Influence of the phase function on determination of the optical properties of biological tissue by spatially resolved reflectance

Spatially resolved reflectance measurements are widely used for determination of the optical properties of biological media. However, the influence of the phase function on these measurements has not been quantified. We show that errors in the derived reduced scattering and absorption coefficients are as great as 100% for both absolute and relative spatially resolved reflectance measurements if a standard solution of the diffusion equation is used in the analysis. In addition, we investigated nonlinear regressions, using Monte Carlo simulations and an additional fitting parameter that characterizes the phase function, and found that the errors in the obtained optical coefficients were 相似文献   

Alternative Oblique-Incidence Reflectometry for Measuring Tissue Optical Properties

To deduce the optical properties, the absorption coefficient SmU_aand reduced scattering coefficient μ’_s, of turbid medium, Lin et al. (Appl. Opt. 34 (1995) 2362) proposed an oblique incidence reflectometry in which the diffusion approximation was assumed. In this paper we propose an alternative method which does not assume the diffusion approximation but uses a Monte Carlo light propagation model. Two features are extracted from the diffuse reflectance distribution detected on the medium surface, and optical properties are then estimated by looking up the predetermined table generated by Monte Carlo simulations. The validity of the proposed method has been confirmed by computer simulations.     

紫激光辐射人肺组织的光学特性

应用光纤探测系统和自行研制的光纤探针深度针,成功实现了Ｋｒ＾＋紫激光辐射新鲜离体人肺组织体表面漫反射率和内部光能流率分布的精确测定,并利用漫射理论和加倍（ＡｄｄｉｎｇＤｏｕｂｌｉｎｇ）方法的部分结论,获得了紫光从肺癌组织和正常组织的主要光学特性参数,与蒙特卡罗法拟合实验的光分布曲线得出的结果一致秋临床实际应用提供了重要的原始参数。     

生物组织光传播的时域特征分析

采用蒙特卡罗方法对脉冲光在组织中传播的时域特征作了研究，并把计算结果与漫射理论的解析解作了比较，文中详细分析了组织光学特性参数与组织表面时间分辨的漫反射光分布之间的关系。结果表明：散射系影响着漫反射光强到达峰值的时间，吸收系数影响着漫反射光强在峰值之后随时间下降的速度，而ｇ因子对响应曲线的前沿影响较大，在后沿阶段基本上没有影响。     

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   

各向异性介质辐射特性参数联合反演

本文基于BP神经网络方法结合蒙特卡洛和BEER定律辐射传输模拟方法建立了联合反演各向异性散射介质的辐射特性参数模型。首先采用半球透射率结合半球反射率反演模型反演了各向同性介质的吸收系数和散射系数,在此基础上增加准直透射率,建立了联合反演各向异性介质的吸收系数、散射系数和散射不对称因子三参数联合反演模型。反演结果表明该模型能准确反演出介质辐射特性参数,具有实用意义。此外,为了检验测量误差对模型的反演准确性的影响,分别在不同程度测量误差情况下进行反演,结果显示测量误差对散射不对称因子反演值影响较大。     

离体正常乳腺组织350～850 nm波段光谱特性

采用带有积分球附件的紫外/可见/近红外分光光度计测量了离体正常乳腺组织在350～850 nm光谱范围的反射率和透射率,运用反向倍加法得到了离体正常乳腺组织在相应光谱范围的光学参数,分析了正常乳腺组织的光学穿透深度随波长的变化情况。实验结果表明：350～850 nm波段正常乳腺组织的约化散射系数μ′_s大于吸收系数μ_a。μ′_s随着波长的增加而减小,即从350 nm波长值为9.731 mm-1～850 nm波长值为1.476 mm-1。μ_a从350 nm波长值为0.798 mm-1～850 nm波长值为0.102 mm-1,410 nm波长处存在一个吸收峰,其值为0.506 mm-1。光学穿透深度随着波长的增加而增大,从350 nm波长值为0.199 mm-1～850 nm波长值为1.439 mm。基于反向倍加法计算获得乳腺组织的光学参数,采用Monte Carlo模拟得到其相应光谱范围的反射率和透射率,并与实际测量值进行比较,二者的一致性较好。实验结果为乳腺组织的光活检及其光学治疗提供重要参考。     

Imaging optically scattering objects with ultrasound-modulated optical tomography

We show the feasibility of imaging objects having different optical scattering coefficients relative to the surrounding scattering medium using ultrasound-modulated optical tomography (UOT). While the spatial resolution depends on ultrasound parameters, the image contrast depends on the difference in scattering coefficient between the object and the surrounding medium. Experimental measurements obtained with a CCD-based speckle contrast detection scheme are in agreement with Monte Carlo simulations and analytical calculations. This study complements previous UOT experiments that demonstrated optical absorption contrast.     

