The optical parameters and structural characteristics of biological tissue are an important basis for medical clinical diagnosis. Biomedical optical imaging technology has gradually become a new pioneering field developing rapidly in the international arena due to its advantages of safety, non-destructive characteristics, high detection sensitivity, and realization of functional imaging in tissue. However, pure optical imaging techniques such as optical coherence tomography and diffusion optical tomography are unable to obtain excellent imaging resolution and large imaging depth at the same time. Therefore, acousto-optical tomography (AOT) which is a hybrid imaging method emerges. It can offer a great spatial resolution of imaging (sub-millimeter level) in deeper tissue (centimeter-level) because the ultrasound has low scattering in tissue to locate the scattered light. Therefore, it is a promising non-destructive imaging technology for biological tissue. However, the research process of AOT faces two main problems, including the research on acousto-optical interaction mechanisms in tissue and the extraction of weak modulated light signals in strong background light. In this paper, COMSOL Multiphysics software is used to simulate the acousto-optical interaction process in various structural tissue. It is a new approach to studying the acousto-optical interaction mechanism. The simulation results can provide an important basis for data analysis and final image processing and reconstruction by AOT.

The acousto-optical interaction mechanism can be divided into coherent modulation mechanism and incoherent modulation mechanism in terms of the choice of light sources and the modulation effect of ultrasound. The incoherent modulation mechanism mainly refers to the change in light energy caused by the periodic change in optical parameters (including absorption coefficient, scattering coefficient, and refractive index) of tissue under the effect of an ultrasonic field. It does not require coherent light as the light source, so it belongs to intensity modulation. On this basis, this study adopts the finite element method for simulation, defines the optical and acoustic fields separately by using the diffuse equation with the extrapolated boundary and the ultrasonic theory, and performs multi-physics field coupling based on the incoherent modulation mechanism. Firstly, the influence of light beams on the distribution of light in the tissue and the characteristics of axial and radial light distributions are analyzed. Then, the relationship between the acousto-optical signal and the ultrasonic signal is discussed, and the simulation results are verified by experiments. Finally, the effect of optical parameters on the acousto-optical signal in monolayer tissue is investigated, and the effect of optical parameters of target and non-target tissue on the acousto-optical signal in multilayer tissue is discussed.

Firstly, the fluence rate distribution under different beam radiation is compared in the simulation. As the beam width increases, the peak fluence rate gets smaller, but the distribution is wider. In addition, the fluence rate decays more rapidly in the radial direction, but this trend decreases as the beam width increases (Fig. 5). The decrease in radial fluence rate slows down with the increase in axial distance. When the radial distance keeps increasing, the decreasing rate of the axial fluence rate also decays, while there is a rising phase of the fluence rate change during the increase in the axial distance and a peak near the equivalent light source (Fig. 6). Secondly, the acousto-optical signal is defined as the cumulative result of modulated fluence rates at various points inside the tissue, and its waveform is very similar to the ultrasonic waveform, with the frequency the same as the ultrasound frequency (Fig. 7). The acousto-optical signal observed in the experiments also has this characteristic (Fig. 7), and the simulation results are in good agreement with the experimental results. Finally, the influencing factors of the acousto-optical signal are evaluated in the tissue. In monolayer tissue, the mean and peak-to-peak values of the acousto-optical signal decrease exponentially, and the modulation depth increases linearly with the increase in the absorption and scattering coefficients, while the mean and peak-to-peak values of the acousto-optical signal increase exponentially, and the modulation depth decreases linearly with the increase in the anisotropy factor (Fig. 10). In multilayer tissue, the relationship between the optical parameters of the target tissue and the acousto-optical signal is no different from that of the monolayer (Fig. 11). However, when the optical parameters of the non-target tissue in multilayer tissue changes, the modulation depth remains stable (Fig. 12).

It is urgent to explore the acousto-optical interaction mechanism in the process of AOT research. Current modeling methods are mostly based on the coherent modulation mechanism and use the traditional Monte Carlo method for numerical simulation. In this paper, a multi-physics field coupling model is constructed by using the finite element method to model the acousto-optical interaction process in different structural tissue based on incoherent modulation. The characteristics of axial and radial light distributions and the influence of optical parameters of monolayer and multilayer tissue on the acousto-optical signal are discussed. The simulation results show that the change in beam size affects the attenuation speed of axial and radial light and the location of the peak value of axial energy flow rate; the peak-to-peak value, average value, and modulation depth of the acousto-optical signal change regularly with the change in optical properties of monolayer tissue. For multilayer tissue, the modulation depth of the acousto-optical signal only depends on the optical properties of the target tissue and has nothing to do with the optical properties of non-target tissue. It has great interference resistance and is only related to the optical parameters of the target tissue, which has a simple linear relationship, or in other words, the difference in the optical parameters of the tissue at the ultrasound localization can directly characterize the difference in modulation depth. Therefore, it is a favorable reference index in the image processing and reconstruction process of AOT.