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Published November 2013 | Published
Journal Article Open

Time-reversed ultrasonically encoded optical focusing using two ultrasonic transducers for improved ultrasonic axial resolution


Focusing light inside highly scattering media is a challenging task in biomedical optical imaging, manipulation, and therapy. A recent invention has overcome this challenge by time reversing ultrasonically encoded diffuse light to an ultrasound-modulated volume inside a turbid medium. In this technique, a photorefractive (PR) crystal or polymer can be used as the phase conjugate mirror for optical time reversal. Accordingly, a relatively long ultrasound burst, whose duration matches the PR response time of the PR material, is usually used to encode the diffuse light. This long burst results in poor focusing resolution along the acoustic axis. In this work, we propose to use two intersecting ultrasound beams, emitted from two ultrasonic transducers at different frequencies, to modulate the diffuse light at the beat frequency within the intersection volume. We show that the time reversal of the light encoded at the beat frequency can converge back to the intersection volume. Experimentally, an acoustic axial resolution of ∼1.1  mm was demonstrated inside turbid media, agreeing with theoretical estimation.

Additional Information

© 2013 Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. Paper 130680LR received Sep. 18, 2013; revised manuscript received Oct. 14, 2013; accepted for publication Oct. 18, 2013; published online Nov. 5, 2013. The authors thank Yan Liu for helpful discussions on resolution. We also thank Professor James Ballard for editing the manuscript. This work was conducted at Washington University in Saint Louis, and sponsored in part by National Academies Keck Futures Initiative grant IS 13, and National Institutes of Health grants DP1 EB016986 (NIH Director's Pioneer Award) and U54 CA136398.

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