Transparent ultrasonic transducers based on relaxor ferroelectric crystals for advanced photoacoustic imaging

Photoacoustic imaging (PAI) is promising as a non-invasive functional imaging modality for various applications, from clinical diagnosis to fundamental research. However, achieving capillary-level resolution, wide field-of-view (FOV) and high imaging frame rate simultaneously in a PAI system remains very challenging. To address this, transparent ultrasonic transducer (TUT) was considered to be an alternative to conventional opaque one in PAI systems. While this concept shows promise, existing TUTs have limitations in bandwidth and sensitivity, hindering their practical application. To overcome these limitations, we propose to use our newly developed transparent relaxor ferroelectric crystals as piezoelectric elements for the TUT design. Owing to their ultrahigh dielectric and piezoelectric properties, the newly designed TUT exhibits a four-fold enhancement of photoacoustic detection sensitivity when compared to conventional LiNbO3-based counterpart. In addition, we implemented a transparent-double-matching-layer design via an innovative transducer fabrication process, resulting in a 31-MHz TUT with an impressive bandwidth of 76%, surpassing existing TUTs (<37%) and comparable to opaque ultrasonic transducers with the same operation frequency. Leveraging the relatively large FOV and significantly improved performances of our newly developed TUT, we achieved a high-speed PAI of the mouse microvasculature at a frame rate of 1 Hz over a 1.5 mm × 1.5 mm area via optical scanning. This advancement surpasses the conventional mechanical-scanning-based PAI techniques by at least 30 times. As a result, we successfully monitored the dynamic change of the mouse cerebral cortex microvasculature during an epileptic seizure. The innovative TUT design holds great potential to drive progress in the field of high-performance and compact photoacoustic devices, particularly for applications in fundamental brain science research.