基于温度不敏感源-探测器距离的测量方法的模拟研究

温度变化是影响近红外无创血糖测量精度的主要因素之一。为降低温度变化对近红外漫反射光谱的影响,提出了一种基于温度不敏感源-探测器距离的测量方法,即在漫反射光强对人体组织温度变化不敏感的源-探测器距离处进行光谱测量。利用Monte Carlo方法模拟了温度为30～40 ℃、葡萄糖浓度为0～300 mmol·L-1的皮肤组织在多个源-探测器距离处的漫反射光强。根据模拟结果,分析了人体皮肤组织模型中温度不敏感源-探测器距离的存在性及其受葡萄糖浓度变化的影响;比较了1 000 nm处温度恒定和温度变动时,不同源-探测器距离处漫反射光强与葡萄糖浓度的相关性;进一步地,利用六个波长(1 000,1 050,1 100,1 150,1 350和1 410 nm)下的温度不敏感源-探测器距离及其他距离处的漫反射光强,建立了葡萄糖的偏最小二乘(PLS)模型,并比较了这些模型在温度恒定和温度变动时的预测精度。结果表明,在1 000～1 440 nm范围内,人体存在温度不敏感源-探测器距离,且葡萄糖浓度变化对该距离的影响可以忽略不计;当组织温度变化时,温度不敏感源-探测器距离处的漫反射光强与葡萄糖浓度的相关性及建模效果均明显优于其他源-探测器距离,基本接近样品温度恒定时的情况。研究表明,基于温度不敏感源-探测器距离的测量方法能有效降低温度变化对漫反射光强的影响,有望提高近红外漫反射无创血糖测量的精度。     

Time-resolved reflectance at null source-detector separation: improving contrast and resolution in diffuse optical imaging

We propose a novel approach to imaging in diffusive media based on time-resolved reflectance measurements at null source-detector separation. This approach yields better spatial resolution and contrast as compared to the classical approach, which typically employs a separation of 20-40 mm. Results are obtained by an analytical perturbation approach to diffusion theory and on Monte Carlo simulations. Practical implementation with state-of-the-art technology and performance of a complementary approach based on the use of small but not null source-detector separation are also discussed.     

Hybrid diffusion approximation in highly absorbing media and its effects of source approximation

A modified diffusion approximation model called the hybrid diffusion approximation that can be used for highly absorbing media is investigated. The analytic solution of the hybrid diffusion approximation for reflectance in two-source approximation and steady-state case with extrapolated boundary is obtained. The effects of source approximation on the analytic solution are investigated, and it is validated that two-source approximation in highly absorbing media to describe the optical properties of biological tissue is necessary. Monte Carlo simulation of recovering optical parameters from refiectant data is done with the use of this model. The errors of recovering μ＇s and μ＇s are smaller than 15% for the reduced albedo between 0.77 and 0.5 with the source-detector separation of 0.4-3 mm.     

Monte Carlo Study on Singly Tagged D Mesons at BES-III

We present Monte Carlo studies on the singly tagged D mesons, which are crucial in the absolute measurements of D meson decays, based on a full Monte Carlo simulation for the BES-III detector, with the BES-III Offline Software System. The expected detection efficiencies and mass resolutions of the tagged D mesons are well estimated.     

Hydrodynamic simulation of electron transport in n-type Hg<Subscript>0.8</Subscript>Cd<Subscript>0.2</Subscript>Te

A hydrodynamic approach based on concentration, velocity and energy conservation equations is developed and used for the simulation of the electron transport in bulk HgCdTe. Both transient and steady-state regimes are simulated using input parameters calculated with a Monte Carlo simulator. The model is validated through a comparison in excellent agreement with Monte Carlo results.     

PIPS-α谱仪探测效率蒙特卡罗模拟及其影响因素

在放射源能谱测量过程中,PIPS-谱仪探测效率的影响因素有几何因子、空气层吸收、死层吸收、本征探测效率等,为了更精确地计算放射源的活度,分析母体核素种类和浓度,探究各因素对探测效率的影响具有重要意义。使用蒙特卡罗软件模拟不同探源距、真空度下的放射源的能谱测量;使用SRIM模拟5 MeV 粒子在空气和死层中的射程及分布;使用PIPS-谱仪测量241Am,238Pu,239Pu三种标准源的实际能谱;根据实验与模拟的结果对各个因素对探测效率的影响进行分析,并计算得到谱仪对不同放射源的本征探测效率。结果表明:在PIPS-谱仪的能谱测量过程中,影响谱仪探测效率的主要因素是几何因子和本征探测效率,空气层和死层对粒子的吸收可以忽略不计;谱仪对241Am,238Pu,239Pu三种标准源的本征探测效率分别为64.84%,49.95%,51.55%。     

Influence of the third-order parameter on diffuse reflectance at small source-detector separations

