The optical properties of lung as a function of respiration

Lung consists of alveoli enclosed by tissue and both structures contribute to volume-dependent scattering of light. It is the purpose of this paper to determine the volume-dependent optical properties of lung. In vivo interstitial fibre measurements of the effective attenuation coefficient mu(eff) at 632.8 nm differed during inspiration (mu(eff) = 2.5 +/- 0.5 cm(-1)) from that during expiration (mu(eff) = 3.2 +/- 0.6 cm(-1)). In vitro measurements on a piglet lung insufflated with oxygen from 50 to 150 mi showed the effective attenuation coefficient at 632.8 nm decreases as a function of oxygen volume in the lung (at 50 mi mu(eff) = 2.97 +/- 0.11 cm(-1), at 100 mi mu(eff) = 1.50 +/- 0.07 cm(-1), and at 150 mi mu(eff) = 1.36 +/- 0.15 cm(-1)). This was explained by combining scattering of alveoli (Mie theory) with optical properties of collapsed lung tissue using integrating sphere measurements. Theory and measured in vitro values showed good agreement (deviation less than or equal to 15%). Combination of these data yields the absorption coefficient and scattering parameters of lung tissue as a function of lung volume. We conclude that the light fluence rate in lung tissue should be estimated using optical properties that include scattering by the alveoli.