEC) system of the mammographic unit. In the simulations the Doppler energy broadening effect was also taken into account. The simulated ESD are about 5%–10% higher than the measured ESD values. The deviation between the measured and simulated MGD values in the phantom is of about 15%. The MGD evaluated using the AEC system is smaller both with respect to the Monte Carlo simulation and experimental result by a factor of about 15% and 25% respectively. Moreover the BackScatter Factor (BSF) estimated by Monte Carlo simulations was used for the MGD calculation according to the Wu’s method. Finally the inclusion of the energy broadening effect on MGD calculation produces negligible variations on the simulated results.