High frequency ultrasound transducer for real time ultrasound biomicroscopy with optoacoustic arrays
Date
2011
Authors
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Publisher
University of Delaware
Abstract
Ultrasound biomicroscopy (UBM) is a high resolution biomedical imaging technique using high frequency ultrasound waves. Fabricating highly populated detector arrays represents a major technical challenge for real-time UBM systems. A potential solution is optoacoustic technology, where high frequency ultrasound is detected with optical methods. The advantages of optoacoustic detection are large bandwidth, good sensitivity, and the capability for large scale parallel read-out. In this thesis, the receiving and transmitting part of a UBM imaging array are investigated separately. Optoacoustic detection is explored with a thin film etalon consisting of two gold films separated by a transparent layer. Simulations and experiments demonstrate that optoacoustic detection sensitivity is maximized with a gold layer thickness of 45 nm. Various transparent layer materials were investigated, including polystyrene microspheres, SU-8 2005 photoresist, and parylene. Experiments demonstrate that parylene is the best material due to its precise thickness control and uniformity. Ideally, the ultrasound transmitter and optoacoustic etalon are integrated into a single device. Piezoelectric materials are the most efficient emitters of ultrasound, but optical transparency is required to facilitate integration with an etalon. Lithium niobate (LiNbO3) is chosen for its high piezoelectricity and excellent optical transparency. Initial efforts with LiNbO3 concentrated on fabricating a "conventional" transducer that is not optically transparent. An unfocused transducer was fabricated that produces 25 MHz ultrasound with a -6 dB bandwidth of 15 MHz and a two-way insertion loss of 27.6 dB. An optically transparent LiNbO3 transducer with indium tin oxide (ITO) electrodes is currently under development. An approach to combine the optically transparent LiNbO3 emitter with an optoacoustic etalon is proposed.