An optically-transparent transducer with a high-NA and wide-bandwidth for photoacoustic microscopy (PAM)

Abstract

In photoacoustic microscopy (PAM), the ultrasound transducer plays a critical role in detecting the PA signals. However, conventional ultrasound transducers are optically opaque, which could hinder the effective delivery of the excitation laser onto the target. Recently, optically-transparent ultrasound transducers have been investigated to address this issue. Nevertheless, the transparent transducers demonstrated so far have either a small numerical aperture (NA) or narrow bandwidth, which limits their acoustic focal spot sizes and therefore achievable spatial resolutions. In this paper, we report a new focused transparent polyvinylidene fluoride (PVDF) transducer with a high NA of 0.64 and a wide acoustic pulseecho bandwidth of 120%. Experiment characterization shows that it has an acoustic center frequency and bandwidth of 36 MHz and 44 MHz, respectively. The acoustic focal diameter and zone are 37.8 μm and 210 μm, respectively. With the new transparent transducer, dual-modal acoustic-resolution PAM (AR-PAM) and pulse-echo ultrasound microscopy (PEUSM) have been demonstrated with a target consisting of black-ink-filled polyimide tubing buried in chicken breast at different depths. The imaging results show that both the (acoustic) lateral and axial resolutions can be maintained even at a penetration depth of larger than 3 mm.