用Monte Carlo方法模拟闪锌矿相ZnS电子的输运特性

用Monte Carlo方法模拟闪锌矿相(zinc blende)ZnS电子的输运特性.实验采用的是非抛物线模型计算电子的能带结构,模拟包含了声学声子散射,极性光学声子散射,压电散射,电离杂质散射,能谷间散射以及自散射等散射机理.通过模拟得到了ZnS材料的平均漂移速度、平均电子能量随电场强度变化的曲线图,以及总散射率随电子能量变化图,并将结果与文献报道的模拟结果[1]进行比较得出:本实验方法具有模型简单,计算速度快,获得结果比较准确的优点.     

基于delta-P1近似模型的空间分辨漫反射一阶散射参量灵敏度研究

以delta-P1近似光学模型为基础, 推导了双点源近似下空间分辨漫反射一阶散射参量μ_s'灵敏度的解析式, 并进行了数值分析和比较. 研究表明, 与混合漫射近似模型和漫射近似模型相比, delta-P1 近似模型能更好地描述强散射较强吸收情况下近光源区域生物组织漫反射光子的分布, 且在有效反照率a'>0.83时, 获得最佳优化距离ρ_opt, ρ_opt 随μ_s' 的增大而减小, 且在距光源约2.7—4个输运平均自由程处μ_s'的变化对测量吸收的影响最小. 这项研究对于优化传感器几何结构以及生物组织光学参量的测量具有重要意义.     

Determination of optical parameters of a medium by the profile of backscattered radiation

The possibilities of determining the scattering coefficient, the absorption coefficient, and the scattering anisotropy parameter of strongly scattering media are analyzed on the basis of comparison of experimentally obtained profiles of backscattered radiation with the profiles calculated with help of the numerical Monte Carlo method.     

Analysis of diffuse scattering of single crystals using monte carlo methods

While conventional crystal structure analysis using Bragg intensities reveals only information about the average structure of the crystal, diffuse scattering contains additional information about the disorder, i.e. departure from the average structure, of the studied material. Two different approaches to the analysis of diffuse scattering based on Monte Carlo methods are described in this paper: the direct Monte Carlo (MC) simulation technique and the Reverse Monte Carlo (RMC) method. The MC method requires the construction of a model for the disorder based on physical and chemical considerations and the selection of a set of near-neighbour interactions. The given model is realized by minimizing the total energy of the crystal via MC simulations. Next, the corresponding diffraction pattern is calculated and compared with the experimental data. By adjusting the near-neighbour interaction and repeating the process, a qualitative “match” between observed and calculated diffuse scattering is obtained. In contrast, the RMC method minimizes the difference between observed and calculated diffuse scattering intensities directly. This method leads to one real space structure consistent with the observed diffuse scattering but does not automatically result in a chemically sensible structure or further insight into the particular disorder problem.

The first example given in this paper demonstrates the viability of the RMC method by refining diffuse scattering data calculated from simulated structures with known disorder parameters. These structures were generated using the MC technique. As a second example MC and RMC simulations of the diffuse scattering of stabilized zirconias (CSZ) are shown, modelling occupational disorder as well as displacements.     

Fiber-Optic Backscattering Profile Measurements for Determination of the Optical Coefficients of Turbid Media

The necessity of using anisotropic approximation in techniques for determining optical parameters from the backscattered radiation profile is shown. A fiber-optic technique for simultaneous determination of the anisotropy parameter as well as the scattering and absorption coefficients of turbid media is proposed. It is based on the comparison of the experimental backscattered radiation profiles and those obtained by the numerical Monte Carlo method.     

Asymmetric diffusion model for oblique-incidence reflectometry

A diffusion theory model induced by a line source distribution is presented for oblique-incidence reflectometry.By fitting to this asymmetric diffusion model,the absorption and reduced scattering coefficients μa and μ’s of the turbid medium can both be determined with accuracy of 10% from the absolute profile of the diffuse reflectance in the incident plane at the negative position-1.5 transport mean free path(mfp’) away from the incident point;particularly,μs’ can be estimated from the data at positive positions within 0-1.0 mfp’ with 10% accuracy.The method is verified by Monte Carlo simulations and experimentally tested on a phantom.     

