Research On Photoacoustic Computed Tomography System Based On A Finite-Aperture Ultrasound Transducer

Photoacoustic computed tomography(PACT)is a novel non-invasive medical imaging technique that uses a pulsed laser as an excitation source to image biological tissues by detecting the ultrasound generated due to light absorption.PACT combines the advantages of optical imaging with high resolution and acoustic imaging with high penetration depth,enabling high sensitivity and specificity detection of pathological tissues.PACT is the most widely used type of photoacoustic imaging in clinical medicine,oncology,and cell biology,providing multi-dimensional biological information from cells,tissues,and organs to whole small animals.In PACT image reconstruction,to simplify the problem,it is usually assumed that the ultrasound transducer is an ideal point transducer with an isotropic response,forming a continuous and complete measurement surface around the target,and without considering the spatial pulse response and electrical pulse response of the transducer affecting the imaging quality.However,in practical applications,this ideal assumption affects the receiving angle and signal delay accuracy,leading to a decrease in imaging resolution and image quality,resulting in the finite aperture effect.Specifically,under a given transducer bandwidth,the tangential resolution of the imaging will decrease when the target is far away from the center of the circular scanning trajectory or close to the transducer surface.This dissertation focuses on four aspects of research: image reconstruction algorithms,system parameter optimization,ultrasound transducer design,and imaging system construction.Addressing the problem of insufficient image resolution caused by the finite aperture effect,this dissertation proposes a back-projection reconstruction algorithm based on spatial pulse response and a fast phase correction back-projection reconstruction algorithm.In terms of solving imaging quality issues,this work systematically studies anti-aliasing methods based on finite aperture transducers.To overcome the problems of imaging speed and image quality,a dual-frequency broadband ultrasound transducer is designed and developed.Based on this,a dual-frequency high-performance system based on finite aperture is constructed.System parameters are optimized to improve system sensitivity and imaging speed,resulting in high-resolution and high signal-to-noise ratio images.The main innovations and contributions of this thesis are as follows:(1)We proposed a spatial pulse response-based back-projection reconstruction algorithm that achieves high imaging resolution for targets at various positions in the tangential direction.To further improve imaging speed and tangential resolution while addressing the limited aperture effect in traditional reconstruction algorithms,we proposed a spatial pulse-echo response-based back-projection reconstruction algorithm.This method comprehensively considered the shape and size of the ultrasound transducer detection surface,as well as the spatial pulse-echo response function of the transducer,to achieve high imaging resolution for targets at various positions in the tangential direction.The stability and imaging speed of this method was verified in 2D photoacoustic computed tomography simulations.Our results demonstrated the effectiveness of the proposed algorithm in addressing the limited aperture effect and improving imaging quality in photoacoustic computed tomography.(2)We proposed a fast phase correction back-projection reconstruction algorithm based on a focused acoustic beam model.Through method discussion,a simulation study on the impact of various system parameters,and the establishment of an image quality evaluation system,we proposed a fast phase correction method based on a focused acoustic field model.This algorithm extends the application range of planar ultrasonic transducers,including pointfocused ultrasonic transducers and divergent ultrasonic transducers.Experimental results show that the fast phase correction backprojection reconstruction algorithm based on the focused acoustic field model has good stability and high imaging speed in both phantoms and in vivo photoacoustic tomography imaging.(3)Two methods were proposed to accurately model the aperture of the transducer in image reconstruction.Therefore,further comparisons were made by numerical simulation and experimental validation to investigate the differences between a limited-aperture photoacoustic computed tomography(PACT)system and an ideal point-source PACT system under different system parameters,including aperture size,center frequency,number of transducers,and scanning radius.In addition,the impact of the limited-aperture transducer on the required number of transducers to combat spatial aliasing noise was studied.The study showed that a limited transducer aperture can produce high-quality imaging regions,and the limited transducer aperture can determine the optimal number of transducers.(4)After correctly modeling the aperture of the transducer and thoroughly investigating the parameters of PACT systems based on limited aperture transducers,a stacked dual-element transducer was designed and fabricated to improve the bandwidth coverage of the transducer.The low-frequency sensing element provided enhanced photoacoustic sensitivity,while the high-frequency sensing element provided excellent spatial resolution for high-resolution imaging.Compared with traditional single-element transducers,the dualelement transducer was able to effectively acquire wideband signals from complex targets.Experimental results demonstrated that the stacked dual-element transducer enhanced the performance of the PACT system,enabling multi-scale analysis of the rat brain vascular network and effective evaluation of blood oxygen saturation using a multi-wavelength imaging scheme.The system could rapidly acquire morphological information about blood vessels,enabling the diagnosis and monitoring of vascular diseases and their effects on surrounding tissues.Additionally,the system could also be used to study and analyze the function and metabolism of blood vessels,such as blood oxygen saturation,providing effective assistance for the treatment and prevention of vascular diseases.