Quantitative analysis of variation in photoacoustic guided wave characteristics with bone optical properties.

Tissue optical and mechanical properties are key indicators of bone health, and influence the generation and propagation of photoacoustic guided waves (PAGWs). The effectiveness of mechanical property characterization based on mode dispersion features of ultrasonic guided waves has been demonstrated in the diagnosis of osteoporosis; however, variation in PAGW characteristics with bone optical properties is less analyzed. It limits the development of ultrasound in precision diagnosis of bone diseases. In this study, the excitation of PAGWs was simulated using bulk thermal flux in the finite element method, taking into account the optical penetration depth in bone. The influence of laser parameters, as well as the absorption and scattering coefficients of bone, on the waveform and mode amplitude of PAGWs was analyzed. Research results indicate the mode components and their amplitude are influenced by the spatial distribution of thermal stress, force source of PAGWs, and wave structure of modes. The signal amplitude of PAGWs increases with the absorption coefficient, the radius and rise time of laser beam, while it decreases with the scattering coefficient. For in-plane and out-of-plane displacements, the optical properties exert different effects on the amplitudes of anti-symmetric and symmetric modes. The amplitude change ratio between A and S modes decreases with increasing optical penetration depth, exhibiting distinct trends with frequency for in-plane and out-of-plane displacements. These results demonstrate the relationship between mode amplitude of PAGWs and the optical properties of 1 mm thick bone plates, thereby enhancing the understanding of the theory of photoacoustic bone assessment.