Ori Katz

Controlling light in scattering media non-invasively using the photoacoustic transmission matrix

An approach is demonstrated that allows the optical transmission matrix to be noninvasively measured over a large volume inside complex samples using a standard photoacoustic imaging set-up. This approach opens the way towards deep-tissue imaging and light delivery utilizing endogenous optical contrast.

Widefield lensless imaging through a fiber bundle via speckle correlations

Flexible fiber-optic endoscopes provide a solution for imaging at depths beyond the reach of conventional microscopes. Current endoscopes require focusing and/or scanning mechanisms at the distal end, which limit miniaturization, frame-rate, and field of view. Alternative wavefront-shaping based lensless solutions are extremely sensitive to fiber-bending. We present a lensless, bend-insensitive, single-shot imaging approach based on speckle-correlations in fiber bundles that does not require wavefront shaping. Our approach computationally retrieves the target image by analyzing a single camera frame, exploiting phase information that is inherently preserved in propagation through convnetional fiber bundles. Unlike conventional fiber-based imaging, planar objects can be imaged at variable working distances, the resulting image is unpixelated and diffraction-limited, and miniaturization is limited only by the fiber diameter.

Super-resolution photoacoustic imaging via flow-induced absorption fluctuations

In deep-tissue photoacoustic imaging, optical-contrast images of deep-lying structures are formed by recording acoustic waves that are generated by optical absorption. Although photoacoustics is perhaps the leading technique for high-resolution deep-tissue optical imaging, its spatial resolution is fundamentally limited by the acoustic wavelength, which is orders of magnitude longer than the optical diffraction limit. Here, we present an approach for surpassing the acoustic diffraction limit in photoacoustics by exploiting inherent temporal fluctuations in the photoacoustic signals due to sample dynamics, such as those induced by the flow of absorbing red blood cells. This was achieved using a conventional photoacoustic imaging system by adapting concepts from super-resolution fluorescence fluctuation microscopy to the statistical analysis of acoustic signals from flowing acoustic emitters. Specifically, we experimentally demonstrate that flow of absorbing particles and whole human blood yields super-resolved photoacoustic images, and provides static background reduction. By generalizing the statistical analysis to complex-valued signals, we demonstrate super-resolved photoacoustic images that are free from common photoacoustic imaging artifacts caused by band-limited acoustic detection. The presented technique holds potential for contrast-agent-free microvessel imaging, as red blood cells provide a strong endogenous source of naturally fluctuating absorption.

Photoacoustic imaging beyond the acoustic diffraction-limit with dynamic speckle illumination and sparse joint support recovery

In deep tissue photoacoustic imaging the spatial resolution is inherently limited by the acoustic wavelength. Recently, it was demonstrated that it is possible to surpass the acoustic diffraction limit by analyzing fluctuations in a set of photoacoustic images obtained under unknown speckle illumination patterns. Here, we purpose an approach to boost reconstruction fidelity and resolution, while reducing the number of acquired images by utilizing a compressed sensing computational reconstruction framework. The approach takes into account prior knowledge of the system response and sparsity of the target structure. We provide proof of principle experiments of the approach and demonstrate that improved performance is obtained when both speckle fluctuations and object priors are used. We numerically study the expected performance as a function of the measurements signal to noise ratio and sample spatial-sparsity. The presented reconstruction framework can be applied to analyze existing photoacoustic experimental data sets containing dynamic fluctuations.

Widefield lensless endoscopy with a multicore fiber

We demonstrate pixelation-free real-time widefield endoscopic imaging through an aperiodic multicore fiber (MCF) without any distal opto-mechanical elements or proximal scanners. Exploiting the memory effect in MCFs, the images in our system are directly obtained without any post-processing using a static wavefront correction obtained from a single calibration procedure. Our approach allows for video-rate 3D widefield imaging of incoherently illuminated objects with imaging speed not limited by the wavefront-shaping device refresh rate.

Beating the photoacoustic imaging diffraction limit using flow-induced absorption fluctuation (Conference Presentation)

The resolution of photoacoustic imaging of blood vasculature is limited at depth by the acoustic diffraction limit. In this work, we propose to exploit the fluctuations caused by flowing absorbers (such as red blood cells in blood vessels) to perform photoacoustic imaging beyond the acoustic diffraction limit: following the super-resolution optical fluctuation imaging (SOFI) method, we analyze the n-th order statistics from the temporal photoacoustic fluctuations induced by flowing particles. We performed a proof-of-concept experiment in a 5-channel microfluidic silicon-based circuit flown with a suspension of RBC-mimicking 10 µm red-tainted polymer spheres (Microparticles, GmbH, Berlin, Germany). The sample was illuminated with a 5 ns pulsed ND-YAG laser (532 nm, Innolas, Krailling, Germany) with a fluence of 3 mJ/cm^2 and imaged at a 20 Hz rate using a L22-8v probe (128 elements, Verasonics, Redmond, WA, USA) coupled to a Verasonics Vantage 256 ultrasound scanner. Whereas the resolution of conventional photoacoustic imaging was too low to resolve individual channels, the nth order statistical analysis of the photoacoustic fluctuations provided images with a resolution enhancement scaling as n^{1/2}, in agreement with the SOFI theory and with numerical simulations. As opposed to our previous work which exploited speckle-based photoacoustic fluctuations to increase the resolution, the approach proposed here based on sample fluctuations do not require coherent light and can be readily applied to conventional photoacoustic imaging setup. Furthermore, in order to discard the oscillatory behavior of the photoacoustic point-spread-function, we extended in this work the SOFI theory to complex-valued photoacoustic images.

Coherent spatio-temporal control of pulsed light through multiple scattering media

Optical imaging through complex media such as biological tissue or white paint remains a daily challenge as spatial information gets mixed because of multiple scattering. Without any ballistic light, common microscopy techniques become useless. Over the last decade, spatial light modulators (SLM) have become the indispensable tool to overcome scattering thanks to their millions of degrees of freedom. Wavefront shaping techniques have enabled the control of a transmitted or a reflected field scrambled after propagation, be it via optimization, digital optical phase conjugation or with the measurement of the optical transmission matrix [1]. However, most technique relies on the use monochromatic light.

Multiple Speckle Illumination in Photoacoustic Imaging

This presentation will illustrate how optical speckle patterns, a manifestation of coherent light discarded by the transport theory for energy, may be exploited to enhance photoacoustic imaging, in particular in terms of visibility and super-resolution. Article not available.