Image reconstruction in a passive element enriched photoacoustic tomography setup

Photoacoustic imaging is a relatively new imaging technology, in which an object is illuminated with optical energy and where in return measurements are taken in the acoustical domain, in order to image the optical absorption distribution inside the object. In this thesis we focus on an experimental setup, named the Passive Element enRiched PhotoAcoustic Tomography (PER-PAT) imaging setup, that is being developed by the Biomedical Photonic Imaging (BMPI) group of the University of Twente. With this setup it is not only possible to image the optical absorption distribution, but also the speed of sound distribution and acoustic attenuation distribution at the same time, by adding carefully positioned passive elements into the setup. These passive elements act as ultrasound point sources when being illuminated with pulsed optical energy. The work that is presented in this thesis deals with the signal processing that is necessary to transform the measured pressure signals with a PER-PAT imaging setup into reconstructions of these three distributions. This work is divided into four parts. The first part is about calibration of the imaging setup. Calibration of the setup is necessary because, in order to do a good reconstruction, the exact geometrical parameters of the setup need to be known. The second part is about the pre-processing step which is necessary for the reconstruction of the speed of sound and acoustic attenuation distributions. In this pre-processing step, we extract the time delay and attenuation factors that are encountered by acoustic signals traveling from these passive elements to our ultrasound detector elements. In the third part we discuss the reconstruction of actual speed of sound and acoustic attenuation distributions from the, in the previous part, extracted time delay and attenuation factors. We propose an algorithm that can be used to reconstruct these distributions from a PER-PAT setup containing one or more passive elements. The last part contains our approach and findings on reconstructing the optical absorption distribution. Altogether, we conclude that we have developed successful algorithms and methods for the image reconstruction problem with the PER-PAT imaging setup.