title: Time-reversed ultrasonically encoded optical focusing using two ultrasonic transducers for improved ultrasonic axial resolution.
abstract: 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.
authors.affiliation: Beijing University of Posts and Telecommunications, State Key Laboratory of Information Photonics and Optical Communications, Beijing 100876, ChinabWashington University in St. Louis, Optical Imaging Laboratory, Department of Biomedical Engineering, St. Louis, Missouri 63130.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Yang
authors.forename: Xiao
authors.initials: X
authors.lastname: Xu
authors.forename: Puxiang
authors.initials: P
authors.lastname: Lai
authors.forename: Daxiong
authors.initials: D
authors.lastname: Xu
authors.forename: Lihong V
authors.initials: LV
authors.lastname: Wang
mesh_terms: D061848:Optical Imaging / Q000295:instrumentation / Q000379:methods; D019047:Phantoms, Imaging; D013016:Sound; D014159:Transducers; D014463:Ultrasonography / Q000295:instrumentation*