背景介绍：基于光的聚焦，高分辨率显微成像在生命科学和医学中有了重要地位。然而，组织的光学强散射使得在组织深处实现光学聚焦成为巨大的挑战。近几年，借助超声导引技术，为该方向的研究开辟了新的道路，研究获得巨大进展，赋予了生物医学光学未来应用的无限遐想。报告人在该领域做了杰出工作，最新进展发表在Nature Photonics.

报告内容摘要(Abstract)

Optical techniques, increasingly important in biomedicine, suffer from the strong scattering of light in biological tissue. At the optimum wavelength, light can penetrate several centimeters into tissue, but it cannot render high-resolution focusing or imaging deeper than ~1 mm below the skin. Noninvasively focusing light into thick biological tissue is highly desirable but challenging.

In this talk, I will present two novel endeavours—time reversed ultrasonically encoded (TRUE) optical focusing and photoacoustically guided wavefront shaping (PAWS) optical focusing—to reach this sought-after goal. In TRUE, diffuse light is encoded by focused ultrasound, providing a virtual internal guide star. Then the ultrasonically encoded (UE) light is selected, phase conjugated, and time-reversed back to the ultrasonic focus. The TRUE optical focus—defined by the ultrasonic wave—is unaffected by multiple scattering of light, and has been validated at depths of up to 5 mm in tissue (in reflection mode). More recently, we have improved the speed of TRUE focusing down to 5 ms—a quantum leap enabling the first optical focusing through thick living tissue. In the second approach, PAWS, photoacoustic (PA) signals are used as feedback for iterative wavefront optimization to compensate for the scattering-induced phase scrambling, thereby forming an optical focus at the ultrasonic focus. In particular, by observing nonlinear PA signals based on the Grueneisen relaxation effect as the feedback, we have successfully broken the acoustic resolution limit and generated optical diffraction limited focusing with a superior peak fluence gain (~6000 times) in scattering media.

Both TRUE and PAWS are still in their infancy. But once they are engineered to respond more efficiently (for TRUE) or faster (for PAWS), optical focusing at depths in vivo will become feasible, which can bring revolutionary advances to a wide range of high-resolution optical applications, including imaging, sensing, therapy, and manipulation. For example, many in vivo optical microscopic technologies can be brought from the ballistic regime (~1 mm) to the diffusive regime (several centimeters in tissue) and even beyond.

报告人简介 (Biography)

Puxiang Lai received his B.A. in Biomedical Engineering in 2002 from Tsinghua University and M.S. in Acoustics in 2005 from the Chinese Academy of Sciences. He received his Ph.D. in Mechanical Engineering from Boston University in 2011, under the supervision of Dr. Ronald Roy and Dr. Todd Murray. Puxiang then joined Dr. Lihong Wang’s Optical Imaging Lab in the Department of Biomedical Engineering at Washington University in St. Louis. In these years, his research has focused on the development of novel biomedical imaging, sensing, and therapy modalities with the synergy of photons and phonons, including 1) noninvasive and nonionizing imaging techniques, such as ultrasonic imaging, acousto-optic imaging (also called ultrasound-modulated optical tomography), photoacoustic/optoacoustic imaging, and fluorescence imaging; 2) focused ultrasound therapy and its real-time monitoring using sound and light; and 3) ultrasonically guided optical focusing in thick scattering media (including biological tissue).