Abstract
Visualization and correct assessment of alveolar volume via intact lung imaging is important to study and assess respiratory mechanics. Optical Coherence Tomography (OCT), a real-time imaging technique based on near-infrared interferometry, can image several layers of distal alveoli in intact, ex vivo lung tissue. However optical effects associated with heterogeneity of lung tissue, including the refraction caused by air-tissue interfaces along alveoli and duct walls, and changes in speed of light as it travels through the tissue, result in inaccurate measurement of alveolar volume. Experimentally such errors have been difficult to analyze because of lack of ’ground truth,’ as the lung has a unique microstructure of liquid-coated thin walls surrounding relatively large airspaces, which is difficult to model with cellular foams. In addition, both lung and foams contain airspaces of highly irregular shape, further complicating quantitative measurement of optical artifacts and correction. To address this we have adapted the Bragg-Nye bubble raft, a crystalline two-dimensional arrangement of elements similar in geometry to alveoli (up to several hundred μm in diameter with thin walls) as an inflated lung phantom in order to understand, analyze and correct these errors. By applying exact optical ray tracing on OCT images of the bubble raft, the errors are predicted and corrected. The results are validated by imaging the bubble raft with OCT from one edge and with a charged coupled device (CCD) camera in transillumination from top, providing ground truth for the OCT.
Highlights
Lung imaging is important for lung physiology and pathology
Serious pathologies associated with sub-optimal mechanical behavior of the lung include acute respiratory distress syndrome (ARDS)[2], chronic obstructive pulmonary disease (COPD)[4], ventilator-induced lung injury (VILI)[3, 5] and extreme cases of asthma[6]
Note that the predicted bottom surface artifact falls on the experimentally-observed Optical Coherence Tomography (OCT) results (Fig 6a) closely
Summary
Lung imaging is important for lung physiology and pathology. For example accurate measurement of alveolar volume under different environmental or clinical conditions can provide information about the stability, interdependence, and mechanism of alveolar collapse and reopening under mechanical ventilation or condition of atelectatic recovery [1]. Serious pathologies associated with sub-optimal mechanical behavior of the lung include acute respiratory distress syndrome (ARDS)[2], chronic obstructive pulmonary disease (COPD)[4], ventilator-induced lung injury (VILI)[3, 5] and extreme cases of asthma[6]. Improved knowledge of lung mechanics, from pressure-volume compliance to alveolar collapse and re-opening, can provide insight into the initiation and progression of such pathologies. Macroscopic imaging techniques are limited by the small size of the important structures such as alveoli, capillaries and alveolar walls. Light penetrates a considerable distance into lung[7] and changes
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