Fast and sensitive diffuse correlation spectroscopy with highly parallelized single photon detection

Abstract

Diffuse correlation spectroscopy (DCS) is a well-established method that measures rapid changes in scattered coherent light to identify blood flow and functional dynamics within a tissue. While its sensitivity to minute scatterer displacements leads to a number of unique advantages, conventional DCS systems become photon-limited when attempting to probe deep into the tissue, which leads to long measurement windows (∽1 sec). Here, we present a high-sensitivity DCS system with 1024 parallel detection channels integrated within a single-photon avalanche diode array and demonstrate the ability to detect mm-scale perturbations up to 1 cm deep within a tissue-like phantom at up to a 33 Hz sampling rate. We also show that this highly parallelized strategy can measure the human pulse at high fidelity and detect behaviorally induced physiological variations from above the human prefrontal cortex. By greatly improving the detection sensitivity and speed, highly parallelized DCS opens up new experiments for high-speed biological signal measurement.

Imaging dynamics beneath turbid media via parallelized single-photon detection

“…The output of the neural network is an image x ∈ R 48×64 , with an image pixel size of 220 × 220µm 2 . This pixel size is a tunable parameter in our reconstruction model, which we select as smaller than the expected achievable resolution [16,33]). Figure 4 depicts our image reconstruction network.…”

“…To generate expected temporal fluctuation variations within living tissue caused e.g. by blood flow, we placed a digital micro-mirror device (DMD) immediately behind this tissue phantom, with which we programmatically created spatiotemporally varying patterns at kilohertz rates [33].…”

“…We guided this light to the liquid phantom surface using a 50µm, 0.22 numerical aperture (NA) multi-mode fiber (MMF). Before the MMF, we ensured that the DPSS laser output was effectively a single transverse mode with a fiber coupler, such that either an MMF or a single-mode fiber (SMF) could serve as the source waveguide [33,34], with MMF being a generally less expensive option. After entering the liquid phantom, the light randomly scatters and decorrelates, and a small fraction of which reaches the DMD placed immediately behind the turbid medium.…”

“…With 768 × 1024 pixels, the entire DMD panel has a screen size of 10.4 × 13.9mm 2 . We chose to use a DMD to generate the spatiotemporal dynamic scattering patterns first because it is easily configurable: light reaching the quickly flipping pixels decorrelates faster than light that does not, and these pixels are digitally addressible and thus can be changed both spatially and temporally without moving the setup; second, because it can meet requisite dynamic variation speeds (we run the DMD between 5-10 kHz), which we have selected to correlate with the response of blood flow at tested depths (5-8 mm) [33]. As the reflected multi-scattered light penetrates on average about 1 /2 − 2 /3 times the source-detector distance(ρ) deep into the phantom tissue [60], we place 12 multi-speckle detection fibers circularly around the source in the center with ρ = 9.0mm.…”

“…This limited correlation measurement rate is quite detrimental -the effects of the subject pulse are challenging to account for, and it precludes observation of dynamic variations at approximately sub-Hz rates, for example. Recent work has demonstrated how parallelized speckle detection across many optical sensor pixels [32,33,34,35,36] can lead to significantly faster correlation sampling rates. We build upon these insights to create a system capable of recording spatially resolved video of temporal decorrelation.…”