Study on photon migration path of ultrashort light pulse in turbid media by Monte Carlo method

The photon density and the photon weight density are obtained by a Monte Carlo method. Based on these two concepts the Gaussian peak value photon paths and the weight mean photon paths of ultrashort light pulse in turbid media are defined and studied. The width of the Gaussian peak value photon path is also given. The influence of the exit angle and time on the photon path and its width are discussed. The relative probability of the photon path is given by the sum of the photon weight densities along the photon path, which could be used to calculate the normalized diffusive intensity approximately. The diffusive reflective intensities will arrive at the maximum at some instant at the place where the photon path reaches on the entrance surface at the same instant. The absorption coefficient has small effect on the photon path and its width in the case of the photon weight density.     

A Monte Carlo study of penetration depth and sampling volume of polarized light in turbid media

Detection depth and sampling volume of polarized light in highly turbid, cylindrically-shaped samples are estimated using pathlength distributions calculated from a polarization-sensitive Monte Carlo model. Due to defined ranges of the polarized light pathlength distribution, the estimated penetration depth and the interrogated volume of the polarization-maintaining photon subpopulation are smaller than those of the whole collected photon population, the latter exhibiting a wider pathlength distribution resulting from multiple scattering. It is also demonstrated that the spatial interrogation extent of polarized light in turbid media is greatly affected by the experimental detection geometry.     

Monte Carlo simulation of photon migration path in turbid media

A new method of Monte Carlo simulation is developed to simulate the photon migration path in a scattering medium after an ultrashort-pulse laser beam comes into the medium.The most probable trajectory of photons at an instant can be obtained with this method.How the photon migration paths are affected by the optical parameters of the scattering medium is analyzed.It is also concluded that the absorption coefficient has no effect on the most probable trajectory of photons.     

Modeling of light propagation in turbid medium using Monte Carlo simulation technique

The aim of the current study is to simulate the laser photon through biological tissue during PDT therapy using Monte Carlo simulation technique. The model is coded using MATLAB. Interaction of laser light with turbid medium e.g. human tissue depends on the optical properties of the medium i.e. refractive index n, absorption coefficient μ _a , scattering coefficient μ _s and anisotropy factor g. Laser light transport through tissue is governed by the radiative transport equations based on absorption and scattering. Direct sampling is used for step-size generation before interaction via absorption or scattering with the transmitting medium, for deflection and azimuthal angle (θ and ϕ) when the scattering even occurs. The tissue medium considered is divided into radial, axial and angular grid elements and an infinite narrow beam with normal incidence on the tissue is considered. The laser light absorbance inside the tissue, reflectance at the top boundary of the tissue and transmittance at the bottom are estimated and these quantities are shown varying radially and angularly. Results of reflectance, transmittance and fluence are compared with the already published results to confirm the authenticity of our coding and these results are found to lie at only 3–4% error.     

Analysis of light propagation in highly scattering media by path-length-assigned Monte Carlo simulations

Numerical analysis of optical propagation in highly scattering media is investigated when light is normally incident to the surface and re-emerges backward from the same point. This situation corresponds to practical light scattering setups, such as in optical coherence tomography. The simulation uses the path-length-assigned Monte Carlo method based on an ellipsoidal algorithm. The spatial distribution of the scattered light is determined and the dependence of its width and penetration depth on the path-length is found. The backscattered light is classified into three types, in which ballistic, snake, and diffuse photons are dominant.     

Numerical modeling of thermal force in a plasma for test-ion transport simulation based on Monte Carlo Binary Collision Model

We present a new numerical model of the thermal force in a plasma, based on the Monte Carlo Binary Collision Model (BCM) [T. Takizuka, H. Abe, J. Comput.Phys. 25 (1977) 205]. This model can be applied for the transport simulation of test ions. The model consists of two steps: (i) choosing a background plasma ion velocity from a distorted Maxwell distribution under the temperature gradient, and (ii) calculating a Coulomb collision between a test particle and the above chosen ion by using the BCM. For the step (i), we developed a velocity sampling method from a distorted Maxwellian, which enables the BCM to bring the thermal force on a test particle in the step (ii).A systematic series of simulations has been performed under various conditions to examine the model. The results of these simulations have been compared with the theoretical values, and it is shown that our model simulates the thermal force correctly for important characteristic features; dependences on the temperature gradient, the test particle velocity, and the background plasma density.     

