Effects of endotoxin lung injury on NMR T2 relaxation

The effects of endotoxin injury on lung NMR relaxation times (T1, CPMG T2, and Hahn decay constant (Hahn T2)) were studied in excised unperfused rat lungs. Blinded histologic examination showed no clear-cut separation between endotoxin and control lungs. Morphometric lung tissue volume density and gravimetric lung water content did not differ significantly between the two groups. In contrast, the values of the fast, intermediate, and slow T2 components, obtained by multiexponential analysis of the CPMG decay curve, increased markedly after endotoxin administration, with minimal overlap between endotoxin and control values. The response of Hahn T2 was, in general, in the same direction as that of CPMG T2; however, Hahn T2 may be more affected by measurement errors and may be less sensitive to the presence of lung injury. T1 showed minimal changes after injury. The present data suggest that CPMG T2 measurements can consistently detect the presence of lung injury even when conventional histologic, morphometric, and gravimetric studies provide negative or equivocal results, and that the CMPG T2 method is superior, in this respect, to the Hahn decay method. T1 does not appear to be sensitive to lung injury in the absence of significant lung water accumulation.