Boundary discretization of light propagation in skin tissue: Problem and countermeasure

In the Monte Carlo (MC) simulation of light propagation in heterogeneous skin tissues, three algorithms have been developed to compute photon deposition including geometry-based MC (GMC), voxel-based MC (VMC) and Tetrahedron-based MC (TMC). In GMC, the interface is defined mathematically without any discretization. It is most accurate but not applicable to more complicated boundaries. In mesh-based methods as VMC or TMC, structured voxels or unstructured tetrahedrons are adopted to approximate the interface. The implement of VMC is simple. However, it may lead to non-neglected errors due to zigzag polygonal interface. Comparing to GMC and VMC, TMC provides a balance between accuracy and flexibility in the treatment of photon-boundary interaction by the shape-changeable tetrahedron cells. The numerical results show that TMC can achieve the same precision as VMC by less than one seventh cells with local refinement. The errors of photon deposition on vessel in VMC and TMC are investigated by the light propagation in two layered skin model with one cylinder vessel. Relative errors by vessel position translation are much more severe in VMC (from-16% to-18%) than those in TMC (from-8% to-12%). For smaller diameter case, the error caused by vessel rotation is much higher in VMC (-10% ~ 3.4%) than TMC (0.5% ~ 2.6%). Unlike VMC, no remarkable fluctuations are observed in TMC when the geometrical parameters change. It is shown that TMC is a superior to the light propagation in skin tissue with better approximation of curved boundary, yielding more accurate computation of photon reflection, refraction and energy deposition.