Lung tissue phantom mimicking pulmonary optical properties, relative humidity, and temperature: a tool to analyze the changes in oxygen gas absorption for different inflated volumes

Abstract

Significance: Gas in scattering media absorption spectroscopy (GASMAS) enables noninvasive gas sensing in the body. It is developing as a tool for diagnosis and monitoring of respiratory conditions in neonates. Phantom models with relevant features to the clinical translation of GASMAS technology are necessary to understand technical challenges and potential applications of this technique. State-of-the-art phantoms designed for this purpose have focused on the optical properties and anthropomorphic geometry of the thorax, contributing to the source-detector placement, design, and optimization. Lung phantom mimicking the alveolar anatomy has not been included in the existent models due to the inherent complexity of the tissue. We present a simplified model that recreates inflated alveoli embedded in lung phantom.

Aim: The goal of this study was to build a lung model with air-filled structures mimicking inflated alveoli surrounded by optical phantom with accurate optical properties (μa = 0.50 cm - 1 and μs'=5.4 cm-1) and physiological parameters [37°C and 100% relative humidity (RH)], and to control the air volume within the phantom to demonstrate the feasibility of GASMAS in sensing changes in pulmonary air volume.

Approach: The lung model was built using a capillary structure with analogous size to alveolar units. Part of the capillaries were filled with liquid lung optical phantom to recreate scattering and absorption, whereas empty capillaries mimicked air filled alveoli. The capillary array was placed inside a custom-made chamber that maintained pulmonary temperature and RH. The geometry of the chamber permitted the placement of the laser head and detector of a GASMAS bench top system (MicroLab Dual O2 / H2O), to test the changes in volume of the lung model in transmittance geometry.

Results: The lung tissue model with air volume range from 6.89 × 10 - 7 m3 to 1.80 × 10 - 3 m3 was built. Two measurement sets, with 10 different capillary configurations each, were arranged to increase or decrease progressively (in steps of 3.93 × 10 - 8 m3) the air volume in the lung model. The respective GASMAS data acquisition was performed for both data sets. The maximum absorption signal was obtained for configurations with the highest number of air-filled capillaries and decreased progressively when the air spaces were replaced by capillaries filled with liquid optical phantom. Further studies are necessary to define the minimum and maximum volume of air that can be measured with GASMAS-based devices for different source-detector geometries.

Conclusions: The optical properties and the structure of tissue from the respiratory zone have been modeled using a simplified capillary array immersed in a controlled environment chamber at pulmonary temperature and RH. The feasibility of measuring volume changes with GASMAS technique has been proven, stating a new possible application of GASMAS technology in respiratory treatment and diagnostics.

Keywords: alveolar structure; gas spectroscopy; inflated volume; lung phantom.

Fig. 1

Diagram of the respiratory airways. The conducting airways warm up and moisten the inhaled air, the gas exchange occurs in the respiratory zone where the alveoli are located.

Fig. 2

(a) Diagram of the experimental set up designed to study the variations in GASMAS signal for different gas volumes in the lung tissue phantom. (b) Zoom in of source and detector placement in transmitance geometry. (c) The capillary array placed in the 3D printed holder, the sampling volume was defined by the size of light source and detector and the length of the capillary array.

Fig. 3

WMS amplitude signals that correspond to the absorption of molecular oxygen tested in the lung model in transmittance geometry. (a) Complete test set 1, the acquisition started with all capillaries filled with air followed by a progressive reduction of air volume. The dashed line exhibits higher absorption compared with the absorption of the maximum sampling volume, therefore these data were removed in (b) for the analysis. (c) Signals from test set 2, the acquisition started with all capillaries filled with optical properties followed by a progressive increase of air volume.

Fig. 4

(a) Peak values of the WMS absorption signals of molecular oxygen as a function of the number of capillaries filled with liquid phantom; and (b) light transmission values for the capillary arrays with optical phantom.