Miniaturized Fabry-Perot Polymer Film Ultrasound Sensor for Photoacoustic Endoscope System

Abstract

Colorectal cancer (CRC) is estimated to be the fourth most common cancer and the second most deadly cancer in 2023. The development of such cancer usually involves a growing polyp into deeper tissue. Thus, depth-resolving imaging modalities will help detect and stage CRC for early prevention and better treatment. Photoacoustic endoscopy (PAE) is an emerging imaging technology that combines the advantages of optics and ultrasound. Various designs of PAE have been proposed, however, there is still a need for highly sensitive, transparent, and miniaturized sensors. A Fabry-Perot (FP) polymer film ultrasound sensor is an excellent candidate to satisfy these demands. The current FP sensors are either bulky substrate-based sensor array, single fiber-based sensor, or fiber bundle-based sensor array. None of them can achieve transparency, miniaturization, design flexibility and array configuration at the same time. Thus, in this dissertation, we propose using thin wafer-based FP sensor to fabricate miniaturized and cost-effective sensor array for PAE applications. A transfer matrix-based model is developed to study the physics of multilayer structure of the miniaturized sensor. An approximate model is then built to simplify the problem, and it reveals that the finite thickness substrate acts as a filter in the frequency domain. Thus, the frequency response of the miniaturized sensor can be back filtered to remove the effect of the substrate, demonstrating the feasibility of miniaturization in theory. The sensor is fabricated by depositing gold and Parylene C films onto a glass wafer. It turns out that the sensitivity is not uniform on the wafer surface. However, by dicing the thin wafer carefully, a relatively uniform region can be selected and treated as a sensor array. In this way a fixed wavelength laser can be used to interrogate this sensor instead of an expensive tunable laser required in other FP sensors. A 2 mm by 9 mm rectangular sensor array is fabricated in this way and a tabletop system is built to conduct tomography imaging. The system clearly shows the structure of a phantom made from a knotted wire and the polyps from a CPC-APC mouse colon tissue ex vivo with IV injection of gold nanoshell. The system has a best NEP of 0.76kPa, a -3dB bandwidth of 16.6 MHz, a lateral resolution of 208 µm and an axial resolution of 32 µm. A 2 mm by 2 mm single element sensor is fabricated to verify the feasibility of being integrated into a PAE probe. A miniaturized PAE probe based on the 2 mm sensor is designed. The sub-probe for 532 nm excitation laser delivery in the PAE design is fabricated to demonstrate the feasibility of miniaturized PAE probe. The imaging performance is demonstrated by phantom imaging and in vivo imaging of the ear of a nude mouse. Another sub-probe using 800 nm is built for deep tissue imaging with gold nanoshell. The system can image phantoms and prolapsed CPC-APC mouse in vivo with IV injection of gold nanoshell. These results show that the proposed miniaturized FP sensor is promising to enable novel PAE designs for better CRC detection and staging.