Escape probability of signal photoelectrons from non-crystalline solids: influence of anisotropy of photoemission

The escape probability of photoelectrons as a function of depth of orgin haa been studied experimentally, analytically and by the Monte Carlo (MS) technique. The depth distribution function (DDF) describing the probability for an electron emitted at a certain depth to leave a surface without being scattered inelastically has been obtained by solving a kinetic equation in the transport approximation. The analytically derived DDF is a universal function of the ratio of the inealstic to the transport mean free paths and the asymmetry parameter. In the directions of minima of the angular distribution, this function is no longer exponential, but it may be essentially nonmonotonic, reaching its maximum value at the depth comparable with the inelastic mean free path. The maximum value of the DDF exceeds its surface value by about 50% for the asymmetry parameter being equal to 2 in the emission directions close to that of X-ray propagation. Under the same conditions, the mean escape depth of electrons may be several times larger than the value predicted by the usual XPS formalism. Such behaviour of the escape probability is explained by elastic scattering of photoelectrons.The solution to the kinetic equation for a uniform target is generalized for a sample with an arbitrary depth profile and depth-dependent elastic and inelastic scattering cross-sections under the condition of the ratio of the inelastic to the transport mean free paths being independent of depth. Analytical formulas for the photoelectron yield from overlayer/substrate structure have been derived and studied in detail. The analytical predictions are compared with the experimental and Monte Carlo simulation data obtained for aluminium oxide/aluminium specimen. A satisfactory agreement is observed between the experimental and theoretical results.     

Estimating optical properties in layered tissues by use of the Born approximation of the radiative transport equation

We use the Born approximation of the radiative transport equation to recover simultaneously the absorption and scattering coefficients in a single layer of a two-layer tissue sample from reflectance data. This method reduces the estimation of both optical properties to a single linear, least-squares problem. It is valid over length scales smaller than a transport mean free path and hence is useful for epithelial tissue layers. We demonstrate the accuracy of this method by using spatially resolved reflectance data computed with Monte Carlo simulations.     

Time-gated optical imaging through turbid media using stimulated Raman scattering: Studies on image contrast

In this paper, we report the development of experimental set-up for time-gated optical imaging through turbid media using stimulated Raman scattering. Our studies on the contrast of time-gated images show that for a given optical thickness, the image contrast is better for sample with lower scattering coefficient and higher physical thickness, and that the contrast improves with decreasing value of anisotropy parameters of the scatterers. These results are consistent with time-resolved Monte Carlo simulations.     

Reflectance spectroscopy of gold nanoshells: computational predictions and experimental measurements

Gold nanoshells are concentric spherical constructs that possess highly desirable optical responses in the near infrared. Gold nanoshells consist of a thin outer gold shell and a silica core and can be used for both diagnostic and therapeutic purposes by tuning the optical response through changing the core–shell ratio as well as the overall size. Although optical properties of gold nanoshells have already been well documented, the reflectance characteristics are not well understood and have not yet been elucidated by experimental measurements. Yet, in order to use gold nanoshells as an optical contrast agent for scattering-based optical methods such as reflectance spectroscopy, it is critical to characterize the reflectance behavior. With this in mind, we used a fiber-optic-based spectrometer to measure diffuse reflectance of gold nanoshell suspensions from 500 nm to 900 nm. Experimental results show that gold nanoshells cause a significant increase in the measured reflectance. Spectral features associated with scattering from large angles (~180°) were observed at low nanoshell concentrations. Monte Carlo modeling of gold nanoshells reflectance demonstrated the efficacy of using such methods to predict diffuse reflectance. Our studies suggest that gold nanoshells are an excellent candidate as optical contrast agents and that Monte Carlo methods are a useful tool for optimizing nanoshells best suited for scattering-based optical methods.     

Retrieval of chromophore concentration in a tissue phantom by diffuse reflectance spectroscopy

This study describes a method for analysis of back reflectance spectroscopic data to estimate the concentration of endogenous or exogenous chromophores of tissue, noninvasively and in real time. In the study, tissue phantoms were prepared using intralipid and two chromophores, indocyanine green and methylene blue. Reduced scattering and absorption coefficients ranges of tissue phantoms were kept in the range of biological tissues. Spectroscopic measurements on the tissue phantoms were carried out using a miniature spectrometer, an optical fiber probe, a halogen tungsten light source and a laptop. Monte Carlo simulations of the experiments were run, and an average optical path lengths of the detected photons were obtained for different absorption and scattering coefficients. The average optical path lengths of the photons were used to estimate concentrations of the chromophores in the tissue phantoms. Scattering and absorption coefficients were estimated with an average error of 4.7 and 5.4%, respectively. The developed method has the potential to be used in diagnosis of pathologic tissues based on the variation of biochemical composition of tissues and in photodynamic therapy to estimate the concentration of photosensitizers.