Photoacoustic Microscopy Imaging Method Based On Improved Back Projection Algorithm

In acoustic resolution based photoacoustic microscopy(ARPAM),although the conventional B-mode imaging algorithm with acoustic linear propagation model is simple and efficient,it can only obtain high spatial resolution within the focus depth of the employed focused transducer,which means its lateral resolution will be seriously reduced in the out-of-focus regions,which greatly limits the imaging depth.In order to overcome the contradiction between depth of focus and lateral resolution in current ARPAM,this thesis proposes an improved back projection reconstruction algorithm,which effectively improves the lateral resolution in the out-of-focus regions.In the reconstruction process,the contribution of the shape of the detection surface of the focus transducer is considered,and the synthetic aperture method is used to calculate the weighted sum of the signals received by the transducer from different angles to get the reconstruction value of a single pixel.In the existing reconstruction algorithms for ARPMA,the synthetic aperture focusing technology(SAFT)also can improve the lateral resolution in the out-of-focus regions.In this thesis,the advantages and disadvantages of the traditional B-mode algorithm,the improved back projection algorithm and the SAFT algorithm will be analyzed and evaluated by numerical simulation and simulation experiments,so as to lay a foundation for the application of ARPAM in biomedicine.The content of this thesis is divided into four parts: firstly,the numerical simulation of the photoacoustic signal in ARPAM is carried out with a linear model,so as to investigate the influence of the parameters of the ultrasonic transducer in ARPAM,as well as the position of the imaging target on the resulted photoacoustic signal.In particular,according to the application conditions of ARPAM in the existing laboratory,the lateral resolution of the system in various depths was investigated within a certain scanning range.The second part is the establishment of the improved back-projection ARPAM algorithm.In this work,the improved back-projection imaging algorithm is used in ARPAM for the first time.The detection surface of focused ultrasonic sensor is divided into finite elements.Each element can be regarded as a point detector,and then the signal is back projected with weighted summation to get the final image,so that the lateral resolution in the out-of-focus regions is improved;then,the traditional B-mode algorithm,SAFT algorithm and the improved back-projection algorithm were used for the reconstruction with the above simulated data,and the results were compared and analyzed;finally,phantom experiments were carried out,with metal wires buried at the different depth as the imaging targets,and the three algorithms were employed for reconstruction with the collected data,and compared.Results show that the improved back-projection algorithm is better than the traditional B-mode algorithm in improving the lateral resolutions,and better than the SAFT algorithm in overall image quality,as validated with both simulation results and experimental results.