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6 Janelia Publications

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    Cui Lab
    10/22/12 | Parallel wavefront measurements in ultrasound pulse guided digital phase conjugation.
    Fiolka R, Si K, Cui M
    Optics Express. 2012 Oct 22;20(22):24827-34. doi: http://dx.doi.org/10.1364/OE.20.024827

    Ultrasound pulse guided digital phase conjugation has emerged to realize fluorescence imaging inside random scattering media. Its major limitation is the slow imaging speed, as a new wavefront needs to be measured for each voxel. Therefore 3D or even 2D imaging can be time consuming. For practical applications on biological systems, we need to accelerate the imaging process by orders of magnitude. Here we propose and experimentally demonstrate a parallel wavefront measurement scheme towards such a goal. Multiple focused ultrasound pulses of different carrier frequencies can be simultaneously launched inside a scattering medium. Heterodyne interferometry is used to measure all of the wavefronts originating from every sound focus in parallel. We use these wavefronts in sequence to rapidly excite fluorescence at all the voxels defined by the focused ultrasound pulses. In this report, we employed a commercially available sound transducer to generate two sound foci in parallel, doubled the wavefront measurement speed, and reduced the mechanical scanning steps of the sound transducer to half.

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    Cui Lab
    10/01/12 | Breaking the spatial resolution barrier via iterative sound-light interaction in deep tissue microscopy.
    Si K, Fiolka R, Cui M
    Scientific Reports. 2012 Oct;2:748. doi: doi:10.1038/srep00748

    Optical microscopy has so far been restricted to superficial layers, leaving many important biological questions unanswered. Random scattering causes the ballistic focus, which is conventionally used for image formation, to decay exponentially with depth. Optical imaging beyond the ballistic regime has been demonstrated by hybrid techniques that combine light with the deeper penetration capability of sound waves. Deep inside highly scattering media, the sound focus dimensions restrict the imaging resolutions. Here we show that by iteratively focusing light into an ultrasound focus via phase conjugation, we can fundamentally overcome this resolution barrier in deep tissues and at the same time increase the focus to background ratio. We demonstrate fluorescence microscopy beyond the ballistic regime of light with a threefold improved resolution and a fivefold increase in contrast. This development opens up practical high resolution fluorescence imaging in deep tissues.

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    Cui Lab
    08/01/12 | Fluorescence imaging beyond the ballistic regime by ultrasound-pulse-guided digital phase conjugation.
    Si K, Fiolka R, Cui M
    Nature Photonics. 2012 Aug;6:657. doi: doi:10.1038/nphoton.2012.205

    Fluorescence imaging has revolutionized biomedical research over the past three decades. Its high molecular specificity and unrivalled single-molecule-level sensitivity have enabled breakthroughs in a number of research fields. For in vivo applications its major limitation is its superficial imaging depth, a result of random scattering in biological tissues causing exponential attenuation of the ballistic component of a light wave. Here, we present fluorescence imaging beyond the ballistic regime by combining single-cycle pulsed ultrasound modulation and digital optical phase conjugation. We demonstrate a near-isotropic three-dimensional localized sound–light interaction zone. With the exceptionally high optical gain provided by the digital optical phase conjugation system, we can deliver sufficient optical power to a focus inside highly scattering media for not only fluorescence imaging but also a variety of linear and nonlinear spectroscopy measurements. This technology paves the way for many important applications in both fundamental biology research and clinical studies.

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    Cui Lab
    05/29/12 | Superpenetration optical microscopy by iterative multiphoton adaptive compensation technique.
    Tang J, Germain RN, Cui M
    Proceedings of the National Academy of Sciences of the United States of America. 2012 May 29;109(22):8434-9. doi: 10.1073/pnas.1119590109

    Biological tissues are rarely transparent, presenting major challenges for deep tissue optical microscopy. The achievable imaging depth is fundamentally limited by wavefront distortions caused by aberration and random scattering. Here, we report an iterative wavefront compensation technique that takes advantage of the nonlinearity of multiphoton signals to determine and compensate for these distortions and to focus light inside deep tissues. Different from conventional adaptive optics methods, this technique can rapidly measure highly complicated wavefront distortions encountered in deep tissue imaging and provide compensations for not only aberration but random scattering. The technique is tested with a variety of highly heterogeneous biological samples including mouse brain tissue, skull, and lymph nodes. We show that high quality three-dimensional imaging can be realized at depths beyond the reach of conventional multiphoton microscopy and adaptive optics methods, albeit over restricted distances for a given correction. Moreover, the required laser excitation power can be greatly reduced in deep tissues, deviating from the power requirement of ballistic light excitation and thus significantly reducing photo damage to the biological tissue.

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    Cui Lab
    04/03/12 | Time-lapse two-color 3D imaging of live cells with doubled resolution using structured illumination.
    Fiolka R, Shao L, Rego EH, Davidson MW, Gustafsson MG
    Proceedings of the National Academy of Sciences of the United States of America. 2012 Apr 3;109(14):5311-5. doi: 10.1073/pnas.1119262109

    Previous implementations of structured-illumination microscopy (SIM) were slow or designed for one-color excitation, sacrificing two unique and extremely beneficial aspects of light microscopy: live-cell imaging in multiple colors. This is especially unfortunate because, among the resolution-extending techniques, SIM is an attractive choice for live-cell imaging; it requires no special fluorophores or high light intensities to achieve twice diffraction-limited resolution in three dimensions. Furthermore, its wide-field nature makes it light-efficient and decouples the acquisition speed from the size of the lateral field of view, meaning that high frame rates over large volumes are possible. Here, we report a previously undescribed SIM setup that is fast enough to record 3D two-color datasets of living whole cells. Using rapidly programmable liquid crystal devices and a flexible 2D grid pattern algorithm to switch between excitation wavelengths quickly, we show volume rates as high as 4 s in one color and 8.5 s in two colors over tens of time points. To demonstrate the capabilities of our microscope, we image a variety of biological structures, including mitochondria, clathrin-coated vesicles, and the actin cytoskeleton, in either HeLa cells or cultured neurons.

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    Cui Lab
    01/01/12 | Complex wavefront corrections for deep tissue focusing using low coherence backscattered light.
    Fiolka R, Si K, Cui M
    Optics Express. 2012;20(15):16532-43. doi: 10.1364/OE.20.016532

    Aberrations and random scattering severely limit optical imaging in deep tissue. Adaptive optics can in principle drastically extend the penetration depth and improve the image quality. However, for random scattering media a large number of spatial modes need to be measured and controlled to restore a diffraction limited focus. Here, we present a parallel wavefront optimization method using backscattered light as a feedback. Spatial confinement of the feedback signal is realized with a confocal pinhole and coherence gating. We show in simulations and experiments that this approach enables focusing deep into tissue over up to six mean scattering path lengths. Experimentally the technique was tested on tissue phantoms and fixed brain slices.

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