Dual-modal photoacoustic and ultrasound microscopy using optically-transparent and high-NA PVDF transducer

Abstract

As two important acoustic imaging modalities, photoacoustic microscopy (PAM) and ultrasound microscopy (UM) provide complementary functional and structural information on biological tissues. At penetration depths beyond the optical diffraction limit, the spatial resolution of both PAM and UM is determined acoustically by the receiving (focused) ultrasound transducer. To obtain good dual-modal imaging resolution, the transducer should have a high center frequency with a wide bandwidth. More importantly, a high acoustic numerical aperture (NA) should be maintained for receiving the PA and ultrasound signals, which is however difficult to achieve as the light blockage by the transducer is more likely to occur at higher NAs. In this paper, we report the dual-modal PAM and UM based on an optically-transparent focused PVDF transducer with a high NA of 0.64. The transducer 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 a central transparent window, the excitation laser pulses can directly pass through the transducer to illuminate the target. This allows a high acoustic NA to be obtained without light blockage. For demonstration, co-registered 3D and 2D PAM/UM imaging experiments have been conducted on a twisted wire target in both water and chicken breast tissue, and in-vivo on a mouse tail, respectively. The imaging results show that high acoustic resolution and sensitivity can be achieved to resolve the target at different depths with a simple and compact dual-modal imaging setup