Abstract
Since changes in lung microstructure are important indicators for (early stage) lung pathology, there is a need for quantifiable information of diagnostically challenging cases in a clinical setting, e.g. to evaluate early emphysematous changes in peripheral lung tissue. Considering alveoli as spherical air-spaces surrounded by a thin film of lung tissue allows deriving an expression for Carr-Purcell-Meiboom-Gill transverse relaxation rates R 2 with a dependence on inter-echo time, local air-tissue volume fraction, diffusion coefficient and alveolar diameter, within a weak field approximation. The model relaxation rate exhibits the same hyperbolic tangent dependency as seen in the Luz-Meiboom model and limiting cases agree with Brooks et al. and Jensen et al. In addition, the model is tested against experimental data for passively deflated rat lungs: the resulting mean alveolar radius of R A = 31.46 ± 13.15 μm is very close to the literature value (∼34 μm). Also, modeled radii obtained from relaxometer measurements of ageing hydrogel foam (that mimics peripheral lung tissue) are in good agreement with those obtained from μCT images of the same foam (mean relative error: 0.06 ± 0.01). The model’s ability to determine the alveolar radius and/or air volume fraction will be useful in quantifying peripheral lung microstructure.
Highlights
Structural and functional changes in pulmonary disease are generally tightly linked to alterations in lung microstructure, most familiar in pulmonary emphysema, where remodeling and/or obliteration of small acini and alveoli, as well as parenchymal tissue destruction, lead to an increasing obstruction of the lung’s airways [1, 2]
Pulmonary function tests are not able to separate between different forms of the underlying tissue pathology, especially when tissue alterations are heterogeneously distributed throughout the entire lung as in early stage emphysema [3, 4]
The Wigner-Seitz foam model for peripheral lung tissue is based on the notion of alveoli as rhombic dodecahedral air-spaces embedded in a surrounding medium [15]
Summary
Structural and functional changes in pulmonary disease are generally tightly linked to alterations in lung microstructure, most familiar in pulmonary emphysema, where remodeling and/or obliteration of small acini and alveoli, as well as parenchymal tissue destruction, lead to an increasing obstruction of the lung’s airways [1, 2]. Pulmonary function tests are not able to separate between different forms of the underlying tissue pathology, especially when tissue alterations are heterogeneously distributed throughout the entire lung as in early stage emphysema [3, 4]. There is a need for quantitative lung imaging to assess the extent of microstructural changes and gain a deeper understanding of the associated pathophysiological.
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