Voxel classification methodology for rapid Monte Carlo simulation of light propagation in complex media

Monte Carlo (MC) method is a statistical method for simulating photon propagation in media in the optical molecular imaging field.However,obtaining an accurate result using the method is quite time-consuming,especially because the boundary of the media is complex.A voxel classification method is proposed to reduce the computation cost.All the voxels generated by dividing the media are classified into three types (outside,boundary,and inside) according to the position of the voxel.The classified information is used to determine the relative position of the photon and the intersection between photon path and media boundary in the MC method.The influencing factors and effectiveness of the proposed method are analyzed and validated by simulation experiments.     

基于蒙特卡罗法的水下激光光幕探测性能研究

为了提高水下激光光幕的探测性能, 根据水下光束传播规律, 构建了水下激光光幕探测模型, 基于蒙特卡罗模拟方法对水下激光光幕探测性能进行研究。基于水下激光光幕探测模型, 利用MATLAB软件进行仿真, 分析海水衰减系数、初始功率及传输距离对水下激光光幕传输的影响。仿真结果表明:海水的衰减系数越小, 水下激光光幕传输率受到的影响越小。海水参数和传输距离一定时, 随着初始功率的增加, 只会影响到达探测端的最终功率, 但对传输率影响不大。当海水衰减系数一定, 传输距离为1 m时, 其传输率约为15%且变化稳定; 当传输距离增加到30 m时, 传输率在5%以下。     

Retrieval of optical characteristics of an inhomogeneous turbid medium based on time-resolved diffuse reflectometry data: Studying by the Monte Carlo method

Formation of a signal in an optical diffuse reflectometry system using pulse probing radiation from a medium with a layered distribution of the absorption coefficient has been simulated by the Monte Carlo method. The influence of absorption inhomogeneities on the pulse shape and photon travel path was studied. Analysis of statistical characteristics (mean propagation time and broadening magnitude) of detected scattered pulses was performed. Variations in the spatial distribution of travel trajectories of photons forming the signal of optical diffuse reflectometry were analyzed. Based on a comparison of the obtained characteristics with theoretical data, conclusions have been drawn on the agreement of the retrieved values of the absorption and scattering coefficients with their true values depending on the distance between the source and receiver.     

A unified Monte Carlo treatment of the transport of electromagnetic energy, electrons, and phonons in absorbing and scattering media

The scalar Boltzmann transport equation (BTE) is often applicable to radiative energy transfer, electron-beam propagation, as well as thermal conduction by electrons and phonons provided that the characteristic length of the system is much larger than the wavelength of energy carriers and that certain interference phenomena and the polarization nature of carriers are ignored. It is generally difficult to solve the BTE analytically unless a series of assumptions are introduced for the particle distribution function and scattering terms. Yet, the BTE can be solved using statistical approaches such as Monte Carlo (MC) methods without simplifying the underlying physics significantly. Derivations of the MC methods are relatively straightforward and their implementation can be achieved with little effort; they are also quite powerful in accounting for complicated physical situations and geometries. MC simulations in radiative transfer, electron-beam propagation, and thermal conduction by electrons and phonons have similar simulation procedures; however, there are important differences in implementing the algorithms and scattering properties between these simulations. The objective of this review article is to present these simulation procedures in detail and to show that it is possible to adapt an existing MC computer code, for instance, in radiative transfer, to account for physics in electron-beam transport or phonon (or electronic thermal) conduction by sorting out the differences and implementing the correct corresponding steps. Several simulation results are presented and some of the difficulties associated with different applications are explained.     

