Time-reversed ultrasonically encoded (TRUE) optical focusing in reflection mode: demonstrations in tissue mimicking phantoms and ex vivo tissue
The problem of how to effectively deliver light dynamically to a small volume inside turbid media has been intensively investigated for imaging and therapeutic purposes. Most recently, a new modality termed Time-Reversed Ultrasonically Encoded (TRUE) optical focusing was proposed by integrating the concepts of ultrasound modulation of diffused light with optical phase conjugation. In this work, the diffused photons that travel through the ultrasound focal region are "tagged" with a frequency shift due to the ultrasound modulation. Part of the tagged light is collected in reflection mode and transmitted to a photorefractive crystal, forming there a stationary hologram through interference with a coherent reference optical beam. The hologram is later read by a conjugated optical beam, generating a phase conjugated wavefront of the tagged light. It is conveyed back to the turbid medium in reflection mode, and eventually converges to the ultrasound focal zone. Optical focusing effects from this system are demonstrated experimentally in tissue-mimicking phantoms and ex vivo chicken breast tissue, achieving effective round-trip optical penetration pathlength (extinction coefficient multiplied by round-trip focusing depth) exceeding 160 and 100, respectively. Examples of imaging optical inclusions with this system are also reported.
Additional Information© 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). This research is sponsored in part by the National Academies Keck Futures Initiative grant IS 13 and the National Institute of Health through grants R01 EB000712 and U54 CA136398. L.W. has a financial interest in Microphotoacoustics, Inc. and Endra, Inc., which, however, did not support this work.
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