Investigating ultrasound–light interaction in scattering media

Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation’s “tagging efficiency.” Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature. Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process. Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component. Results: Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4  mm⁻¹ for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other. Conclusion: Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques

Analyzing the relationship between decorrelation time and tissue thickness in acute rat brain slices using multispeckle diffusing wave spectroscopy

Novel techniques in the field of wavefront shaping have enabled light to be focused deep inside or through scattering media such as biological tissue. However, most of these demonstrations have been limited to thin, static samples since these techniques are very sensitive to changes in the arrangement of the scatterers within. As the samples of interest get thicker, the influence of the dynamic nature of the sample becomes even more pronounced and the window of time in which the wavefront solutions remain valid shrinks further. In this paper, we examine the time scales upon which this decorrelation happens in acute rat brain slices via multispeckle diffusing wave spectroscopy and investigate the relationship between this decorrelation time and the thickness of the sample using diffusing wave spectroscopy theory and Monte Carlo photon transport simulation

Seeing Through the Fog: Using Scattered Light to Peer Deeper into Biological Tissue

Optical scattering is a fundamental problem in biomedical optics and limits most optical techniques to shallow operating depths less than 1 millimeter. However, although the scattering behavior of tissue scrambles the information it contains, it does not destroy it. Therefore, if you can unscramble the scattered light, it increases the accessible imaging depths up the absorption limit of light (several centimeters deep). One such way to beat optical scattering is using wavefront shaping. Borrowing ideas from adaptive optics in astronomy and phased arrays in radar and ultrasonic imaging, the basic concept of wavefront shaping is to control the phase and amplitude of the light field in order to harness scattered light. Using wavefront shaping techniques, scattered light can be used to form focal spots or transmit information through or inside optically scattering media. Furthermore, even without correcting for scattering directly by shaping the input light field, the properties of the scattered light can be analyzed to recover information about the structure and dynamic properties of a sample using methods from diffuse optics. The main contributions of this thesis are along these two lines of research: moving wavefront shaping toward more practical applications and developing new techniques to recover useful physiological information from scattered light. This is developed through three main projects: (1) an investigation of how dynamic samples impact the scattering process and the practical implications of these dynamics on wavefront shaping systems, (2) the development of a wavefront shaping system combining light and ultrasound to focus light inside acute brain slices to improve light delivery for optogenetics, (3) a novel method to sensitively detect the dynamics of scattered light and use it to tease out information about the flow of blood within the tissue sample of interest.</p

Refractive index measurement using an optical cavity based biosensor with a differential detection

We proposed a low cost optical cavity based biosensor with a differential detection for point-of-care diagnosis. Two lasers at different wavelengths are used for the differential detection. This method enhances the sensitivity through higher responsivity and noise cancelation. To reduce noise further, especially due to the unstable low cost laser diode output, we employed a referencing method in which a reference pixel value in each CMOS image frame is subtracted from all other pixels. To validate the designed structure and demonstrate the sensitivity of it, we perform refractive index measurements of fluids with our design. In this presentation, we will discuss our design, simulation results, and measurement results

Interferometric speckle visibility spectroscopy for improved measurement of blood flow dynamics

The dynamics of blood flow within tissue is a key indicator of metabolic function, providing functional information about physiological activity [1]. Speckle visibility spectroscopy (SVS) [2,3] is an emerging technique which allows for blood flow dynamics to be measured non-invasively by analyzing the statistical properties of a captured optical speckle field which has interacted with blood in a volume of interest. Blurring of the speckle field caused by the dynamic scattering of the blood cells contains information about the blood flow dynamics. However, at weak signal light intensities, the impact of camera noise prevents accurate measurements of the sample dynamics using SVS. This means that longer camera exposures are required in order to accumulate enough signal photons to accurately determine the dynamics of the sample, which leads to reduced measurement refresh rates. In this poster we will present an optical measurement method which enables high-speed measurement of the optical field dynamics with shot-noise limited sensitivity. This method, termed interferometric speckle visibility spectroscopy (iSVS), enables sensitive, non-invasive monitoring of hemodynamic activity, even when dealing with very weak signal light intensities. Furthermore, the interferometric nature of the measurement allows for calculations to be performed with the electric field autocorrelation function g1(t) directly, avoiding the errors typically encountered when relating the intensity autocorrelation function g2(t) to the blood flow signal of interest and enabling accurate measurements in samples where the scattering dynamics are non-ergodic. In this poster we will develop the theoretical advantages of iSVS compared to other methods for measuring blood flow in dynamic samples and also present some proof-of-concept in-vivo blood flow data collected from rodent models. References: [1] Durduran, Turgut, and Arjun G. Yodh. Diffuse correlation spectroscopy for non-invasive, micro-vascular cerebral blood flow measurement. Neuroimage 85 (2014): 51-63. [2] Dixon, P. K., and Douglas J. Durian. Speckle visibility spectroscopy and variable granular fluidization. Physical review letters 90.18 (2003): 184302. [3] Bandyopadhyay, Ranjini, et al. Speckle-visibility spectroscopy: A tool to study time-varying dynamics. Review of scientific instruments 76.9 (2005): 093110

