Research On Axial High-resolution Photoacoustic Microscopy Imaging Method

In recent years,photoacoustic microscopy has gradually become an important tool for biological microcirculation network research due to its high optical absorption contrast and ultrasound penetration depth,without the need of exogenous labels,imaging the structure and function of biological tissues.In traditional photoacoustic microscopy,the lateral resolution of the system is determined by the lateral size of the focused light spot,which can reach sub-micron level;the axial resolution is determined by the detection bandwidth of the ultrasonic transducer,which is often worse than ten microns,making it hard for the system to perform three-dimensional high-resolution imaging.In recent years,a variety of technologies have been proposed to improve the limited axial resolution,but there are still some drawbacks,such as the need to replace with a higher-performance probe or imaging medium,causing light damage to the tissue,and sacrificing the lateral resolution in exchange for the increase in axial resolution.In view of the shortcomings of the current technology,the following researches are carried out to focus on the limited axial resolution in photoacoustic microscopy.To avoid the shortcomings of optical damage in the existing solutions to improve the axial resolution of photoacoustic microscopy,this thesis establishes and optimizes a set of axial structured light modulated photoacoustic(PA)signal methods Based on the Moiréeffect,by combining the principle of structured light modulation with the linear photoacoustic microscopy system,the theoretical model of PA signal modulated by the axial structured light is established.By exciting the sample with a series of cosine-structured beams with different frequencies,the high-frequency information of the PA signals can be gradually modulated to the low-frequency detection band of the ultrasonic transducer.Without facing the tissue’s strong loss of high-frequency information,the encoded highfrequency information is obtained.Simultaneously,a set of reconstruction methods suitable for PA signals modulated by structured light are provided to restore high frequency information,broaden the detection range of frequency domain of the detector,and improve the axial resolution of the system.Furthermore,with the help of k-wave toolbox,the physical transmission process of the PA signal modulated by the axial structured illumination is simulated and verified.The results show that,after being reconstructed by the algorithm,the axial resolution of the PA signal modulated by the axial structured light is improved by nearly 1.6 times to that of the PA signal excited by the uniform illumination.To avoid the problem of poor lateral resolution inherent in the orthogonal axial structured light modulation PA signal scheme,this thesis provides a coaxial axial structured light realization scheme based on the double ring slit method.By using the Collins diffraction integral method,the three-dimensional light field of the axial structured illumination generated by the double ring slit method is derived and optimized.And the physical process has been simulated and verified by Zemax optical software.Furthermore,combing with the lens and grating method,the phase diagrams of the double ring slits corresponding to axial structured light with different frequencies and phases are obtained.Finally,a set of illumination system was built to obtain the axial structured light,and the performance of axial structured beam with different frequencies and phases was measured.To study and verify the real performance of the structured light modulation PA signal scheme,a set of transmissive axial high-resolution photoacoustic microscopy system is developed.By combining the principle of axial structured light modulation with the ultrasonic detection,the limitation of the axial resolution of the photoacoustic microscopy determined by the detection bandwidth of the ultrasonic transducer is broken,and the axial high-resolution PA signals are obtained.Under the 9 ns pulse width,through quantitative analysis of the hilbert envelope of the PA signal,it is found that the axial structured light modulation method can increase the axial resolution of the system by about 1.5 times compared with uniform light.The frequency spectrum of the reconstructed signal has also been broadened accordingly.The above experimental results show that the method of PA signal modulated by the axial structured light has great potential in the field of photoacoustic microscopy.