Optoacoustic detector arrays for medical imaging applications.

Better image resolution would expand the frontiers of ultrasound for both clinical diagnostics and medical research, particularly with spatial resolutions at the subcellular level (≤10mum). The limit on spatial resolution for a particular ultrasound imaging system is imposed by the system electronics together with the physical and electrical characteristics of the array elements. In general, the system electronics is not the technologically limiting factor, and the array element characteristics depend on the method used to generate and detect ultrasound: piezoelectric, capacitive, or optical. Optoacoustic (OA) detectors have been much-touted for their potential as high resolution, high bandwidth transducers with the possibility of miniaturization, selectable operating frequency, and simple fabrication. For medical applications, the reduced sensitivity of existing OA array detectors with respect to piezoelectric detectors is a considerable deterrent. We have studied stabilized resonant structures for high sensitivity optical detection of ultrasound in medical imaging applications. The theoretical performance of Fabry-Perot (FP) resonant optical elements is well documented, and we establish the parameters required of an ideal FP-based OA detector for sensitivity equivalence to ideal piezoelectric array elements. High-finesse structures are required to achieve the desired sensitivities. Not only are there challenges associated with fabricating FP resonators of high finesse, but the goal of using such a device as an imaging array with an acceptable dynamic range is shown to lead to the requirement of mirror surface imperfections and thickness uniformity constrained to better than that possible with state-of-the-art fabrication methods. We have studied active stabilization of the FP element as a means of circumventing this constraint. The methods available to effect stabilization include control of (1) laser wavelength, (2) physical path length, and (3) index of refraction. With the promise of sufficient tuning range, high tuning sensitivity, excellent bandwidth, high stability and ease of implementation, index of refraction control is the most attractive approach. Simulations of thermo-optic and electro-optic stabilization for resonant optoacoustic detectors indicate excellent potential for use as array detectors. We have fabricated prototypes of index of refraction stabilized resonant optoacoustic detectors and demonstrate their potential for use as sensitive optoacoustic detectors.Ph.D.Applied SciencesBiomedical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123956/2/3106167.pd