共查询到20条相似文献,搜索用时 15 毫秒
1.
Xichang Wang Yanjun Gong DongCao Song Zhensen Wu 《International Journal of Infrared and Millimeter Waves》2004,25(10):1545-1556
Near-IR radiation is often utilized to detect the properties in tissues, up to now, a semi-infinite medium photon migration model and a two-layered turbid medium model are applied widely. But the solution is approximate. In this paper, According to the diffusion equation, employing the extrapolated boundary condition, we analyze the diffusion of photons of a three-layered matched medium, set up the accurate solution of the diffuse equation. In order to validate our solution, we apply the Monte-Carlo simulation of the time domain and the steady-state, we find that the solution of a three-layered matched medium diffusion equation not only accord with the Monte-Carlo simulation. The solution can still solve the problems of a two-layered turbid medium model and a semi-infinite medium photon migration model 相似文献
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平稳状态下匹配介质三层漫射方程的空间分辨漫反射 总被引:6,自引:1,他引:5
如果应用漫射方程分析和确定生物表面组织的光学特性,就能够确定生物活体是否发生病变,实现生物活体的无损探测。目前,光在生物体内传播常用的基本模型是半无限厚模型和两层介质模型。而实际上,人体组织为多层介质。根据漫射方程理论,在平稳状态条件下,使用外推边界条件,精确给出了匹配介质中平稳状态下三层体系光的漫射方程格林函数的解。通过此解,可以算出空间分辨漫反射。使用蒙特卡罗方法来验证三层体系的漫射方程,发现我们给出的理论解,不仅可以包括与Kienle的结论一致的两层体系模型的空间分辨漫反射的解,而且三层体系漫射方程的空间分辨漫反射解和蒙特卡罗方法基本一致。 相似文献
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O. Lehtikangas V. Kolehmainen S.R. Arridge 《Journal of Quantitative Spectroscopy & Radiative Transfer》2010,111(10):1406-1417
In diffuse optical tomography, light transport theory is used to describe photon propagation inside turbid medium. A commonly used simplification for the radiative transport equation is the diffusion approximation due to computational feasibility. However, it is known that the diffusion approximation is not valid close to the sources and boundary and in low-scattering regions. Fokker-Planck equation describes light propagation when scattering is forward-peaked. In this article a numerical solution of the Fokker-Planck equation using finite element method is developed. Approach is validated against Monte Carlo simulation and compared with the diffusion approximation. The results show that the Fokker-Planck equation gives equal or better results than the diffusion approximation on the boundary of a homogeneous medium and in turbid medium containing a low-scattering region when scattering is forward-peaked. 相似文献
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The paper focuses on the determination of statistical characteristics of photon distributions in a semi-infinite turbid medium, specifically the photon average trajectory and the root-mean-square deviation of photons from the average trajectory, with an approach based on the diffusion approximation to the radiative transfer equation. We show that the Dirichlet and Robin boundary conditions used for this purpose give close results. We derive exact analytical expressions for the case of the Dirichlet boundary condition. To demonstrate the practical value of our results we consider approximate solution of the inverse problem of time-domain diffuse optical tomography with the flat layer transmission geometry. The problem is solved with the method of photon average trajectories which are constructed with analytical expressions derived for a semi-infinite medium. 相似文献
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Xichang Wang Zhen hua Yanjun Gong 《International Journal of Infrared and Millimeter Waves》2005,26(8):1195-1205
Infrared ray (IR) has great potential in medical diagnosis and therapy. In order to detect tumor in skin, we set up the steady-state and time domain IR diffusion model of an n-layered matched medium with an infinitely thick. We utilize the diffuse equation to solve a five-layered infinite matched medium and obtain the accurate solution of a matched medium of the steady state and time domain in tissue. We compare the steady-state spatially resolved reflectance calculated with Monte-Carlo simulations. The Monte-Carlo simulation shows that the solution is valid. Our equation can be used to obtain the tumor information in medical diagnosis and therapy. 相似文献
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A photon-transport forward model for image reconstruction in turbid media is derived that treats weak inhomogeneities through a Born approximation of the Boltzmann radiative transfer equation. This model can conveniently replace the commonly used diffusion approximation in optical tomography. An analytical expression of the background Green's function is obtained from the cumulant solution of the Boltzmann equation. Our model provides the correct behavior of photon migration at early times and reduces at long times to the center-moved diffusion approximation. Numerical comparisons between this model and the standard and center-moved diffusion models are presented. 相似文献
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The steady-state and time-domain analytical solutions of the diffusion equation for photon migration through a highly scattering n-layered mismatched medium have been obtained. The effect of the refractive-index mismatch is taken into account, and the extrapolated boundary condition has been considered. A Monte Carlo code for photon migration through an n-layered mismatched medium has also been developed. Comparisons with the results of Monte Carlo simulations showed that the analytical solutions correctly describe the photon migration, while reflectance is observed. 相似文献
