We investigate photoacoustic (PA) signal magnitude variation to an absorption coefficient of localized absorbing objects measured by spherically focused ultrasound transducers (US TDs). For this investigation, we develop the PA simulation method that directly calculates Green function solutions of the Helmholtz PA wave equation, considering grid-like elements on absorbing objects and US TDs. The simulation results show that the PA signal amplitude in the PA imaging is nonlinearly varied to the absorption coefficient of localized objects, which are distinct from the known PA saturation effect. For spherical objects especially, the PA amplitude shows a maximum value at a certain absorption coefficient, and decreases even though the absorption coefficient further increases from that point. We suggest conceptual and mathematical interpretations for this phenomenon by analyzing the characteristics of PA spectra combined with US TD transfer functions, which indicates that the combined effect of US TD spatial and temporal filtering plays a significant role in the PA signal magnitude nonlinearity.

中文翻译：

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在光声成像中信号幅度非线性对吸收系数的影响。

我们调查光声（PA）信号幅度变化到由球形聚焦超声换能器（US TDs）测量的局部吸收物体的吸收系数。为了进行这项研究，我们开发了一种PA模拟方法，该方法直接计算了Helmholtz PA波动方程的格林函数解，考虑了吸收物体和US TD上的网格状元素。仿真结果表明，PA成像中的PA信号幅度随局部物体的吸收系数非线性变化，这与已知的PA饱和效应不同。特别是对于球形物体，PA振幅在一定的吸收系数下显示出最大值，即使从该点开始吸收系数进一步增加，PA振幅也会减小。