Photoacoustic Tomography In Biological Tissues

Photoacoustic tomography (PAT) is a novel biomedical imaging based on photoacoustic effect. It can combine the excellent spatial resolution in ultrasound imaging and the high contrast in optical imaging, and it has been attracting more and more attention. In PAT, pulsed laser is employed to radiate tissues to generate ultrasound, and the ultrasound is recorded by transducer to reconstruct the distribution of optical absorption within the tissues. It is usually assumed that transducer could record the complete data by full-view scanning and the tissues are acoustically homogeneous. However, in practical, transducer cannot scan in full-view way but limited-view way, and the tissues are inhomogeneous. To this end, the dissertation focuses on the limited-view reconstruction and PAT of acoustical scattering tissues. The major work of this dissertation is described as follows:This dissertation quantitatively studied the influence of the limited-view scanning on the imaging resolution and depth of the back-projection method. It was found that the limited-view scanning could give rise to artifact and distortion which makes two individual absorbers connect with each other and not be separated in the reconstructed image, therefore degrading the resolution. Moreover, the limited-view scanning could also make the reconstructed intensity of deep absorber much weaker than that of the shallow absorber so that the deep absorber cannot be clearly indicated and become indiscernible in the image. The concept of effective scanning angel is proposed to analyze the degradation of imaging depth, and one method based on that concept is proposed to improve imaging depth of limited-view PAT.It also was found that methods based on homogeneity assumption could bring about artifact and distortion when imaging acoustically scattering tissues. This dissertation proposed a method based on the time reversal invariance of the propagation of photoacoustics. In this method, the recorded photoacoustic signals are time-reversed and reemitted into a model medium which has the same deposition of scatterers as the realistic medium, the eventual time reversal field could reveal the initial pressure induced by photoacoustic effect and therefore could be used for imaging. The mismatch between the model medium and the realistic medium was also considered to verify its practicality and robustness; it was shown that the mismatch will degrade the image quality, but within a wide range of mismatch, the method is still superior to the homogeneity-based methods like delay-and-sum method. Ultrasound imaging is a good candidate for establishing such medium model, and it is possible that the deep scatterers cannot be accurately determined and can even be neglected; however, further investigation showed that the method could also provide satisfactory result using a medium model ignoring the deep scatterers.Acoustical scattering is generally considered as nuisance in PAT because it distorts the propagation of photoacoustic waves. However, this dissertation showed that scattering could also contain useful information for imaging and proposed to use backscattering to improve limited-view PAT. The method employed native scatterers or artificial ones behind the region of interest to make the back-propagating signals accessible to the transducer after backscattering. After time reversal, the backscattered waves could converge to the source and provide backside information lost by direct waves, therefore improving image quality. The practicality and robustness of this method is verified by both simulations and experiments, it was shown that this method could significantly improve the limited-view reconstruction, especially when the aperture of the transducer array is quite limited.