大空间生物组织透射的Monte Carlo模拟方法的加速

乳腺等类型生物组织的透射Monte Carlo模拟(简称MC模拟)耗时很长，这主要是由于被模拟的组织较厚和Monte Carlo模拟自身统计特性引起的所需光子数量多造成的。针对光源与接收面在同一直线上，采用光纤接收透射光的模型进行分析，通过考虑生物组织自身的光学特性，进行了边界约束和有效后向散射次数的限制来缩短仿真时间，经过多次实验验证，以两次散射所到位置为分界，并充分利用组织光学参数计算约束边界，计算相对比较简单，同时考虑到入射与出射的实际情况进行微扩，从而得到的约束边界。对光学参数相同但厚度不同的组织进行仿真，分析进入同一位置光纤的光子数所发生后向散射次数，发现后向散射次数随着组织厚度与散射系数的增大而增大，随着吸收系数与各向异性因子的增大而减小，通过限定后向散射次数来节省时间。仿真结果表明，在传统计算机上，该方法能够显著缩短MC模拟的运算时间50%左右，尤其适用于较厚且边界较为复杂的高散射物质的MC模拟。该提速方法主要应用于光源与接收面处于同一直线，且生物组织较厚，散射系数较大的情况，能够有效节约仿真所需时间，有利于分析组织透射成像。

The Acceleration of Monte Carlo Simulation for Optical Transmission in Large Space Biological Tissue

Typically,it is time-consuming to use Monte Carlo simulation to model light propagation in turbid media,such as breasts,mainly because of the thick simulated tissue and the large number of photons required by the stochastic nature of MC simulations.In this paper,in view of the light source and the receiver in a straight line,the transmitted light received by optical fiber model is analyzed;in terms of the optical properties of biological tissue itself,implementation of boundary constraint condi-tions and restriction of the number of backscatter events have been developed to reduce the simulation time.It is verified that the calculation is relatively simple by many experiments,when the position after two scattering is regarded as the demarcation and the boundary constraint conditions are efficiently solved by optical parameters of tissue respectively.At the same time,consider-ing the actual situation in incident and emergent position,the boundary constraint conditions are expanded.The number of back-scatter of every photon that received by optical fiber in the same position are recorded and tissues are selected different thickness and the similar optical parameters in the simulations.Additionally,it is found that the number of backscatter would increase as the depth of tissue or scattering coefficient increases,or as absorption coefficient or anisotropic factor decreases.Therefore,the time is saved further through limiting the number of backscatter.The simulation results show the combination of those new methods reduces the operation time of the MC simulation by 50% on a typical desktop computer.Those new methods are espe-cially suitable for MC simulations of the thick tissue or the scatterer with a complex boundary.This acceleration method is appli-cablewhen light source and receiver are in a straight line the biological tissue is thicker and scattering coefficient is large.It can effectively save needed time and be beneficial to analyze the transmission imaging.