Itay Remer

Background-free Brillouin spectroscopy in scattering media at 780 nm via stimulated Brillouin scattering.

We demonstrate the effectiveness of stimulated Brillouin scattering for background-free Brillouin spectroscopy in scattering media within the biological spectral window. Using two nearly counter-propagating continuous-wave diode laser beams at 780 nm, we acquired transmission stimulated Brillouin point spectra in 10 mm and 500 μm thick Intralipid tissue phantoms with ∼100  μm and ∼16  μm diameter focal points, respectively. Stimulated gain spectra with high signal-to-noise ratio (8.7-30.7 dB) and frequency accuracy (6-72 MHz) were obtained at 20  MHz/10  ms and 20  MHz/100  ms through 0.24-3.36 mean-free paths of tissue phantoms. Our results suggest that stimulated Brillouin gain can be useful for imaging of Brillouin resonances in submillimeter-thick scattering samples.

Laser speckle spatiotemporal variance analysis for noninvasive widefield measurements of blood pulsation and pulse rate on a camera-phone

Photoplethysmography is a well-established technique for the noninvasive measurement of blood pulsation. However, photoplethysmographic devices typically need to be in contact with the surface of the tissue and provide data from a single contact point. Extensions of conventional photoplethysmography to measurements over a wide field-of-view exist, but require advanced signal processing due to the low signal-to-noise-ratio of the photoplethysmograms. Here, we present a noncontact method based on temporal sampling of time-integrated speckle using a camera-phone for noninvasive, widefield measurements of physiological parameters across the human fingertip including blood pulsation and resting heart-rate frequency. The results show that precise estimation of these parameters with high spatial resolution is enabled by measuring the local temporal variation of speckle patterns of backscattered light from subcutaneous skin, thereby opening up the possibility for accurate high resolution blood pulsation imaging on a camera-phone. Camera-phone laser speckle imager along with measured relative blood perfusion maps of a fingertip showing skin perfusion response to a pulse pressure applied to the upper arm. The figure is for illustration only; the imager was stabilized on a stand throughout the experiments.

Static laser speckle contrast analysis for noninvasive burn diagnosis using a camera-phone imager

Abstract. Laser speckle contrast analysis (LASCA) is an established optical technique for accurate widefield visualization of relative blood perfusion when no or minimal scattering from static tissue elements is present, as demonstrated, for example, in LASCA imaging of the exposed cortex. However, when LASCA is applied to diagnosis of burn wounds, light is backscattered from both moving blood and static burn scatterers, and thus the spatial speckle contrast includes both perfusion and nonperfusion components and cannot be straightforwardly associated to blood flow. We extract from speckle contrast images of burn wounds the nonperfusion (static) component and discover that it conveys useful information on the ratio of static-to-dynamic scattering composition of the wound, enabling identification of burns of different depth in a porcine model in vivo within the first 48 h postburn. Our findings suggest that relative changes in the static-to-dynamic scattering composition of burns can dominate relative changes in blood flow for burns of different severity. Unlike conventional LASCA systems that employ scientific or industrial-grade cameras, our LASCA system is realized here using a camera phone, showing the potential to enable LASCA-based burn diagnosis with a simple imager.

High-speed stimulated Brillouin scattering spectroscopy at 780 nm

We demonstrate a high-speed stimulated Brillouin scattering (SBS) spectroscopy system that is able to acquire stimulated Brillouin gain point-spectra in water samples and Intralipid tissue phantoms over 2 GHz within 10 ms and 100 ms, respectively, showing a 10-100 fold increase in acquisition rates over current frequency-domain SBS spectrometers. This improvement was accomplished by integrating an ultra-narrowband hot rubidium-85 vapor notch filter in a simplified frequency-domain SBS spectrometer comprising nearly counter-propagating continuous-wave pump-probe light at 780 nm and conventional single-modulation lock-in detection. The optical notch filter significantly suppressed stray pump light, enabling detection of stimulated Brillouin gain spectra with substantially improved acquisition times at adequate signal-to-noise ratios (∼25 dB in water samples and ∼15 dB in tissue phantoms). These results represent an important step towards the use of SBS spectroscopy for high-speed measurements of Brillouin g...

In vivo burn diagnosis by camera-phone diffuse reflectance laser speckle detection.

Burn diagnosis using laser speckle light typically employs widefield illumination of the burn region to produce two-dimensional speckle patterns from light backscattered from the entire irradiated tissue volume. Analysis of speckle contrast in these time-integrated patterns can then provide information on burn severity. Here, by contrast, we use point illumination to generate diffuse reflectance laser speckle patterns of the burn. By examining spatiotemporal fluctuations in these time-integrated patterns along the radial direction from the incident point beam, we show the ability to distinguish partial-thickness burns in a porcine model in vivo within the first 24 hours post-burn. Furthermore, our findings suggest that time-integrated diffuse reflectance laser speckle can be useful for monitoring burn healing over time post-burn. Unlike conventional diffuse reflectance laser speckle detection systems that utilize scientific or industrial-grade cameras, our system is designed with a camera-phone, demonstrating the potential for burn diagnosis with a simple imager.

