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Deep Tissue Fluorescence Imaging with Time-Reversed Light

Citation

Wang, Ying Min (2013) Deep Tissue Fluorescence Imaging with Time-Reversed Light. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YNSN-8960. http://resolver.caltech.edu/CaltechTHESIS:04282013-103938118

Abstract

Advances in optical techniques have enabled many breakthroughs in biology and medicine. However, light scattering by biological tissues remains a great obstacle, restricting the use of optical methods to thin ex vivo sections or superficial layers in vivo. In this thesis, we present two related methods that overcome the optical depth limit—digital time reversal of ultrasound encoded light (digital TRUE) and time reversal of variance-encoded light (TROVE). These two techniques share the same principle of using acousto-optic beacons for time reversal optical focusing within highly scattering media, like biological tissues. Ultrasound, unlike light, is not significantly scattered in soft biological tissues, allowing for ultrasound focusing. In addition, a fraction of the scattered optical wavefront that passes through the ultrasound focus gets frequency-shifted via the acousto-optic effect, essentially creating a virtual source of frequency-shifted light within the tissue. The scattered ultrasound-tagged wavefront can be selectively measured outside the tissue and time-reversed to converge at the location of the ultrasound focus, enabling optical focusing within deep tissues. In digital TRUE, we time reverse ultrasound-tagged light with an optoelectronic time reversal device (the digital optical phase conjugate mirror, DOPC). The use of the DOPC enables high optical gain, allowing for high intensity optical focusing and focal fluorescence imaging in thick tissues at a lateral resolution of 36 µm by 52 µm. The resolution of the TRUE approach is fundamentally limited to that of the wavelength of ultrasound. The ultrasound focus (~ tens of microns wide) usually contains hundreds to thousands of optical modes, such that the scattered wavefront measured is a linear combination of the contributions of all these optical modes. In TROVE, we make use of our ability to digitally record, analyze and manipulate the scattered wavefront to demix the contributions of these spatial modes using variance encoding. In essence, we encode each spatial mode inside the scattering sample with a unique variance, allowing us to computationally derive the time reversal wavefront that corresponds to a single optical mode. In doing so, we uncouple the system resolution from the size of the ultrasound focus, demonstrating optical focusing and imaging between highly diffusing samples at an unprecedented, speckle-scale lateral resolution of ~ 5 µm. Our methods open up the possibility of fully exploiting the prowess and versatility of biomedical optics in deep tissues.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Biomedical optics; Fluorescence imaging; Imaging through turbid media; Optical phase conjugation;
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Bioengineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Yang, Changhuei
Thesis Committee:
  • Guo, Chin-Lin (chair)
  • Yang, Changhuei (co-chair)
  • Fraser, Scott E.
  • Choo, Hyuck
  • Gradinaru, Viviana
Defense Date:20 March 2013
Record Number:CaltechTHESIS:04282013-103938118
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:04282013-103938118
DOI:10.7907/YNSN-8960
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7640
Collection:CaltechTHESIS
Deposited By: Ying Min Wang
Deposited On:07 May 2013 21:39
Last Modified:26 Apr 2019 18:20

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