Spatially resolved reflectance close to source has received a great deal of attention recently. This research is considered to develop a new noninvasive technique for measuring the optical properties of biological media. Using Monte Carlo simulations, we investigated the influence of third-order parameter 5 on diffuse reflectance and found that the reflectance decreased with an increase of delta at a short source-detector separation of approximately 0.7-2 transport mean free paths. We show that the effects of two parameters, y and second-order parameter delta, on the reflectance are contrary. As a result the influence of the second-order parameter y on the reflectance is irregular when the condition delta delta相似文献   

Computation of the string tension in four-dimensional Yang–Mills theory using large N reduction

Continuum reduction and Monte Carlo simulation are used to calculate the heavy quark potential and the string tension in large N Yang–Mills theory in four dimensions. The potential is calculated out to a separation of nine lattice units on a lattice with extent six in each direction.     

基于漫反射光谱法下层组织信息的最佳探测位置的研究

由于人体组织结构的复杂性和个体差异性,采用光谱分析技术实现组织内部信息的检测方法中,仍存在着检测位置不明确的问题。采用多位置空间分辨漫反射光谱法对脂肪肌肉组织中内部信息的最佳探测位置进行了研究。由于目标组织为肌肉组织,为了消除由于不同人体皮下脂肪层厚度的差异以及目标层中多次后向散射光对测量结果引起的较大误差,并根据光在复杂生物组织中的传输模型,通过增加约束条件——两条理想的“香蕉型”路径—来定义有效光子比SNR,并以SNR作为评价最佳探测位置SDS_best的标准,对优化后的Monte Carlo仿真结果进行统计分析,分别研究了脂肪层厚度h_f、脂肪层吸收系数μ_af和肌肉层吸收系数μ_am与光源探测器间距SDS之间的关系,并以h_f为自变量与SDS_best建立线性回归模型。研究表明,(1)当0<h_f<0.6 cm时,μ_af和μ_am对SDS_best几乎没有影响,而h_f与SDS_best的建模相关系数R2为0.991 8;(2)任取脂肪层厚度值0.12与0.22 cm作预测,得到预测误差分别为0.030 14和0.020 16,预测误差均能控制在5%以内。该方法能更方便、快速地为测量组织内部信息确定最佳检测位置,并有效削弱浅表组织以及目标层中多次背向散射光的干扰。     

Monte Carlo Simulation of Near Infrared Reflectance Spectroscopy in the Wavelength Range from 1000 nm to 1900 nm

We have calculated the characteristics of reflected near infrared light, which is incident on the skin surface, scattered in the skin tissue and detected, in the wavelength range from 1000 nm to 1900 nm. We have used a Monte Carlo method to calculate the propagation path, the average photon visit depth, the average photon path length, etc. for the cases of short source-detector distances, to which the isotropic scattering approximation cannot be applied. An experiment to validate the simulation results was also conducted using a liquid phantom.     

Determination of optimal source-detector separation in measuring chromophores in layered tissue with diffuse reflectance

Based on analysis of the relation between mean penetration depth and source-detector separation in a threelayer model with the method of Monte-Carlo simulation, an optimal source-detector separation is derived from the mean penetration depth referring to monitoring the change of chromophores concentration of the sandwiched layer. In order to verify the separation, we perform Monte-Carlo simulations with varied absorption coefficient of the sandwiched layer. All these diffuse reflectances are used to construct a calibration model with the method of partial least square (PLS). High correlation coefficients and low root mean square error of prediction (RMSEP) at the optimal separation have confirmed correctness of the selection. This technique is expected to show light on noninvasive diagnosis of near-infrared spectroscopy.     

基于蒙特卡罗方法的α放射源能谱测量模拟北大核心CSCD

运用蒙特卡罗方法建立PIPS-α谱仪的腔室模型,模拟谱仪对α面源(238Pu)的能谱测量,通过改变放射源与探头之间的距离,模拟不同条件下α粒子的射线能谱,同时,对不同探源距条件对α射线能谱的影响进行分析;另一方面,使用谱仪在不同探源距的情况下进行α能谱实验测量,获得不同条件下α射线能谱;最后,将模拟与实验的结果进行对比,分析实验测量结果与模拟结果的相对误差,并对模拟过程及模型参数进行校正,对校正后的模型进行实验验证,使其能够应用到其他α放射源的能谱测量中,并且使其可用于单能α粒子的能谱研究,研究结果也可为α能谱测量技术提供依据,为α能谱解谱技术提供数据支持。     

小体积比两相分离早期过程的三维格子气模型研究

采用Monte Carlo模拟方法研究了小体积比三维格子气模型中的相分离过程, 比较了sc, fcc, bcc 等3种晶格类型和T/T_c=0.45–0.85等 5个相对温度的系统演化行为, 计算了系统的结构因子函数, 发现结构因子在相分离早期不满足标度关系, 生长指数等于1/6, 小于经典Lifshitz-Slyozov理论的值. 关键词： 格子气模型 相分离过程 Monte Carlo模拟 生长律     

Monte Carlo simulation of joint density of states in one-dimensional Lebwohl-Lasher model using Wang-Landau algorithm

Monte Carlo simulation using the Wang-Landau algorithm has been performed in an one-dimensional Lebwohl-Lasher model. Both one-dimensional and two-dimensional random walks have been carried out. The results are compared with the exact results which are available for this model.     

