Alveolar air-tissue interface and nuclear magnetic resonance behavior of the lung

摘要

The nuclear magnetic resonance (NMR) properties of lung are markedly affected by the alveolar air-tissue interface, which produces internal magnetic field inhomogeneity because of the different magnetic susceptibilities of air and water. This internal magnetic field inhomogeneity results in a marked shortening of the free induction decay (FID) (in the time domain) and in inhomogeneous NMR line broadening (in the frequency domain). The signal loss due to internal magnetic field inhomogeneity can be measured as the difference $Delta between the spin-echo signals obtained using temporally symmetric and asymmetric spin-echo sequences; the degree of asymmetry of the asymmetric sequence is characterized by the asymmetry time t$-a$/. In accordance with predictions based on the analysis of theoretical models, experiments in excised rat lungs (studied at various inflation levels) have shown that $Delta depends on t$-a$/ and is very low in degassed lungs. When measured at t$-a$/ $EQ 6 ms, the difference signal ($Delta$-6ms$/) increases markedly with alveolar opening but does not vary significantly during the rest of the inflation-deflation cycle. In edematous (oleic acid-injured) lungs, the values of $Delta$-6ms$/ measured at low inflation levels are significantly below those observed in normal lungs. These results suggest that $Delta$-6ms$/ is very sensitive to alveolar recruitment and relatively insensitive to alveolar distension. Therefore, measurements of $Delta$-6ms$/ may provide a means of assessing the relative contributions of these two factors to the pressure-volume behavior of lung. Such measurements may contribute to the characterization of pulmonary edema (for example, by detecting the loss of alveolar air-tissue interface due to alveolar flooding, by differentiating interstitial from alveolar pulmonary edema, and by assessing the effects of positive airway pressures). NMR lineshape measurements can also provide valuable information regarding lung geometry and the characterization of pulmonary edema.