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The study of the relationship between the spectral characteristics of the photoacoustic signal and the size, density and shape of the absorber is of great significance to image reconstruction and is of great practical significance to the better application of photoacoustic imaging in the medical field. This paper uses the simulation software COMSOL Multiphysics to design a two-dimensional simulation model based on finite element to study the relationship between the spectral characteristics of the photoacoustic signal and the properties of the absorber. In this study, the model consists of three parts: 1) water layer; 2) short pulse laser source (wavelength of 840nm); 3) gastric tumor tissue. The laser point source is located in the middle of the upper water layer. By solving the diffusion equation and the biological heat equation, respectively, the propagation of light in the water layer and the temperature change in biological tissues are simulated. When the absorber is irradiated by a Gaussian pulsed laser, due to the extremely short time, the absorber can be regarded as adiabatic expansion after absorbing energy, thereby generating ultrasonic waves. Using the finite element analysis method, the complex situation of photoacoustic imaging is transformed into the coupling of multiple physical fields, and the photoacoustic signal is obtained by numerical calculation of partial differential equations. Analyzing the simulation results, it is found that the spectral characteristics of the photoacoustic signal are closely related to the size of the absorber. The relationship obtained in this simulation experiment is: the spectral intercept and the size of the absorber have a power function relationship, the larger the size, the larger the spectral intercept , And the growth rate increases with the increase of the size; the spectral slope and the size of the absorber also have a power function relationship, the larger the size, the smaller the spectral slope, and the rate of change of the slope decreases with the increase of the size. For the relationship between the spectral characteristics and density of the photoacoustic signal, the intercept and slope characteristics are opposite. As the density of the photoacoustic absorber increases, the spectral slope of the photoacoustic signal increases, and the spectral intercept of the photoacoustic signal decreases. The photoacoustic signals and spectrograms of different shapes of absorbers have their own characteristics. The research results in this paper can promote the research of photoacoustic imaging and can better apply photoacoustic imaging in the medical field.
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