SignificanceInterstitial fiber-based spectroscopy is gaining interest for real-time in vivo optical biopsies, endoscopic interventions, and local monitoring of therapy. Different from other photonics approaches, time-domain diffuse optical spectroscopy (TD-DOS) can probe the tissue at a few cm distance from the fiber tip and disentangle absorption from the scattering properties. Nevertheless, the signal detected at a short distance from the source is strongly dominated by the photons arriving early at the detector, thus hampering the possibility of resolving late photons, which are rich in information about depth and absorption.AimTo fully benefit from the null-distance approach, a detector with an extremely high dynamic range is required to effectively collect the late photons; the goal of our paper is to test its feasibility to perform TD-DOS measurements at null source–detector separations (NSDS).ApproachIn particular, we demonstrate the use of a superconducting nanowire single photon detector (SNSPD) to perform TD-DOS at almost NSDS ( ≈ 150 μm ) by exploiting the high dynamic range and temporal resolution of the SNSPD to extract late arriving, deep-traveling photons from the burst of early photons.ResultsThis approach was demonstrated both on Monte Carlo simulations and on phantom measurements, achieving an accuracy in the retrieval of the water spectrum of better than 15%, spanning almost two decades of absorption change in the 700- to 1100-nm range. Additionally, we show that, for interstitial measurements at null source–detector distance, the scattering coefficient has a negligible effect on late photons, easing the retrieval of the absorption coefficient.ConclusionsUtilizing the SNSPD, broadband TD-DOS measurements were performed to successfully retrieve the absorption spectra of the liquid phantoms. Although the SNSPD has certain drawbacks for use in a clinical system, it is an emerging field with research progressing rapidly, and this makes the SNSPD a viable option and a good solution for future research in needle guided time-domain interstitial fiber spectroscopy.
KEYWORDS: In vivo imaging, Single photon detectors, Superconductors, Nanowires, Spectroscopy, Signal to noise ratio, Sensors, Picosecond phenomena, Liquids, Tissues
Diffuse correlations spectroscopy (DCS) is a non-invasive optical technique that, studying the speckle intensity fluctuations of light diffused through a biological tissue, measures its microvascular blood flow. Typically, a long coherence length continuous wave source is used, which limits the possibility to resolve the photon path lengths. Recently, time-domain (TD) DCS was proposed, where a pulsed yet coherent light source is used to resolve the speckle fluctuations at different time-of-flights. Due to the constraint of single-speckle detection and time-resolved acquisition, the technique has a limited throughput which limits depth sensitivity. Here, we demonstrate TD DCS with a superconducting nanowire single-photon detector (SNSPD). The SNSPD has a high quantum efficiency and temporal resolution, while maintaining a very low background and no after-pulsing.We report results on phantom and in vivo experiments, which show the potentiality of the proposed detection system for highly accurate TD DCS experiments.
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