Interstitial null-distance time-domain diffuse optical spectroscopy using a superconducting nanowire detector

Abstract

Significance: Interstitial 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.

Aim: To 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).

Approach: In particular, we demonstrate the use of a superconducting nanowire single photon detector (SNSPD) to perform TD-DOS at almost NSDS $(\approx 150\mu m)$ 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.

Results: This 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.

Conclusions: Utilizing 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: biophotonics; diffuse optics; interstitial fiber; spectroscopy; superconducting nanowire detector; time domain.

Fig. 1

Schematic of the experimental setup.

Fig. 2

Comparision of the IRFs of the SNSPD and SiPM with the MC fluence and experimental DTOF (SNSPD) at ${\mu }_a=0.01{\mathrm{cm}}^{-1}$ and ${\mu }_s^{\prime }=10{\mathrm{cm}}^{-1}$.

Fig. 3

MC simulations (solid colored lines), and analytical solutions of the DE (black lines). (a), (b) The same curves but at shorter and longer time scales, respectively.

Fig. 4

Linearity test of fitted versus expected absorption with MC simulations. The rows contain absorption linearity spectra from DTOFs convolved with different IRFs, and the columns change as the start of the fitting range on the falling edge of the DTOF. Colored frames represent the best (green) and worst (red) cases.

Fig. 5

(a) Comparision of errors arising due to different IRFs. CV, coefficient of variation; RE, relative error. (b) Linearity measurements with the SNSPD at 700 and 860 nm.

Fig. 6

(a) Absorption spectrum of water obtained from three different experiments from liquid phantoms with ${\mu }_s^{\prime }=10{\mathrm{cm}}^{-1}$. (b) Effect of scattering on ${\mu }_a$ retrieval, where different curves represent different ${\mu }_s^{\prime }$ values (${\mathrm{cm}}^{-1}$).