Retrospectively gated ultrashort-echo-time MRI T mapping reveals compromised pulmonary microvascular NO-dependent function in a murine model of acute lung injury.

This study sought to develop noninvasive, in vivo imaging schemes that allow for quantitative assessment of pulmonary microvascular functional status based on the combination of pulmonary T mapping and dynamic contrast-enhanced (DynCE) imaging. Ultrashort-echo-time (UTE) imaging at 9.4 T of lung parenchyma was performed. Retrospective gating was based on modulation of the first point in each recorded spoke. T maps were obtained using a series of five consecutive images with varying RF angles and analyzed with the variable flip angle approach. The obtained mean T lung value of 1078 ± 38 ms correlated well with previous reports. Improved intersession variability was observed, as evident from a decreased standard deviation of motion-resolved T mapping (F-test = 0.051). Animals received lipopolysaccharide (LPS) and were imaged at t = 2, 6, and 12 h after administration. The nitric oxide (NO)-dependent function was assessed according to changes in lung T after L-NAME injection, while microvascular perfusion and oxidant stress were assessed with contrast-enhanced imaging after injection of gadolinium or 3-carbamoyl-proxyl nitroxide radical, respectively. Retrospectivel gated UTE allowed robust, motion-compensated imaging that could be used for T mapping of lung parenchyma. Changes in lung T after L-NAME injection indicated that LPS induced overproduction of NO at t = 2 and 6 h after LPS, but NO-dependent microvascular function was impaired at t = 12 h after LPS. DynCE imaging at t = 6 h after LPS injection revealed decreased microvascular perfusion, with increased vascular permeability and oxidant stress. MRI allows to visualize and quantify lung microvascular NO-dependent function and its concomitant impairment during acute respiratory distress syndrome development with high sensitivity. UTE T mapping appears to be sensitive and useful in probing pulmonary microvascular functional status.