Research On Acoustic Heterogeneity In Biological Endoscopic Photoacoustic Tomography

Biological photoacoustic tomography(PAT)is an ultrasound-based imaging technique.By considering the complexity of biological tissues,the acoustic properties of the imaged tissues are often assumed to be ideal during image reconstruction.For example,the speed of sound(SOS)in tissue is assumed to be a constant.The ultrasonic waves are assumed to propagate in free space and no scattering occurs.However,in actual applications,acoustic properties of biological tissues such as SOS,acoustic scattering and attenuation are usually unknown in priori.Furthermore,tissues with different composition always have different acoustic characteristics.The difference of SOS in soft tissues can even reach 10%.Consequently,the ideal assumption of homogeneous acoustic properties may lead to blurred details,misalignment of targets and artifacts in reconstructed PAT images..The purpose of this thesis is to compensate the acoustic heterogeneity including variable SOS and acoustic scattering in reconstructing endoscopic PAT(EPAT)images.First,the variable SOS is compensated with a delay compensation method according to the relationship between the ultrasonic propagation time.Second,the image artifact caused by acoustic scattering is reduced with an adaptive focusing method based on time reversal(TR)(AFM-TR)according to the time reversal invariance of the photoacoustic signals.The validity of the methods was demonstrated with computer-simulated arterial vessel-mimicking phantoms.The performance of the methods was compared with that of the methods under the assumption of acoustic homogeneity.The advantages and limits were discussed.The results show that the image quality can be improved and the misalignment of the target tissues caused by the variable SOS can be corrected with the method of delay compensation.Most image artifact caused by acoustic scattering can be effectively reduced with the AFM-TR.