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

Abstract Non-invasive in vivo sensing of the lung with light would help diagnose and monitor pulmonary disorders (caused by e.g., emphysema, immature lung tissue in infants, Covid-19). 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 (7-9 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 long photon propagation times. Yet, intra- and inter-subject variability and inconsistencies, possibly alluring to competing scattering and absorption effects, prevented a straightforward interpretation. Aspects to be further investigated to gain a deeper insight are discussed.