| 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 synthetic 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 um 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. We also developed a tomographic technique based on incoherent summation of multiple angle-diverse images by utilizing image registration to increase our depth of imaging and our results were validated by utilizing the inflated lung phantom.;In this thesis also, an experimental apparatus for macro-scale mechanical probing of lung with in-situ micro-scale imaging of alveolar deformation was analyzed. Specifically, the OCT system was combined with a transparent, rigid tip enabling spherical indentation of the pleural surface of inflated, excised lung and real-time imaging of distal alveolar compression and shear. This apparatus, first presented in a previous study, was modeled using ray tracing methods to predict imaging artifacts (distortion) arising from tip refraction. Predicted artifacts are compared with experimental observations on inflated lung and lung phantom, and suggest that distortion (i) occurs typically away from the primary region of interest and (ii) becomes less apparent as a tip with a larger radius of curvature is used. |
