Background: Simulation of optical light in objects with high heterogeneity, such as biological tissue, has been a long-standing challenge in the field of biomedical engineering. Many of the recent groundbreaking bioengineering technologies utilize some form of biomarker, many of which emit optical light, such as fluorescence, as their signal. Thus simulating the nature of the light travelling through biological tissue has very significant implications for the future of biomedical engineering research.

TIM-OS: For many years, the main limitation to light (photon) simulation has been the highly variable nature of photon propagation in biological tissue (scattering), which makes the determination of the ray-boundary intersection prohibitively expensive from a computational perspective. The "TIM-OS" algorithm we proposed is an unique algorithm for light propagation in highly heterogeneous domain. In this algorithm, the photon propagation is guided by a tetrahedral mesh. Since the photon is highly scattered, the average step size of a photon is much smaller than the size of a tetrahedron, thus only one tetrahedron requires analysis per step. And, each tetrahedron has only four triangular surfaces, so it is a fairly straightforward geometry to compute the photon surface interaction. 

Features of Tetrahedron-based simulation algirhtm:

  • Support highly complex tissues and organs.
  • Deal with complex optical property condition.
  • Fast and efficient photon-tetrahedron intersection algorithm.
  • Validated accuracy in simple and general cases.


  1. H. Shen and G. Wang. Tetrahedron-based inhomogeneous Monte-Carlo optical simulator. Phys. Med. Biol. 55:947-962, 2010.
    Featured Article in Physics in Medicine & Biology 2010
  2. Y. Lu, B. Zhu, H. Shen, J.C. Rasmussen, G. Wang, E.M. Sevick-Muraca. Parallel Adaptive Finite Element Simplified Spherical Harmonics Approximations Solver for Frequency-domain Fluorescence Molecular Tomography. Physics in Medicine and Biology, 55:4624-4645 (2010).
    Featured Article in Physics in Medicine & Biology 2010
  3. W.C. Vogt, H. Shen, G. Wang, and C.G. Rylander. Parametric study of tissue optical clearing by localized mechanical compression using combined finite element and Monte Carlo simulation. Journal of Innovative Optical Health Sciences, 3(3):203-211, (2010).
  4. H. Shen, G. Wang. A study on tetrahedron-based inhomogeneous Monte-Carlo optical simulation. Biomedical Optics Express, 2(1): 44-57 (2011).
  5. H. Shen and G. Wang. Reply to “Comment on ‘A study on tetrahedron-based inhomogeneous Monte-Carlo optical simulation’”. Biomedical Optics Express. 2(5):1265-1267 (2011).

For a brief overview, please read this poster: TIM-OS: A General Monte Carlo Optical Simulator for Biomedical Optics.

For detail information about how to implement a Monte Carlo simulation, please ref to the original " MCML" papers by Dr. Lihong Wang. 

Contact: Dr. Haiou Shen (hhshen at and Dr. Ge Wang (wangg at