Author's School

School of Engineering & Applied Science

Author's Department/Program

Biomedical Engineering

Language

English (en)

Date of Award

5-24-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Lihong Wang

Abstract

Ultrasound modulated optical tomography is a developing hybrid imaging modality that combines high optical contrast and good ultrasonic resolution to image soft biological tissue. We developed a photorefractive crystal-based, time-resolved detection scheme with the use of a millisecond long ultrasound burst to image both the optical and mechanical properties of biological tissues, with improved detection efficiency of ultrasound-tagged photons. We also applied spectral-hole burning: SHB) aided detection in ultrasound-modulated optical tomography: UOT) to image optical heterogeneities in thick tissue-mimicking phantom samples and chicken breast tissue. The efficiency of SHB was improved by using a Tm3+: YAG crystal of higher doping concentration: 2.0-atomic%) and a double-pass pumping configuration. With the improved SHB-UOT system, we imaged absorbing, scattering, and phase contrast objects that were embedded in the middle plane of a 30-mm thick phantom sample. The imaging resolution was 0.5 mm in the lateral direction, as defined by the focal width of the ultrasonic transducer, and 1.5 mm in the axial direction, as determined by the ultrasonic burst length. We also imaged two absorbing objects embedded in the middle plane of a 32-mm thick chicken breast sample. The results suggest that the improved SHB-UOT system is one step closer to a practical optical imaging application in biological and clinical studies. Light focusing plays a central role in biomedical imaging, manipulation, and therapy. In optical scattering media such as biological tissue, light propagation is randomized by multiple scattering. Beyond one transport mean free path, where photon propagation is in the diffusive regime, direct light focusing becomes infeasible. Although various methods have been developed to overcome this optical diffusion limit, all are limited by the lack of a practical internal "guide star." Here we proposed and experimentally validated a novel concept, called Time-Reversed Ultrasonically Encoded: TRUE) optical focusing, to deliver light dynamically into any predefined location inside a scattering medium. First, diffused coherent light is encoded by an ultrasonic wave focused to a predefined location; then, the encoded component of the diffused light is time-reversed and consequently converges back to the ultrasonic focus. The ultrasonic encoding noninvasively provides a virtual internal "guide star" for the time reversal. The TRUE optical focus--dynamically defined by the ultrasonic focus--is unaffected by multiple scattering of light, which is especially desirable in biological tissue where ultrasonic scattering is ~1000 times weaker than optical scattering. Various fields, such as biomedical, colloidal, atmospheric, and ocean optics, can benefit from TRUE optical focusing. Further, the concept can be generalized for non-optical waves.

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Permanent URL: http://dx.doi.org/10.7936/K7W093WM

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