Efficient Monte Carlo simulation of confocal microscopy in biological tissue

A variance-reduction technique is described that greatly improves the efficacy of Monte Carlo simulations of reflection-mode confocal microscopy in anisotropically scattering media. The efficiency gain is large enough that the performance of confocal microscopes probing as deep as 5 scattering lengths can be simulated with a desktop computer. We use the technique to simulate the response of a true confocal microscope probing biological tissue, a problem that has been impractical to undertake by using conventional Monte Carlo methods. Our most important finding is that operation of a confocal microscope in the true confocal mode enables much more effective rejection of undesired scattered light than operation in the partially coherent mode, but the maximum probing depths of microscopes operated in either mode are similar (2-3) scattering lengths) in practice because of sensitivity limitations.