Evaluation of finite-element-based simulation model of photoacoustics in biological tissues

A finite element (FE)-based simulation model for photoacoustic (PA) has been developed incorporating light propagation, PA signal generation, and sound wave propagation in soft tissues using a commercial FE simulation package, COMSOL Multiphysics. The developed simulation model is evaluated by comparing with other known simulation models such as Monte Carlo method and heat-pressure model. In this in silico simulation, FE model is composed of three parts of 1) homogeneous background soft tissues submerged in water, 2) target tissue inclusion (or PA contrast agents), and 3) short pulsed laser source (pulse length of 5-10 ns). The laser point source is placed right above the tissues submerged in water. This laser source light propagation through the multi-layer tissues using the diffusion equation is compared with Monte Carlo solution. Photoacoustic signal generation by the target tissue inclusion is simulated using bioheat equation for temperature change, and resultant stress and strain. With stress-strain model, the process of the PA signal generation can be simulated further in details step by step to understand and analyze the photothermal properties of the target tissues or PA contrast agents. The created wide-band acoustic pressure (band width > 150 MHz) propagates through the background tissues to the ultrasound detector located at the tissue surface, governed by sound wave equation. Acoustic scattering and absorption in soft tissues also have been considered. Accuracy and computational time of the developed FE-based simulation model of photoacoustics have been quantitatively analyzed.