Multi-mode fiber-based speckle contrast optical spectroscopy: analysis of speckle statistics

Abstract

Speckle contrast optical spectroscopy/tomography (SCOS/T) provides a real-time, non-invasive, and cost-efficient optical imaging approach to mapping of cerebral blood flow. By measuring many speckles (n>>10), SCOS/T has an increased signal-to-noise ratio relative to diffuse correlation spectroscopy, which measures one or a few speckles. However, the current free-space SCOS/T designs are not ideal for large field-of-view imaging in humans because the curved head contour cannot be readily imaged with a single flat sensor and hair obstructs optical access. Herein, we evaluate the feasibility of using cost-efficient multi-mode fiber (MMF) bundles for use in SCOS/T systems. One challenge with speckle contrast measurements is the potential for confounding noise sources (e.g., shot noise, readout noise) which contribute to the standard deviation measure and corrupt the speckle contrast measure that is central to the SCOS/T systems. However, for true speckle measurements, the histogram of pixel intensities from light interference follows a non-Gaussian distribution, specifically a gamma distribution with non-zero skew, whereas most noise sources have pixel intensity distributions that are Gaussian. By evaluating speckle data from static and dynamic targets imaged through an MMF, we use histograms and statistical analysis of pixel histograms to evaluate whether the statistical properties of the speckles are retained. We show that flow-based speckle can be distinguished from static speckle and from sources of system noise through measures of skew in the pixel intensity histograms. Finally, we illustrate in humans that MMF bundles relay blood flow information.

Validation of the Openwater wearable optical system: cerebral hemodynamic monitoring during a breath-hold maneuver

“…Instead, it uses speckle contrast to derive blood flow, which is based on an integral of the autocorrelation function. Speckle contrast flow monitoring with low-cost CMOS/CCD has been studied by several groups, 27 – 32 , 74 – 81 including with wearable probes without fibers, similar to the Openwater system 33 – 35 , 81 Concurrent speckle contrast and DCS monitoring of relative CBF changes have been compared in murine and neonatal swine models, 33 , 35 as well as in human skeletal muscle during cuff-induced forearm ischemia 27 , 29 , 30 .…”

Comparing the performance potential of speckle contrast optical spectroscopy and diffuse correlation spectroscopy for cerebral blood flow monitoring using Monte Carlo simulations in realistic head geometries

“…In the absence of noise, the measured contrast is proportional to the DCS autocorrelation function though 29 Kf2=2βTexp∫0Texpg12(τ)(1−τTexp)dτ,where Kf is the fundamental speckle contrast and Texp is the exposure time. In contrast to the single/few-mode fibers used in DCS measurements, light detection from the tissue in SCOS makes it possible to use large multimode fibers or multi-mode fiber bundles, greatly increasing the optical throughput, and, due to the number of pixels that are typically present in imaging sensors, allows for incredibly scalable, massively parallelized speckle channel detection 18 , 19 …”

“…In contrast to the single/few-mode fibers used in DCS measurements, light detection from the tissue in SCOS makes it possible to use large multimode fibers or multi-mode fiber bundles, greatly increasing the optical throughput, and, due to the number of pixels that are typically present in imaging sensors, allows for incredibly scalable, massively parallelized speckle channel detection. 18,19 2.3 Simulation Models…”

“…14,15 Sampling the spatial speckle contrast measured in an area some distance away from the illumination point provides high SNR potential as megapixel CMOS sensors are now available with low read noise and high frame rates. The latter approach, inspired by the laser speckle contrast imaging technique used for superficial perfusion imaging, 16 has been termed speckle contrast optical spectroscopy/tomography (SCOS/SCOT) 13,17 and has recently been demonstrated to allow light collection through multi-mode fiber bundles, 18,19 and offer more than an order of magnitude improvement in SNR with a lower price for cerebral blood flow (CBF) monitoring versus DCS measurements at the same source-detector separation. 18 Given the differences in how SCOS and DCS measurements derive blood flow from the fundamental scatterer motion, it is necessary to understand how both the sensitivity to cerebral perfusion, and the overall noise performance depend on system components and operating parameters for both methods.…”

Anatomical Modeling and Optimization of Speckle Contrast Optical Tomography

“…17,[21][22][23][24][25][26] These HD-DOT designs can be leveraged for designing SCOT systems. Recent SCOS studies show that cost-efficient multimode fibers (MMFs) are suitable for SCOS/SCOT, [27][28][29] wherein speckle statistics are preserved through the MMFs and flow information is relayed, e.g., for pulsatile blood flow. 29 HD-DOT imaging utilizes a linear forward model 10,11,15,30 relating changes in oxy-hemoglobin (HbO) and deoxy-hemoglobin (HbR) concentrations to differential diffuse optical measurements, with images of changes in HbO and HbR estimated by regularized inversion of the measurements.…”