Tissue characterization of pneumonia and irradiated rat lungs with magnetic resonance relaxation times.

To interpret the MRI signal intensities, the T1 and T2 values were determined in vitro for rat lungs with radiation pneumonitis, fibrosis or pneumonia, and also for muscle and fat tissues. The transverse magnetization decays mentioned above exhibited two components, a T2 fast (T2f) and a T2 slow (T2s) component. Lungs from rats with pneumonia had significantly longer T2f and T2s values than lungs from rats with radiation pneumonitis and fibrosis. The T2f and T2s values for a "pneumonia lung" were not significantly different from those of muscle. The measured values of T2f and T2s suggested that radiation pneumonitis and fibrosis likely exhibit lower signal intensities than do muscle and that a "pneumonia lung" likely exhibits a similar signal intensity to that of muscle on T2-weighted MR images.