In vivo noninvasive visualization of retinal perfusion dysfunction in murine cerebral malaria by camera-phone laser speckle imaging

Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection associated with impaired cerebral blood flow. Visualization of the eye vasculature, which is embryologically derived from that of the brain, is used clinically to diagnose the syndrome. Here, we introduce camera-phone laser speckle imaging as a new tool for in vivo, noncontact two-dimensional mapping of blood flow dynamics in the experimental cerebral malaria (ECM) murine model of Plasmodium berghei ANKA. In a longitudinal study, we show that the camera-phone imager can detect an overall decrease in the retinal blood-flow-speed (BFS) as ECM develops in P. berghei ANKA infected mice, with no similar change observed in uninfected control mice or mice infected with a non-ECM inducing strain (P. berghei NK65). Furthermore, by analyzing relative alterations in the BFS of individual retinal vessels during the progression of ECM, we illustrate the strength of our imager in identifying different BFS-change heterogeneities in the retinas of ECM and uninfected mice. The technique creates new possibilities for objective investigations into the diagnosis and pathogenesis of CM noninvasively through the eye. The camera-phone laser speckle imager along with measured spatial blood perfusion maps of the retina of a mouse infected with P. berghei ANKA-a fatal ECM model-on different days during the progression of the infection (top, day 3 after infection; middle, day 5 after infection; and bottom, day 7 after infection).

High-speed Brillouin imaging via continuous-wave stimulated Brillouin scattering (Conference Presentation)

Brillouin spectroscopy is a noncontact technique for characterizing the mechanical properties of materials. Typically, Brillouin spectrometers have been realized using scanning Fabry–Perot spectrometers that measure, with long acquisition times, spontaneous Brillouin scattering from the samples. In the last few years, the use of virtually imaged phase array (VIPA) etalons for constructing Brillouin spectrometers has enabled to acquire spontaneous Brillouin spectra <1,000-fold faster than with scanning Fabry–Perot spectrometers, opening up new means for high-speed Brillouin analysis of materials. In this talk, we will present a different approach for high-speed Brillouin material analysis. The method uses continuous-wave stimulated Brillouin scattering (CW-SBS) to measure stimulated Brillouin gain (SBG) spectra of materials at <100 milliseconds – up to 100-fold faster than with existing CW-SBS spectrometers. The SBS spectrometer comprises two nearly counter-propagating single-frequency lasers at 780 nm whose frequency detuning is scanned through the material Brillouin shift. SBG is detected via an ultra-narrowband hot rubidium-85 vapor notch filter and a lock-in detector, resulting in an improved signal-to-noise ratio that enables to significantly shorten acquisition times. We will show that this improvement, combined with micrometer-step-size spatial scanning of the sample, provides precise Brillouin profiles of layered liquids at 30-milliseconds pixel-dwell-time, facilitating Brillouin profilometry analysis of materials at high speed.

High-speed Continuous-wave Stimulated Brillouin Scattering Spectrometer for Material Analysis

Recent years have witnessed a significant increase in the use of spontaneous Brillouin spectrometers for non-contact analysis of soft matter, such as aqueous solutions and biomaterials, with fast acquisition times. Here, we discuss the assembly and operation of a Brillouin spectrometer that uses stimulated Brillouin scattering (SBS) to measure stimulated Brillouin gain (SBG) spectra of water and lipid emulsion-based tissue-like samples in transmission mode with <10 MHz spectral-resolution and <35 MHz Brillouin-shift measurement precision at <100 ms. The spectrometer consists of two nearly counter-propagating continuous-wave (CW) narrow-linewidth lasers at 780 nm whose frequency detuning is scanned through the material Brillouin shift. By using an ultra-narrowband hot rubidium-85 vapor notch filter and a phase-sensitive detector, the signal-to-noise-ratio of the SBG signal is significantly enhanced compared to that obtained with existing CW-SBS spectrometers. This improvement enables measurement of SBG spectra with up to 100-fold faster acquisition times, thereby facilitating high spectral-resolution and high-precision Brillouin analysis of soft materials at high speed.

Continuous-wave stimulated Brillouin spectroscopy in scattering media at 780 nm(Conference Presentation)

Quantitative probing of the mechanical properties of scattering media by Brillouin spectroscopy is an emerging field of research. At present, Brillouin spectrometers typically detect spontaneous Brillouin backscattered signals from the sample using setups that comprise virtually imaged phased arrays (VIPAs) cascaded in cross-axis configuration or heated molecular absorption cells prior to the VIPA. These experimental arrangements are necessary in order to significantly suppress the strong elastic scattering background from the medium. In this talk, we present a different approach for Brillouin spectroscopy of scattering matter based on stimulated Brillouin scattering (SBS) amplification. Unlike spontaneous Brillouin scattering, SBS amplification does not show elastic scattering background due to the resonant nature of the amplification process, thereby providing excellent spectral contrast. We demonstrate that the use of two continuous-wave distributed feedback lasers at 780 nm in a counter-propagating SBS amplifier geometry is useful for acquiring high signal-to-noise ratio SBS spectra of Intralipid solutions at concentrations that yield up to ~3 scattering events for photons propagating through the sample. Potential applications of SBS spectroscopy in mechanical characterization of thin tissue sections and biopolymers will be discussed.

Burn depth assessment by a cell-phone

We develop and test a novel cellphone-based laser speckle analyzer for assessing burn depth in vivo. The results show a potential for monitoring burn recovery and that superficial-dermal and deep-dermal burns are distinguishable 32-hours-post-injury.