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Photoacoustic imaging (PAI) has gradually developed into a new and important imaging technology, which combines the high contrast of optical imaging and the high resolution of ultrasonic imaging to achieve a deeper imaging of biological tissue, and has been widely used in biological imaging. The basic principle of photoacoustic imaging is that a short pulse of laser is used to illuminate biological tissue. As photons pass through biological tissue, some of them are absorbed by biological molecules such as hemoglobin, DNA-RNA, fat, water, melanin and cytochrome. In PAI, the light absorption mechanisms generally include electron absorption, vibration absorption, stimulated Raman absorption and surface plasmon resonance absorption. The absorbed light energy is usually completely converted to heat energy by the non-radiative relaxation of the excited molecules, and the pressure wave caused by the heat energy is transmitted in the tissue to form ultrasonic wave, namely the photoacoustic signal. The photoacoustic signal reconstruction generated by the detection of ultrasonic transducer can be obtained from biological tissue. The absorption of blood in visible band is much higher than that of other tissues (except melanin) to achieve high resolution imaging of blood vessels without exogenous markers. Due to the characteristics of biological tissue, it has different degrees of absorption to different wavelengths of light, resulting in different heat and pressure waves generated, the final received ultrasonic wave is different, which will affect the final image results. Based on the finite element analysis method, this paper establishes four complete coupling modules in COMSOL software: coefficient form partial differential equation module, biological heat transfer module, solid mechanics module and transient pressure acoustic module. By setting different optical parameters to explore the degree of absorption of different light waves by biological tissues, so as to find the best wavelength of light for the experiment.
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