High-resolution photoacoustic and ultrasound endoscope based on transparent ultrasound transducer

Photoacoustic (PA) imaging has become one of the promising biomedical imaging technologies in the past decade, thanks to its advantages of structural, functional, imaging capabilities and seamless integration with conventional ultrasound imaging. Endoscopic photoacoustic and ultrasound (ePAUS) is the combination of PA imaging technology and endoscopic ultrasound (EUS). In the design of the ePAUS, it is ideal to align the optical beam of the laser and the acoustic beam of the transducer on the same axis to achieve high spatial resolution and long imaging range. Existing ePAUS uses a ring transducer or a beam combiner to obtain a coaxial or rather an off-axis arrangement. However, the ring transducer has a problem in that the diameter and acoustic side lobes are large, and the beam combiner has a disadvantage in that the structure is complicated and the acoustic loss due to multiple acoustic reflections is large. Our approach to solving this problem is the development of ePAUS based on a miniaturized transparent ultrasonic transducer (TUT). In this study, lead-magnesium- niobate lead-titanate and Indium Tin Oxide-based ultra-small TUT was fabricated, and the performance of center frequency of 28.1 MHz and bandwidth of 51.5% was obtained. Thereafter, quasi-focus was used by combining a multimode optical fiber and a gradient index lens, and coaxial alignment was achieved by arranging the optical axis perpendicular to the optically transparent TUT. This results in high spatial resolution and long imaging distances, and the imaging performance of the probe is demonstrated by imaging the rectum and vagina of the rat in vivo.