Magnetic resonance assessment of parenchymal elasticity in normal and edematous, ventilator-injured lung

Magnetic resonance elastography (MRE) is a MR imaging method capable of spatially resolving the intrinsic mechanical properties of normal lung parenchyma. We tested the hypothesis that the mechanical properties of edematous lung exhibit local properties similar to those of a fluidfilled lung at transpulmonary pressures (P_tp) up to 25 cm H₂O. Pulmonary edema was induced in anesthetized female adult Sprague-Dawley rats by mechanical ventilation to a pressure of 40 cm H₂O for ∼30 min. Prior to imaging the wet weight of each ex vivo lung set was measured. MRE, high-resolution T₁-weighted spin echo and T₂* gradient echo data were acquired at each P_tp for both normal and injured ex vivo lungs. At P_tps of 6 cm H₂O and greater, the shear stiffness of normal lungs was greater than injured lungs (P ≤ 0.0003). For P_tps up to 12 cm H₂O, shear stiffness was equal to 1.00, 1.07, 1.16, and 1.26 kPa for the injured and 1.31, 1.89, 2.41, and 2.93 kPa for normal lungs at 3, 6, 9, and 12 cm H₂O, respectively. For injured lungs MRE magnitude signal and shear stiffness within regions of differing degrees of alveolar flooding were calculated as a function of P_tp. Differences in shear stiffness were statistically significant between groups (P < 0.001) with regions of lower magnitude signal being stiffer than those of higher signal. These data demonstrate that when the alveolar space filling material is fluid, MRE-derived parenchymal shear stiffness of the lung decreases, and the lung becomes inherently softer compared with normal lung.