Addressing the Need for Non-Invasive Lung Assessment with Time-Resolved Diffuse Optics

Abstract Monitoring lungs functions is key for detecting several morbidities and pathologies. Photons in the 600-1300 nm range might have the potentiality to reach lungs and provide compositional and functional information. Yet, few optical techniques have been challenged non-invasively so far. In this paper, we investigate the conditions to probe lungs using Time Domain Diffuse Optical spectroscopy (TD-DO). Counterintuitively, we discovered from Monte Carlo simulations that a higher absorption coefficient in the chest wall as compared to lungs increases sensitivity to deeper structures. In vivo measurements on the thorax of healthy volunteers during a protocol of forced breathing, complemented with information on lung composition and in vivo spectra of swine lung, suggest that such condition is met above 1100 nm. Multiple experimental setups were exploited to cover the 600-1300 nm spectral range and test different source-detector distances (3-7 cm). All measurements exhibit oscillations consistent with the breathing rhythm, suggesting detection of lung expansion and compression. However, diverse responses from subjects and also a complex dependence of the detected signal on the photon time-of-flight seem to allure to a non-trivial role of photon propagation through lungs, related – for instance – to the presence of alveoli. The unceasing development of time-resolved single-photon detectors with increasing performances above 1000 nm – e.g. InGaAs Solid State Photomultipliers or large-area Ge diodes – with a better understanding of lung optics – e.g. anomalous diffusion models – might unravel the information from late, deep-travelling photons and lead to a novel photonic tool to probe the lungs non-invasively.