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I. V. Krasnikov A. Yu. Seteikin B. Roth M. Meinhardt-Wollweber 《Optics and Spectroscopy》2018,124(2):180-186
A search for an effective method of modelling of the Raman-spectroscopy problem in turbid (scattering) media has been performed taking into account the corresponding parameters of the detector and sample volume. A solution of the radiative-transfer equation by Monte-Carlo method underlies the proposed model. Two fundamental approaches to numerical modeling of Raman scattering are considered: the direct transport problem of Rayleigh and Raman photons at each point of the medium and the two-step model, in which a photon flux in the medium is calculated in the first stage, followed by generation of the corresponding number of Raman photons at each point. 相似文献
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Matcher SJ 《Optics letters》1999,24(23):1729-1731
The question of whether a unique distribution of the absorption coefficient, the scattering coefficient, and the refractive index of a turbid medium can be reconstructed by optical tomography is considered. A recent publication [Opt. Lett. 23, 882 (1998)] established that such reconstruction is not possible when photon transport is well modeled by the diffusion equation. A simple proof is offered that, when measurements at high modulation frequencies are included, the more exact P1 model of light transport suggests that this reconstruction may be possible. 相似文献
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Venkatesh Gopal Sushil Mujumdar Hema Ramachandran A. K. Sood 《Optics Communications》1999,170(4-6):331-345
We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance up to which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of that predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results. 相似文献
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We present a generalized delta-Eddington phase function to simplify the radiative transfer equation to an integral equation with respect to the photon flux vector. The solution of the integral equation is highly accurate to model the photon propagation in the biological tissue over a broad range of optical parameters, especially in the visible light spectrum where the diffusion approximation breaks down. The methodology is validated in the Monte Carlo simulation and can be applied in various optical imaging applications. 相似文献
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Ultrasound tagging of light provides a unique way to probe photon density inside turbid media. We show that this technique allows one to probe the well-known banana-shaped photon density noninvasively, giving rise to a new tool for modeling diffusive photon propagation. Moreover, we show that this technique is quantitative and allows one to get a precise determination of the absorbing constituents inside the turbid medium. 相似文献
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针对扩散光学层析在小动物成像中的应用问题并基于混浊介质空间光子三维散射的实际物理效应,提出的二维稳态辐射传输方程的有限差分数值求解新方法.在此基础上,研究了不同的空间剖分网格和角度离散密度对求解准确度的影响,并通过将所提方法与蒙特卡洛模拟进行比对,验证方法的正确性.研究表明:在均匀组织体内,当离散角度达到一定数量时,由辐射传输方程的有限差分解获得的透射面和侧面的光子密度对空间网格大小并不敏感,而在反射面上光子密度计算则需要较密的空间网格才能够达到一定准确度.本研究为发展基于辐射传输方程的扩散光学层析理论奠定了基础. 相似文献
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The recent controversy over whether the photon diffusion coefficient depends on absorption is addressed by use of the analytical solution of the photon transport equation in an infinite homogeneous scattering medium. The diffusion coefficient is found to be independent of absorption but temporally dependent. After a long period of time, the photon diffusion coefficient approaches D=1/3mu(s)(?) , which supports a claim made by Furutsu and Yamada [Phys. Rev. E 50, 3634 (1994)]. At early times, the diffusion coefficient is smaller than D=1/3mu(s)(?) , but this reduction cannot be expressed as D=1/3(mu(s)(?)+mu(a)) , since the time-dependent diffusion coefficient is found to be unrelated to absorption. 相似文献
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Chernomordik V Gandjbakhche AH Hassan M Pajevic S Weiss GH 《Optics Communications》2010,283(23):4832-4839
We develop an analytic model of time-resolved fluorescent imaging of photons migrating through a semi-infinite turbid medium bounded by an infinite plane in the presence of a single stationary point fluorophore embedded in the medium. In contrast to earlier models of fluorescent imaging in which photon motion is assumed to be some form of continuous diffusion process, the present analysis is based on a continuous-time random walk (CTRW) on a simple cubic lattice, the objective being to estimate the position and lifetime of the fluorophore. This can provide information related to local variations in pH and temperature with potential medical significance. Aspects of the theory were tested using time-resolved measurements of the fluorescence from small inclusions inside tissue-like phantoms. The experimental results were found to be in good agreement with theoretical predictions provided that the fluorophore was not located too close to the planar boundary, a common problem in many diffusive systems. 相似文献
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建立有限元模型,通过求解瞬态辐射传输方程模拟短脉冲激光在半透明介质中的传输.针对散射占优性半透明介质内辐射传输求解效率较差的问题,采用扩散综合加速迭代算法,提高计算效率,缩短计算时间.结果表明:采用精确解析式描述脉冲激光散射源项的求解策略可以获得准确的计算结果,精确地模拟快速变化的波前,不会产生数值扩散和数值振荡.此外,扩散综合迭代算法的计算时间仅为源项迭代的50%~60%. 相似文献