25 April 2005 Computations of acoustically induced phase shifts of optical paths in acousto-photonic imaging
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Abstract
Ultrasound modulated optical signals from scattered laser light in biological tissue have been investigated in order to improve optical imaging quality, a technique we call acousto-photonic imaging (API). Recent experiments using a photorefractive crystal based detection system have shown that there is a large DC-offset to the acoustically modulated AC optical signal, an effect that is second order in the large acoustically-induced optical phase shifts. We present numerical calculations for the size of the phase shifts of various photons in an API set-up. The calculations use a Monte Carlo simulation for the propagation of laser photons in an optically diffuse medium with properties similar to tissue and a finite-difference time domain simulation of a realistic focused ultrasound beam in that medium. The phase shifts of photons which passed through different regions around the ultrasound focus were compared. Quantities which characterize the AC and DC signal components were evaluated using the calculated phase shifts. It was found that the DC term dominates as it does in experiments, even though it is second order in the phase shifts. This is due to the fact that the contributions to the DC signal from each photon add coherently. In addition, the significant contributions to the DC signal are limited to those photons that passed near the ultrasound focal region, which has important implications for improving the imaging resolution in API.
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Florian J. Blonigen, Charles A. DiMarzio, "Computations of acoustically induced phase shifts of optical paths in acousto-photonic imaging", Proc. SPIE 5697, Photons Plus Ultrasound: Imaging and Sensing 2005: The Sixth Conference on Biomedical Thermoacoustics, Optoacoustics, and Acousto-optics, (25 April 2005); doi: 10.1117/12.592028; https://doi.org/10.1117/12.592028
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