Magnetic resonance behavior of normal and diseased lungs: spherical shell model simulations

The alveolar air-tissue interface affects the lung NMR signal, because it r esults in a susceptibility-induced magnetic field inhomogeneity. The air-ti ssue interface effect can be detected and quantified by measuring the diffe rence signal (Delta) from a pair of NMR images obtained using temporally sy mmetric and asymmetric spin-echo sequences. The present study describes a m ulticompartment alveolar model (consisting of a collection of noninteractin g spherical water shells) that simulates the behavior of Delta as a functio n of the level of lung inflation and can be used to predict the NMR respons e to various types of lung:injury. The model was used to predict Delta as a function of the inflation level (with the assumption of sequential alveola r recruitment, partly parallel to distension) and to simulate pulmonary ede ma by deriving equations that describe ii for a collection of spherical she lls representing combinations of collapsed, flooded, and inflated alveoli. Our theoretical data were compared with those provided by other models and with experimental data obtained from the literature. Our results suggest th at NMR a measurements can be used to study the mechanisms underlying the lu ng pressure-volume behavior, to characterize lung injury, and to assess the contributions of alveolar recruitment and distension to the lung volume ch anges in response to the application of positive airway pressure (e.g., pos itive end-expiratory pressure).