Krishnamurthy Ganesan

Quantitative assessment of pulmonary edema by nuclear magnetic resonance methods.

Considerable progress has been made in the application of nuclear magnetic resonance (NMR) imaging and nonimaging techniques to the quantitative assessment of pulmonary edema. NMR measurements offer the advantages of being noninvasive, relatively rapid, and easily repeatable. In addition, NMR imaging is suitable for the determination of lung water distribution. Studies of various animal models have shown that NMR techniques can adequately detect and quantify relative changes in lung water content and distribution in various types of experimental lung injury. Preliminary observations in humans suggest that NMR measurement of relative lung water changes in clinical pulmonary edema should be feasible. Although the application of NMR to the assessment of pulmonary edema appears to be very promising, it also poses significant problems that must be solved before it can be established as a standard experimental and clinical method.

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

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 Δ 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 τa. In accordance with predictions based on the analysis of theoretical models, experiments in excised rat lungs (studied at various inflation levels) have shown that Δ depends on τa and is very low in degassed lungs. When measured at τa equals 6 ms, the difference signal (Δ6ms) 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 Δ6ms measured at low inflation levels are significantly below those observed in normal lungs. These results suggest that Δ6ms is very sensitive to alveolar recruitment and relatively insensitive to alveolar distension. Therefore, measurements of Δ6ms 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.