Simulation study of second-harmonic microscopic imaging signals through tissue-like turbid media.

We establish, for the first time, a simulation model for dealing with the second-harmonic signals under a microscope through a tissue-like turbid medium, based on the Monte Carlo method. With this model, the angle-resolved distribution and the signal level eta of second-harmonic light through a slab of the turbid medium are demonstrated and the effects of the thickness (d) of the turbid medium, the numerical aperture (NA) of the objective as well as the size (rho) of the scatterers forming the turbid medium are explored. Simulation results reveal that the use of a small objective NA results in a narrow angle distribution but strong second-harmonic signals. A turbid medium consisting of large scattering particles has a strong influence on the angle distribution and the signal level eta, which results in a low penetration limit for second-harmonic signals made up of ballistic photons. It is approximately 30 microm in our situation.