Needle hydrophone-based photoacoustic microscopy with experimentally measured impulse response for improved depth of focus.

Reconstruction-based acoustic-resolution photoacoustic microscopy (AR-PAM) has been developed to extend the depth of field (DOF), enabling simultaneous observation of structures at multiple depths. However, conventional AR-PAM systems, which rely on focused transducers, face inherent limitations in effectively increasing the DOF. To address this issue, we developed a needle hydrophone (NH)-based AR-PAM system that enables deep imaging with enhanced resolution and improved DOF. The proposed system was validated using tissue-mimicking phantoms and ex Ovo chick embryo imaging. Our results demonstrated a DOF exceeding 20 mm, a lateral resolution comparable to the NH diameter (∼400 µm) at shallow depth (10 mm) and 870 µm at deep depth (30 mm), and an axial resolution of 250 µm. Furthermore, we investigated the impact of different reconstruction techniques, including the measured impulse response function (MIRF), simulated impulse response function (SIRF), and coherence factor (CF). Our comparative analysis revealed that MIRF-based reconstruction provided superior performance in maintaining resolution and image quality across varying depths, making it the most effective approach for multi-depth imaging.