A Monte Carlo approach for the calculation of polarized light: application to an incident narrow beam

Monte Carlo approaches to compute multiple scattering of polarized light are examined. A Backward Monte Carlo (BMC) method is developed to solve the Stokes vector of the multiple scattered light for an inhomogeneous scattering medium with boundaries. A generalized form of the BMC method in vector notation is proposed. This method can determine the scattered light with sufficient accuracy in both intensity and polarization compared to the same calculation using the doubling-adding method for a plane parallel medium.For application to a narrow incident beam and an inhomogeneous medium, a modified BMC method is developed, borrowing a concept from the Forward Monte Carlo (FMC) method for the first scattering events. Furthermore, a modification of the total scattering matrix, i.e., the combination of the derived scattering matrix with its time inverse, is discussed. This BMC method can be used successfully for model calculations of lidar and other laser measurements of polarized light.     

Multiple passages of light through an absorption inhomogeneity in optical imaging of turbid media

Multiple passages of light through an absorption inhomogeneity of finite size deep within a turbid medium are analyzed for optical imaging by use of the self-energy diagram. The nonlinear correction becomes more important for an inhomogeneity of a larger size and with greater contrast in absorption with respect to the host background. The nonlinear correction factor agrees well with that from Monte Carlo simulations for cw light. The correction is approximately 50%-75% in the near infrared for an absorption inhomogeneity with the typical optical properties found in tissues and five times the size of the transport mean free path.     

Shadow hybrid Monte Carlo: an efficient propagator in phase space of macromolecules

Shadow hybrid Monte Carlo (SHMC) is a new method for sampling the phase space of large molecules, particularly biological molecules. It improves sampling of hybrid Monte Carlo (HMC) by allowing larger time steps and system sizes in the molecular dynamics (MD) step. The acceptance rate of HMC decreases exponentially with increasing system size N or time step δt. This is due to discretization errors introduced by the numerical integrator. SHMC achieves an asymptotic O(N^1/4) speedup over HMC by sampling from all of phase space using high order approximations to a shadow or modified Hamiltonian exactly integrated by a symplectic MD integrator. SHMC satisfies microscopic reversibility and is a rigorous sampling method. SHMC requires extra storage, modest computational overhead, and a reweighting step to obtain averages from the canonical ensemble. This is validated by numerical experiments that compute observables for different molecules, ranging from a small n-alkane butane with four united atoms to a larger solvated protein with 14,281 atoms. In these experiments, SHMC achieves an order magnitude speedup in sampling efficiency for medium sized proteins. Sampling efficiency is measured by monitoring the rate at which different conformations of the molecules' dihedral angles are visited, and by computing ergodic measures of some observables.     

Light transport in laser irradiated turbid media, application in medicine and image processing

Much effort is being made in the study of light propagation in human tissue due to the development and wide use of lasers for surgical and therapeutic applications. When a picosecond light pulse is incident on biological tissue, part of the light is reflected back owing to specular surface reflection and the rest of the light penetrates the tissue and encounters multiple scattering and absorption processes. An analysis has been presented for modeling of light distribution in human tissue. The influence of the refractive index, scattering phase function and scattering albedo on light distribution in laser irradiated tissue has been demonstrated.     

Determination of g and mu using multiply scattered light in turbid media

We propose an inversion scheme to reconstruct the scattering coefficient mu and the anisotropy factor g that characterize the optical properties of a turbid medium. It is based on a theory for the scattering of light inside the medium from an angularly collimated light source. We demonstrate the feasibility of this method using light scattering data obtained from a Monte Carlo simulation.     

Monte Carlo algorithm for drift and diffusion of ions in anisotropic,non-homogeneous media

A new Monte Carlo algorithm for ion transport in two-dimensional anisotropic media is reported. It is based on physical considerations of drift and diffusion in anisotropic media with or without an impermeable boundary. Inhomogeneities in the medium and electric field can be taken into account by averaging along the ion trajectory. The algorithm has been applied to the calculation of ion transport in liquid crystal displays and has been successfully compared with a finite difference program on a one-dimensional liquid crystal structure.