  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.