RWMCS – An alternative random walk Monte Carlo code to simulate light scattering in biological suspensions

In biological suspensions, the forward light scattering is done mainly by the contribution of the suspended cells. The multiple scattering is almost always unavoidable, its contribution being described by Monte Carlo simulations or by approximate analytical formula. The Monte Carlo approach we propose moves one photon at a time and checks all scattering centers to find, at each simulation step, which one will scatter the photon. The validation of the simulation results is performed by comparing the obtained angular distribution with the predictions of the effective phase function theory reported recently in the literature and with the angle resolved experimental measurements performed on human red blood cells (RBCs) in suspensions at different hematocrits. The results show a good agreement in the small concentration range.     

Grand Canonical Ensemble Monte Carlo Simulation of Depletion Interactions in Colloidal Suspensions

Depletion interactions in colloidal suspensions confined between two parallel plates are investigated by using acceptance ratio method with grand canonical ensemble Monte Carlo simulation. The numerical results show that both the depletion potential and depletion force are affected by the confinement from the two parallel plates. Furthermore, it is found that in the grand canonical ensemble Monte Carlo simulation, the depletion interactions are strongly affected by the generalized chemical potential.     

Monte Carlo Simulation of Optical Properties of Wake Bubbles

Based on Mie scattering theory and the theory of multiple light scattering, the light scattering properties of air bubbles in a wake are analysed by Monte Carlo simulation. The results show that backscattering is enhanced obviously due to the existence of bubbles, especially with the increase of bubble density, and that it is feasible to use the Monte Carlo method to study the properties of light scattering by air bubbles.     

Simulations of censoring effect in XRF analysis

The quantitative aspects of a use of random left‐censoring as a statistical approach accounting for the detection limit effects in x‐ray fluorescence (XRF) analysis were investigated using the Monte Carlo simulations. More precisely, the performance of the Kaplan–Meier method applied to the estimation of the original concentration distributions from detection limit censored concentration measurements are discussed. The simulations were performed for assumed log‐normal and log‐stable concentration distributions, which are known to model fairly well both concentrations and detection limits for biomedical and environmental samples. In particular, the question of the accuracy of the estimation of the mean value and median for the discussed concentration distributions using the Kaplan–Meier estimator was addressed. It is demonstrated that both the mean value and median of the concentration distribution can be estimated from censored data fairly precisely, typically within 4% for the log‐normal and within 15% for log‐stable models, even for substantial censoring levels (up to 80%). Moreover, the estimation of the median is much more precise than that of the mean value, in particular for stable distributions. Finally, the simulations show that random left‐censoring can be recommended as a standard tool for analysing the detection limit censored concentration measurements of trace elements in XRF analysis. Copyright © 2004 John Wiley & Sons, Ltd.     

基于舰船尾流激光雷达的Monte Carlo模型及方差消减方法研究

对舰船尾流激光散射机理及探测过程的数值模拟是研究舰船尾流激光探测与制导的重要基础.首先分析了Monte Carlo方法引入到尾流激光雷达数值模拟领域研究的可行性.结合自行研制的尾流激光雷达实际参数,建立了尾流激光探测的Monte Carlo数值仿真模型.通过对仿真结果进行统计,深入剖析了仿真结果方差大及光子利用率低等问题的成因.为解决该问题,基于有偏采样理论和分裂-轮盘赌基本原理提出了接收光学视场内光子强行碰撞方法和以光子自由程总长度为准则的光子分裂方法,并进行了两方法的融合.仿真与实验结果的对比分析表明,提出的模型仿真结果与实验数据符合较好,验证了模型的正确性;提出的接收光学视场内光子强行碰撞方法和光子分裂方法能有效减小方差并提高光子利用率;解决了Monte Carlo方法引入到尾流激光雷达模拟过程中的适用性问题.     

含葡萄糖的混浊介质的Mueller矩阵模拟与分析

利用矢量Monte Carlo算法结合葡萄糖的旋光特性模拟了含糖混浊介质的背散射Mueller矩阵.数值模拟表明:含糖混浊介质的背散射Mueller矩阵元素中除Mll、M14、M41、M44外的其它元素花样均发生龙卷风状的不同程度的扭曲,其中M24、M34、M42、M43的扭曲程度更为显著.研究发现M24、M34、M4...