Investigating ultrasound–light interaction in scattering media

Significance: Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation’s “tagging efficiency.” Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature. Aim: The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process. Approach: We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component. Results: Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4  mm⁻¹ for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other. Conclusion: Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques

Improving light delivery for optogenetics using wavefront shaping

New developments in neuroscience are enabling us to understand the brain at unprecedented temporal and spatial resolution. One of these exciting new techniques is optogenetics, which allows select neuronal populations of the brain to be targeted to express light sensitive ion channels. These enable optical control of the electrophysiological state of the cell, enabling neurons to be activated or deactivated using light. However, due to the strongly scattering nature of biological tissue in the brain, tightly focusing light to a specific voxel is not possible with conventional optical techniques. In this poster we will present the results of our recent work to develop new optical wavefront shaping tools which enable us to focus light inside strongly scattering media and discuss the outlook for such tools for improving light delivery for techniques such as optogenetics. The focus of our work is to use an optical wavefront shaping technology termed Time-Reversed Ultrasound- Encoded (TRUE) focusing [1,2]. This strategy uses ultrasound to form an ultrasonic focus at depths beyond the optical diffusion limit. This ultrasound focus modulates photons passing through it via the acousto-optic effect, shifting their frequency by the ultrasound frequency. Then, by detecting these ultrasound-tagged photons, we can measure the optical wavefront corresponding to the tagged photons and selectively time-reverse this optical field using a technique called Digital Optical Phase Conjugation (DOPC) [3]. This wavefront is then used to send photons back into the scattering tissue in such a way that they travel in a time-reversed fashion, constructively interfering at the location of the ultrasound focus. This allows us to focus light in highly scattering media beyond the optical diffusion limit at ultrasonic resolution (~30 micrometers at 50 MHz). In this poster we will present results from recent work using the TRUE focusing technique to perform optogenetic stimulation. We demonstrate in 300 and 500 micrometer thick living brain slices that the TRUE focusing technique can be used to improve the spatial resolution of optogenetic stimulation compared to conventional optical methods. Furthermore, we will discuss the outlook and challenges facing the development of wavefront shaping techniques such as TRUE focusing for applications in neuroscience and other areas of biotechnology. References: [1] Xu, Xiao, Honglin Liu, and Lihong V. Wang. Time-reversed ultrasonically encoded optical focusing into scattering media. Nature photonics 5.3 (2011): 154-157. [2] Wang, Ying Min, et al. Deep-tissue focal fluorescence imaging with digitally time-reversed ultrasound- encoded light. Nature communications 3 (2012): 928. [3] Cui, Meng, and Changhuei Yang. Implementation of a digital optical phase conjugation system and its application to study the robustness of turbidity suppression by phase conjugation. Optics express 18.4 (2010)

Glare suppression by coherence gated negation

Imaging of a weak target hidden behind a scattering medium can be significantly confounded by glare. We report a method, termed coherence gated negation (CGN), that uses destructive optical interference to suppress glare and allow improved imaging of a weak target. As a demonstration, we show that by permuting through a set range of amplitude and phase values for a reference beam interfering with the optical field from the glare and target reflection, we can suppress glare by an order of magnitude, even when the optical wavefront is highly disordered. This strategy significantly departs from conventional coherence gating methods in that CGN actively 'gates out' the unwanted optical contributions while conventional methods 'gate in' the target optical signal. We further show that the CGN method can outperform conventional coherence gating image quality in certain scenarios by more effectively rejecting unwanted optical contributions.Comment: main article (14 pages) and appendices (3 pages

Interferometric speckle visibility spectroscopy (ISVS) for human cerebral blood flow monitoring

Infrared light scattering methods have been developed and employed to non-invasively monitor human cerebral blood flow (CBF). However, the number of reflected photons that interact with the brain is low when detecting blood flow in deep tissue. To tackle this photon-starved problem, we present and demonstrate the idea of interferometric speckle visibility spectroscopy (ISVS). In ISVS, an interferometric detection scheme is used to boost the weak signal light. The blood flow dynamics are inferred from the speckle statistics of a single frame speckle pattern. We experimentally demonstrated the improvement in the measurement of fidelity by introducing interferometric detection when the signal photon number is low. We apply the ISVS system to monitor the human CBF in situations where the light intensity is ∼100-fold less than that in common diffuse correlation spectroscopy (DCS) implementations. Due to the large number of pixels (∼2 × 10⁵) used to capture light in the ISVS system, we are able to collect a similar number of photons within one exposure time as in normal DCS implementations. Our system operates at a sampling rate of 100 Hz. At the exposure time of 2 ms, the average signal photoelectron number is ∼0.95 count/pixel, yielding a single pixel interferometric measurement signal-to-noise ratio (SNR) of ∼0.97. The total ∼2 × 10⁵ pixels provide an expected overall SNR of 436. We successfully demonstrate that the ISVS system is able to monitor the human brain pulsatile blood flow, as well as the blood flow change when a human subject is doing a breath-holding task