Research On Image Reconstruction Method For Endoscopic Photoacoustic Tomography Considering Characteristics Of Ultrasonic Detectors

Endoscopic photoacoustic tomography(EPAT)is a functional imaging modality for biological cavity.For simplicity,the ultrasonic detector is usually assumed to be an ideal point detector with omnidirectional response in the image reconstruction methods.The influence of the spatial impulse response(SIR)and electrical impulse response(EIR)of the detectors on reconstruction quality is not taken into account.However,in practice,these ideal assumptions will lead to the degradation of image quality and the reduction in the imaging accuracy.This thesis aims at image reconstruction for EPAT considering the characteristics of ultrasonic detector in order to improve the quality of reconstructed images.The main work includes two aspects.First,a model-based image reconstruction method is proposed considering the responses of the detectors.A forward imaging model is built physically describing the generation of the acoustic pressure due to optical absorption and thermoelastic expansion,the propagation of acoustic waves in tissues,and the acoustic measurement.The images representing the distribution maps of the optical absorption energy density on cross-sections of the imaged luminal structures are reconstructed through iterative inversion of the forward model.Second,an image reconstruction method considering the directivity of the detectors is presented.The theoretical sampling voltage signals output by the detector are obtained according to the relationship between the photoacoustic signals and the directivity and responses of the detectors.The image reconstruction is then achieved through iterative optimization minimizing the error between the theoretical and measured sampling voltage signal.The validity of the methods is verified through computer simulation and phantom demonstration.Comparison demonstrations are also conducted comparing the proposed methods with the conventional back projection(BP),time reversal(TR)and different iterative optimization algorithms.The results indicate that the proposed methods can improve the image quality effectively compared with the standard reconstruction methods.