Adapted directivity approach for photoacoustic imaging reconstruction

In photoacoustic imaging, upon short laser pulse irradiation, absorbers generate N-shaped pulses which can be detected by ultrasound transducers. Radio frequency signals from different spatial locations are then reconstructed taking into account the ultrasound transducer angular response. Usually, the directivity is part of the "a priori" characterization of the transducer and it is assumed to be constant in the reconstruction algorithm. This approach is valid in both transmission and reflection ultrasound imaging, where any echo resembles the transducer frequency response. Center frequency and bandwidth of any echo are almost the same, and the ultrasound transducer collect signals with the same "fixed" acceptance angle. In photoacoustics, instead, absorbers generate echoes whose time duration is proportional to the absorber size. Large absorbers generate low frequency echoes, whereas small absorber echoes are centered at higher frequencies. Thus for different absorber sizes, different pulse frequencies are obtained and different directivities need to be applied. For this purpose once a radio-frequency signal is aquired, it is pre-processed with a sliding window: every segment is Fourier transformed to extract the central frequency. Then, a proper directivity can be estimated for each segment. Finally signals can be reconstructed via a backprojection algorithm, according to the system's geometry. Echoes are backprojected over spheres with the angular extension being adapted to the frequency content of the photoacoustic sources. Simulation and experimental validation of this approach are discussed showing promising results in terms of image contrast and resolution.