A Macro-Monte Carlo method for the simulation of diffuse light transport in tissue.

Abstract

The Monte Carlo (MC) method of calculating light distributions in turbid media such as tissue has become the gold standard, especially in complex geometries and heterogeneous tissue. The utility of the MC method, however, is limited by is computational intensity. In an effort to reduce the time needed for MC calculations, we have adapted a macro-Monte Carlo (MMC) method (Neuenschwander, et al. 1995, Phys. Med. Biol. 40, 543-574) to the solution of tissue optics problems. Traditional MC routines trace individual photons step-by-step through the tissue. Instead, the MMC approach relies on a data set consisting of spheres in which the light absorbed in each voxel is pre-calculated using a traditional MC routine. At each MMC step, the pre-calculated absorbed light dose in the appropriate sphere, aligned to the current position and direction of the sphere, is recorded in the dose matrix. The position and direction of the photon exiting the sphere are chosen from the exit distribution of the pre-calculated sphere, and the process is repeated. By choosing the size of the pre-calculated sphere appropriately, arbitrarily complex boundary geometries can be simulated. We compare the accuracy and calculation time of the MMC method with a traditional MC algorithm for a variety of tissue optical properties and geometries. We find that the MMC algorithm can increase the speed of calculation by as much as two orders of magnitude, depending on the optical properties being simulated, without a significant loss in accuracy.