A study of reconstruction in photoacoustic tomography with a focused transducer

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So far most rigorous reconstruction algorithms for photoacoustic tomography (PAT), e.g., the modified back-projection algorithm, have been developed based on ideal point detectors. However, a flat unfocused transducer is commonly used in PAT, thus suffering from the finite aperture effect - tangential resolution deteriorates as the imaging point moves away from the scanning center. Based on a virtual-point-detector concept, we propose a PAT reconstruction with a focused transducer to improve the degraded tangential resolution. We treat the focal point of the focused transducer as a virtual-point detector, which means that delays applied in reconstruction are relative to the focal point. The geometric focus defines propagation path of photoacoustic signals. The simulation results show that compared with PAT with an unfocused transducer, PAT with a focused transducer having an f-number of 2.5 significantly improves tangential resolution by 29 microns up to 791 microns at the imaging positions of at least 4 mm away from the scanning center. The farther the imaging positions away from the scanning center, the larger the improvement. In the region of 4 mm away from the scanning center, PAT with a focused transducer slightly degrades the tangential resolution by up to 70 microns. The improvement in tangential resolution comes with a compromise of loss in radial resolution by 26 microns up to 79 microns depending on the distance from the scanning center. In terms of the significant improvement in tangential resolution, the loss in radial resolution is tolerable, especially for imaging of big objects, e.g., breast.