Fast time-domain diffuse correlation spectroscopy with superconducting nanowire single-photon detector: system validation and in vivo results

Abstract

Time-domain diffuse correlation spectroscopy (TD-DCS) has been introduced as an advancement of the "classical" continuous wave DCS (CW-DCS) allowing one to not only to measure depth-resolved blood flow index (BFI) but also to extract optical properties of the measured medium without using any additional diffuse optics technique. However, this method is a photon-starved technique, specially when considering only the late photons that are of primary interest which has limited its in vivo application. In this work, we present a TD-DCS system based on a superconducting nanowire single-photon detector (SNSPD) with a high quantum efficiency, a narrow timing response, and a negligibly low dark count noise. We compared it to the typically used single-photon avalanche diode (SPAD) detector. In addition, this system allowed us to conduct fast in vivo measurements and obtain gated pulsatile BFI on the adult human forehead.

Figure 1

Experimental system (BS, beam splitter; VA, variable attenuator; PD, photodiode; MMF, multimode fiber; SMF, single mode fiber; TCSPC, time-correlated single-photon counting), schematically divided in source, detection and data acquisition units (red, grey and green boxes respectively).

Figure 2

Scheme on how photon arrival times are registered, windowed with overlapping segments and then processed with calculating intensity autocorrelation function.

Figure 3

Intralipid phantom measurement with the SPAD detector and the SNSPD: black—results from the SPAD, blue—results from the SNSPD; (a) measured normalized IRF, (b) measured DTOF, (c) ungated auto-correlation function averaged over 30 curves, shaded region represents a standard deviation.

Figure 4

Gated analysis of the intralipid phantom measurements with the SNSPD. (a) IRF (gray) and DTOF (blue) of the SNSPD with gates, (b) example of one $g_2$ curve for each gate, (c) fitted $\alpha D_b$ averaged over 30 values with standard deviation for each gate.

Figure 5

(a) IRF (gray), DTOF (blue), and fitted DTOF curve (red) of Subject 4 with shaded early and late gates; (b) pulsatile rBFI of Subject 4; (c) Power spectral density estimate (PSD) calculated over 7-minute of Resting state period for all 4 subjects. Colours correspond to different gates: black—ungated case, blue—early gate, red—late gate.

Figure 6

(a) Example of slow (bold line) and fast (thin pulsating line) rBFI signals to VM exercise (grey region highlights the VM time period); (b) rBFI response to VM exercise averaged over 6 repetitions (2 subjects $\times$ 3 repetitions), numbers show the location of each phase.