Large source-detector distance time-resolved measurements to probe adult lungs non-invasively

Nowadays, the origin and the onset of lung diseases are mainly investigated through computed tomography (CT), chest x-ray (CXR) and biopsy (B). However, these techniques cannot provide any physiological insight (CT, CXR) or they cannot be performed on a routine basis given their expensiveness (CT) and invasiveness (CT, CXR, B). This work delves into the design of a strategy to probe adult human lungs in vivo through time domain diffuse optical spectroscopy (TD-DOS), based on its capability to characterize non-invasively biological tissues down to a depth of a few centimeters, especially when a large source-detector distance is used. This is particularly important in the case of the lungs, given the stratified structure of the chest, where they occupy the deepest position after skin, lipids, pectoral muscle, ribs and pleural membranes. Simulations and measurements on phantoms were conducted to test different scenarios in terms of wavelength and geometry to sense the optical properties of the lungs. Then, a breathing protocol was employed on healthy subjects to ascertain whether re-emitted pulses have effectively propagated through pulmonary tissue. More experimental setups were used to cover a broad spectral range (600–1300 nm) and test different source-detector distances. In this work, we will focus on large source-detector distance measurements performed in vivo. These initial results lay the groundwork for further research on how to best exploit the potential of TD-DOS to probe the lungs non-invasively.