Broadband All-Optical Ultrasound Transducers for High-Resolution Ultrasound Imaging.

Abstract

The use and development of high-frequency ultrasound has dramatically expanded the frontiers of ultrasound imaging, making better image resolution possible in various clinical applications. Real-time 3D high-resolution ultrasound imaging requires 2D high-frequency (>30 MHz) transducer arrays, which are not available with conventional piezoelectric technology. Optoacoustic transduction relying on optical generation and detection of ultrasound is an attractive alternative, which utilizes two laser beams as input/output vectors instead of electronic signals. Previously, the most efficient optoacoustic transmitter is a 25 microns black PDMS film, and the state of the art optoacoustic detector utilizes an 11 microns Fabry-Perot polymer etalon. Both devices operate at below 50 MHz, and an integrated structure combining both optoacoustic transmitters and detectors has not been designed and fabricated. This thesis addresses the development of integrated broadband all-optical ultrasound transducers. Optimized black PDMS films and innovative gold nanostructure films are first presented as efficient optoacoustic transmitters, both of which are capable of generating ultrasound in the 50-100 MHz frequency range with surface acoustic pressure far exceeding 100 MPa. Thinner polymer etalons are then demonstrated for building ultrasound detection arrays at above 50 MHz, and 3D photoacoustic imaging experiments using etalon arrays have shown axial and lateral resolutions better than 20 microns. The first integrated all-optical ultrasound transducer combining the etalon structure and the gold nanostructure has been designed, fabricated, and evaluated. Pulse-echo signals display bandwidths of 57 MHz. High-resolution ultrasound imaging capabilities have been demonstrated using a 1-D synthetic aperture formed by mechanically scanning the imaging target, where the -6 dB lateral resolution is 38 microns. In addition, a theta-array imaging system combining black PDMS and etalon structures have been developed for 3D ultrasound imaging. Bandwidths of pulse-echo signals are over 50 MHz, and reconstructed ultrasonic images capture the imaging targets with reasonable accuracy. Optical and acoustical characterization of the optoacoustic devices, as well as high-resolution imaging results, strongly suggest that all-optical ultrasound transducers are suitable for 2D high-frequency arrays enabling real-time 3D high-resolution ultrasound imaging.