Optical fluence distribution study in tissue in dark-field confocal photoacoustic microscopy using a modified Monte Carlo convolution method.

We have modified the existing convolution method of the Monte Carlo simulation for finite photon beams with both translational and rotational invariance. The modified convolution method was applied to simulate the optical fluence distribution in tissue in dark-field confocal photoacoustic microscopy. We studied the influence of the size of the dark field and the illumination incident angle on the depth position of the effective optical focus (the region with the highest fluence) and the fluence ratio (the ratio of the optical fluence at the effective optical focus inside the tissue to the optical fluence on the tissue surface along the ultrasonic axis). Within the reach of diffuse photons, the depth position of the effective optical focus increases with the size of the dark field and is much less sensitive to the incident angle. The findings show that, while the fluence at the effective optical focus decreases, the fluence ratio increases with the size of the dark field. The incident angle has a weaker influence on the fluence ratio than the size of the dark field does. An incident angle between 30 and 50 degrees gives the highest fluence at the effective optical focus.