@article {5278, title = {Breaking the spatial resolution barrier via iterative sound-light interaction in deep tissue microscopy.}, journal = {Scientific Reports}, volume = {2}, year = {2012}, month = {2012 Oct}, pages = {748}, abstract = {

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.

}, doi = {doi:10.1038/srep00748}, author = {Si, Ke and Fiolka, Reto and Cui, Meng} } @article {5102, title = {Complex wavefront corrections for deep tissue focusing using low coherence backscattered light.}, journal = {Optics Express}, volume = {20}, year = {2012}, month = {2012}, pages = {16532-43}, abstract = {

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.

}, doi = {10.1364/OE.20.016532}, author = {Fiolka, Reto and Si, Ke and Cui, Meng} } @article {5101, title = {Fluorescence imaging beyond the ballistic regime by ultrasound-pulse-guided digital phase conjugation.}, journal = {Nature Photonics}, volume = {6}, year = {2012}, month = {2012 Aug}, pages = {657}, abstract = {

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{\textendash}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.

}, doi = {doi:10.1038/nphoton.2012.205}, author = {Si, Ke and Fiolka, Reto and Cui, Meng} } @article {5389, title = {Parallel wavefront measurements in ultrasound pulse guided digital phase conjugation.}, journal = {Optics Express}, volume = {20}, year = {2012}, month = {2012 Oct 22}, pages = {24827-34}, abstract = {

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.

}, issn = {1094-4087}, doi = {http://dx.doi.org/10.1364/OE.20.024827}, author = {Fiolka, Reto and Si, Ke and Cui, Meng} }