Study Of Photoacoustic Signals In Biological Tissues Including Tumors With Different Physical Properties

The laser ultrasound generation by thermoelastic mechanism and the influence factor on the laser ultrasound are studied by using the finite element method and experiment. The elastic modulus, the optical absorption and the size of tumor are introduced into the models as the influence factors.Taking into account the effects of thermal diffusion, thermoelastic effect, as well as optical penetration, a numerical model dealing with laser-generated ultrasound in soft tissue is presented. And the transient temperature distributions and the generation of laser ultrasonic force in the soft tissues are simulated.On this basis, the relationship between the elastic modulus of tumor and the generation and propagation of ultrasound waveforms is analyzed. Theoretical and experimental results show that there is a linear positive correlation between the maximum compressive stress and the elastic modulus of tumor; the full width at half maximum and the rise time of laser ultrasound signal waveform are negative connected with the elastic modulus of tumor. Meanwhile, the percentage of the received ultrasonic signal at higher frequency region increases as the elastic modulus of tumor become higher. We put forward a conclusion that it is possible and necessary to add elastic modulus as the imaging parameter, and it is a complement of the optical absorption coefficient parameter to improve the diagnosis rate of tumor in imaging system.The relationship between the optical absorption of tumor and the generation of laser ultrasonic force power source, the waveform and the frequency spectrum of the photoacoustic signals are analyzed. We obtain that there is a linear correlation between the maximum compressive stress and the optical absorption of tumor. We found that the optical absorption of tumor not only has influence on the photoacoustic signal amplitude but also has effect the polarity of the signal. The waveform becomes bipolar when the biological tissue containing tumor with larger optical absorption.Considering the differences of the physical parameters between the tumor and adipose tissue, the effect of tumor size on photoacoustic signal waveform is obtained, thus we put forward that the photoacoustic signal amplitude is correlated not only with the optical absorption of tumor, but also with tumor size. Based on the comprehensive analysis of the effect of elastic modulus of tumor on the photoacoustic signal, the existing studies on the relationship between the duration time of the photoacoustic signal and tumor size is amended.Results of this study suggest that this study can provide theoretical and experimental basis for further research in photoacoustic imaging.