Initial non-invasive in vivo sensing of the lung using time domain diffuse optics

TL;DR

This study explores the feasibility of using time domain diffuse optics for non-invasive in vivo lung sensing, demonstrating potential depth reach and observing breathing-related signal changes, but highlighting variability challenges.

Contribution

First application of time domain diffuse optics for in vivo lung sensing, assessing depth and breathing signals in humans.

Findings

Probed lung tissue at depths up to 3.9 cm.

Detected signal changes during breathing cycles.

Identified variability and scattering effects affecting interpretation.

Abstract

Non-invasive in vivo sensing of the lung with light would help diagnose and monitor pulmonary disorders (caused by e.g. COVID-19, emphysema, immature lung tissue in infants). We investigated the possibility to probe the lung with time domain diffuse optics, taking advantage of the increased depth (few cm) reached by photons detected after a long (few ns) propagation time. An initial study on 5 healthy volunteers included time-resolved broadband diffuse optical spectroscopy measurements at 3 cm source-detector distance over the 600-1100 nm range, and long-distance (6-9 cm) measurements at 820 nm performed during a breathing protocol. The interpretation of the in vivo data with a simplified homogeneous model yielded a maximum probing depth of 2.6-3.9 cm, suitable to reach the lung. Also, signal changes related to the inspiration act were observed, especially at high photon propagation times. Yet, intra- and inter-subject variability and inconsistencies, possibly alluring to competing scattering and absorption effects, prevented a simple interpretation. Aspects to be further investigated to gain a deeper insight are discussed.