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

Coherent fluctuations in time-domain diffuse optics

Near infrared light pulses, multiply scattered by random media, carry useful information regarding the sample key constituents and their microstructures. Usually, the photon diffusion equation is used to interpret the data, which neglects any interference effect in the detected light fields. However, in several experimental techniques, such as diffuse correlation spectroscopy or laser speckle flowmetry, the effect of light coherence is exploited to retrieve the information on the sample dynamical properties. Here, using an actively mode-locked Ti:Sapphire laser, we report the observation of temporal fluctuations in the diffused light pulse, which cannot be described by the diffusion theory. We demonstrate the sensitivity of these fluctuations on the sample dynamical properties and on the number of detected coherence areas (i.e., speckles). In addition, after interpreting the effect as a time-resolved speckle pattern, we propose a simple statistical method for its quantification. The proposed approach may enable the simultaneous monitoring of the static (absorption and scattering coefficients) and dynamical (Brownian diffusion coefficient) properties of the sample, and also provide physical insight on the propagation of optical waves in random media

Diffuse Correlation Spectroscopy (DCS) for Assessment of Tissue Blood Flow in Skeletal Muscle: Recent Progress

Near-infrared diffuse correlation spectroscopy (DCS) is an emerging technology for monitoring blood flow in various tissues. This article reviews the recent progress of DCS for the assessment of skeletal muscle blood flow, including the developments in technology allowing use during dynamic exercise and muscular electrical stimulation, the utilization for diagnosis of muscle vascular diseases, and the applications for evaluating treatment effects. The limitations of current DCS studies and future perspective are finally discussed

High Frequency Sampling of TTL Pulses on a Raspberry Pi for Diffuse Correlation Spectroscopy Applications

Diffuse Correlation Spectroscopy (DCS) is a well-established optical technique that has been used for non-invasive measurement of blood flow in tissues. Instrumentation for DCS includes a correlation device that computes the temporal intensity autocorrelation of a coherent laser source after it has undergone diffuse scattering through a turbid medium. Typically, the signal acquisition and its autocorrelation are performed by a correlation board. These boards have dedicated hardware to acquire and compute intensity autocorrelations of rapidly varying input signal and usually are quite expensive. Here we show that a Raspberry Pi minicomputer can acquire and store a rapidly varying time-signal with high fidelity. We show that this signal collected by a Raspberry Pi device can be processed numerically to yield intensity autocorrelations well suited for DCS applications. DCS measurements made using the Raspberry Pi device were compared to those acquired using a commercial hardware autocorrelation board to investigate the stability, performance, and accuracy of the data acquired in controlled experiments. This paper represents a first step toward lowering the instrumentation cost of a DCS system and may offer the potential to make DCS become more widely used in biomedical applications.Radiation Monitoring Devices, Inc

Transcranial diffuse optical assessment of the microvascular reperfusion after thrombolysis for acute ischemic stroke

In this pilot study, we have evaluated bedside diffuse optical monitoring combining diffuse correlation spectroscopy and near-infrared diffuse optical spectroscopy to assess the effect of thrombolysis with an intravenous recombinant tissue plasminogen activator (rtPA) on cerebral hemodynamics in an acute ischemic stroke. Frontal lobes of five patients with an acute middle cerebral artery occlusion were measured bilaterally during rtPA treatment. Both ipsilesional and contralesional hemispheres showed significant increases in cerebral blood flow, total hemoglobin concentration and oxy-hemoglobin concentration during the first 2.5 hours after rtPA bolus. The increases were faster and higher in the ipsilesional hemisphere. The results show that bedside optical monitoring can detect the effect of reperfusion therapy for ischemic stroke in real-time.Peer ReviewedPostprint (published version