Optical and thermal properties of lung tissue undergoing thermal treatment

Minimally invasive photothermal and thermal ablation techniques represent promising strategies for the treatment of tumors affecting lung tissue. In order to monitor and increase the effectiveness of these therapies, it is pivotal to characterize the optical–thermal response of lung tissue as a function of thermal treatment. In this study, we present the analysis of the optical and thermal properties of ex vivo calf and porcine lung tissue samples undergoing a homogenous thermal treatment from room to ablative temperature.

Optical properties, specifically the absorption coefficient (μ_a) and reduced scattering coefficient (μ’_s), were estimated over a broadband spectral range from 657 nm to 1107 nm, using time-domain diffuse optical spectroscopy. Furthermore, the transient hot-wire technique was utilized for the measurements of the thermal properties, i.e., thermal conductivity (k) and thermal diffusivity (D), by means of a dual needle probe.

Concerning optical properties, the thermal treatment induced changes in the μ_a spectra, such as the rise of two more evident peaks at the wavelengths of 770 nm and 830 nm and a dip at ~910 nm at temperatures >75 °C. Conversely, for μ’_s, related to the lung microstructure, minimal variations were observed during the thermal treatment.

Regarding thermal properties, both k and D experienced an exponential increment with tissue temperature. At ablative temperatures (~90 °C), the measured thermal properties reached average values more than 13 times higher than the nominal values estimated for the tissue samples at room temperature.

Overall, the characterization of the optical and thermal response of lung tissue subjected to thermal treatment could represent a step forward in the optimization of photothermal and thermo-ablative techniques for pulmonary tissue treatment.