两层生物组织光学特性参数无损测量的模拟研究

Simulation Study of Noninvasive Determination of the Optical Properties of Two-layered Turbid Media

生物组织的光学特性参数是与疾病的无损光诊断有关的重要的光学参数 ,一般的测量所应用的模型是假设生物组织为半无限厚的均匀结构。为了更能反映生物组织的真实特性 ,本文应用两层结构模型 ,即假设第一层为有限厚的均匀组织 ,第二层为半无限厚 ,由漫射方程经过傅立叶变换得到传输方程的漫射解。为检验漫射解的准确性 ,将漫射解的表面漫射光分布与MonteCarlo模拟结果进行了比较 ,结果表明两者符合得很好。为了研究由漫射解是否可以获得两层组织的光学特性参数 ,将漫射解与MonteCarlo模拟数据进行了非线性拟合 ,反演获得组织的光学特性参数 ,结果表明 ,由漫射解可以获得两层组织的有效散射系数和吸收系数。

Much effort has been devoted in recent years to quantify the optical properties of superficial biological tissue from spatially resolved reflectance measurement, as they can be used to infer important physiological information. Some examples include the noninvasive measurement of the concentration of chemotherapy drugs or exogenous chromophores used for photodynamic cancer treatment and the assessment of hemoglobin oxygenation in tissue. The diffusion approximation to the Boltzmann equation has become the current standard for making theoretical predictions of spatially resolved reflectance because of its higher mathematical tractability compared with the general transport equation. In this regime, the parameters of interest are the absorption (μ α) and the effective isotropic transport scattering ( μ′ s) coefficients. To a first crude approximation, superficial tissue can be considered as a semi-infinite turbid medium. Diffuse reflectance models have been developed for that geometry. In reality, superficial tissue is a multi-layered structure with each component having a different μ α and μ′ s. Recovery of the complete spatial distribution of μ α and μ′ sis the goal of optical tomography, and it is an open question as to whether this can be accomplished with sufficient accuracy. In an attempt to mimic tissue structure more realistically than the semi-infinite medium model, an improved, yet still crude, two-layer turbid medium model of tissue structure can be utilized. In this model, a top-layer thickness of 1～10mm that includes the epidermis and the dermis lies on top of a semi-infinite homogeneous medium that could be fat or muscle. In this article, a solution of the diffusion equation for a two-layered turbid medium having a semi-infinite second layer is derived by using the Fourier transform. Naturally, the assumptions that are inherent in each model will be reflected in the accuracy of its predictions and, consequently, the accuracy of the optical properties that can be inferred from it. The comparisons of the reflectance in the steady-state showed that the derived solutions of the two-layer model are close to the results obtained from two-layer Monte Carlo simulations. Thus, for many applications the reflectance calculated with the derived solution is exact enough to replace the time-consuming Monte Carlo simulations. Nonlinear regressions of these solutions of the diffusion equation to reflectance data obtained from two-layer Monte Carlo simulations in the steady-state showed the absorption and the effective isotropic transport scattering coefficients of the two layers can, in principle, be obtained. It was also shown by fitting the solutions of the diffusion equation to results obtained from Monte Carlo data that the optical parameters are obtained with different accuracy.