Photoacoustic tomography of heterogeneous media using a model-based time reversal method

In photoacoustic (also called optoacoustic or thermoacoustic) tomography acoustic pressure waves are generated by illumination of a semitransparent sample with pulsed electromagnetic radiation. Subsequently the waves propagate toward the detection surface enclosing the sample. The inverse problem consists of reconstructing the initial pressure sources from those measurements. By combining the high spatial resolution of ultrasonic imaging with the high contrast of optical imaging it offers new potentials in medical diagnostics. In certain applications of photoacoustic imaging one has to deal with media with spatially varying sound velocity, e.g. bones in soft tissue. These inhomogeneities have a strong influence on the propagation of photoacoustically generated sound waves. Image reconstruction without any compensation of this effect leads to a poor image quality. It is therefore essential to develop reconstruction algorithms that take spatially varying sound velocity into account and are able to reveal small structures in acoustically heterogeneous media. A model-based time reversal reconstruction method is presented that is capable of reconstructing the initial pressure distribution despite variations of sound speed. This reconstruction method calculates the time reversed field directly with a second order embedded boundary method by retransmitting the measured pressure on the detector positions in reversed temporal order. With numerical simulations the effect of heterogenous media on sound propagation and the consequences for image reconstruction without compensation are shown. It is demonstrated how time reversal can lead to a correct reconstruction if the distribution of sound speed is known. Corresponding experiments with phantoms consisting of areas with spatially varying sound velocity are carried out and the algorithm is applied to the measured signals.