Investigation of the far-field approximation for modeling a transducer's spatial impulse response in photoacoustic computed tomography

When ultrasonic transducers with large detecting areas and/or compact measurement geometries are employed in photoacoustic computed tomography (PACT), the spatial resolution of reconstructed images can be significantly degraded. Our goal in this work is to clarify the domain of validity of the imaging model that mitigates such effects by use of a far-field approximation. Computer-simulation studies are described that demonstrate the far-field-based imaging model is highly accurate for a practical 3D PACT imaging geometry employed in an existing small animal imaging system. For use in special cases where the far-field approximation is violated, an extension of the far-field-based imaging model is proposed that divides the transducer face into a small number of rectangular patches that are each described accurately by use of the far-field approximation.