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The study of the relationship between the spectral characteristics of the photoacoustic signal and the shape and size of the absorber has important practical significance for image reconstruction. Using the commercial finite element simulation software COMSOL Multiphysics, a two-dimensional simulation model based on finite element was designed, which studied the relationship between the spectral characteristics of the photoacoustic signal and the shape and size 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. Simulate the propagation of light in the water layer by solving the diffusion equation. The temperature changes in biological tissues are obtained by solving the biothermal equation. When the absorber is irradiated by Gaussian pulses, 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 numerical calculation of partial differential equations to obtain the photoacoustic signal. Fitting the simulation results shows that the spectral characteristics of the photoacoustic signal change regularly with the size of the absorber. The size of the absorber obtained in this paper has a power function relationship with the spectral intercept. The larger the size, the larger the spectral intercept, and the growth rate increases with the increase of the size. The size of the absorber and the spectral slope also have a power function relationship. The slope of the large spectrum is smaller, and the rate of change of the slope decreases as the size increases. At the same time, analyzing the photoacoustic spectrum of absorbers of different shapes also shows that absorbers of different shapes have their own characteristics. This research is helpful to understand the relationship between spectral characteristics and the shape and size of the absorber, and has certain theoretical guiding significance for image reconstruction.
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