Monte Carlo simulation of light transport in dark-field confocal photoacoustic microscopy

A modified MC convolution method for integration extension of MC simulation is developed for finite photon beam with random shape of translational or rotational invariance, which is proven consistent with the conventional convolution extension of MC simulation for normal incident finite beam. The method is applied to analyze the positions of fluence foci and ratios of fluence at the focus and surface which are two key factors in the application of dark-field confocal and some interesting points are presented including: 1) The fluence profile has a saddle-like shape with highest peak in the bright field and low valley near the surface and a second rise in the center of dark field which is defined as the effective optical focus; 2) Besides a little peak near zero inner radius, the ratio of fluences at the focus and surface increases linearly with the inner radius, suggesting the large inner radius more advantageous to image at the effective optical focus; 3) The position of effective optical foci deepens linearly with the increase of the inner radius, suggesting that to get a high quality image of deeper target, a dark-field with larger size is more beneficial. But the position of fluence foci are far away from the foci of geometrical laser beam in high scattering tissue, so aligning the foci of geometrical laser beam and acoustic transducer doesn't guarantee that effective optical focus is accurately overlapping with the acoustic focus. An MC simulation with integration extension presented in this paper maybe helpful to determine where the acoustic focus should be to maximize the SNR in tissue imaging; 4) incident angle makes little difference to ratio of fluences at the focus and surface and an incident angle between 30 and 50 degrees gives the highest fluence at the effective optical focus; 5) the depth of fluence focus is insensitive to the incident angle.