Spatial over-sampling and its influence on spatial resolution for photoacoustic tomography with finite sized detectors

Detector arrays enable parallel detection for faster photoacoustic imaging than by moving a single detector, but the detector spacing for arrays cannot be smaller than the size of an array element. Spatial over-sampling is scanning with a step-size smaller than the size of the detector element and is possible only for a moving single detector. For a detector with finite sized surface the measured acoustic signal is a spatial average of the pressure field over the detector surface. If the reconstruction is performed assuming point-like detection over-sampling brings no advantage as e. g. for spherical or cylindrical detection surfaces the blurring caused by a finite detector size is proportional to the distance from the rotation center and is equal to the detector size at the detection surface. Iterative reconstruction algorithms or inverting directly the imaging matrix can take the finite size of real detectors directly into account, but the numerical effort is significantly higher compared to direct algorithms assuming point-like detection. Another reconstruction with less numerical effort is to use a direct algorithm assuming point-like detectors and run a deconvolution algorithm for deblurring afterwards. For such reconstruction methods spatial over-sampling makes sense because it reduces the blurring significantly. The effect of step size on the reconstructed image is systematically examined using simulated and experimental data. Experimental data are obtained on a plastisol cylinder with thin holes filled with an absorbing liquid. Data acquisition is done by utilization of a piezoelectric detector (PVDF stripe) which is rotated around the plastisol cylinder.