The aim of this study was to assess the utility of (3)He MRI to noninvasively probe the effects of positive end-expiratory pressure (PEEP) maneuvers on alveolar recruitment and atelectasis buildup in mechanically ventilated animals. Sprague-Dawley rats (n = 13) were anesthetized, intubated, and ventilated in the supine position ((4)He-to-O(2) ratio: 4:1; tidal volume: 10 ml/kg, 60 breaths/min, and inspiration-to-expiration ratio: 1:2). Recruitment maneuvers consisted of either a stepwise increase of PEEP to 9 cmH(2)O and back to zero end-expiratory pressure or alternating between these two PEEP levels. Diffusion MRI was performed to image (3)He apparent diffusion coefficient (ADC) maps in the middle coronal slices of lungs (n = 10). ADC was measured immediately before and after two recruitment maneuvers, which were separated from each other with a wait period (8-44 min). We detected a statistically significant decrease in mean ADC after each recruitment maneuver. The relative ADC change was -21.2 ± 4.1 % after the first maneuver and -9.7 ± 5.8 % after the second maneuver. A significant relative increase in mean ADC was observed over the wait period between the two recruitment maneuvers. The extent of this ADC buildup was time dependent, as it was significantly related to the duration of the wait period. The two postrecruitment ADC measurements were similar, suggesting that the lungs returned to the same state after the recruitment maneuvers were applied. No significant intrasubject differences in ADC were observed between the corresponding PEEP levels in two rats that underwent three repeat maneuvers. Airway pressure tracings were recorded in separate rats undergoing one PEEP maneuver (n = 3) and showed a significant relative difference in peak inspiratory pressure between pre- and poststates. These observations support the hypothesis of redistribution of alveolar gas due to recruitment of collapsed alveoli in presence of atelectasis, which was also supported by the decrease in peak inspiratory pressure after recruitment maneuvers.

Purpose To investigate the feasibility of describing the impact of any flip angle–TR combination on the resulting distribution of the hyperpolarized xenon‐129 (HXe) dissolved‐phase magnetization in the chest using a single virtual parameter, TR90°,equiv. Methods HXe MRI scans with simultaneous gas‐ (GP) and dissolved‐phase (DP) excitation were performed using 2D projection scans in mechanically ventilated rabbits. Measurements with DP flip angles ranging from 6–90° and TRs ranging from 8.3–500 ms were conducted. DP maps based on acquisitions of similar radio frequency pulse‐induced relaxation rates were compared. Results The observed distribution of the DP magnetization was strongly affected by acquisition flip angle and TR. However, for flip angles up to 60°, measurements with the same radio frequency pulse‐induced relaxation rates, resulted in very similar DP images despite the presence of significant macroscopic gas transport processes. For flip angles approaching 90°, the downstream signal component decreased noticeably relative to acquisitions with lower flip angles. Nevertheless, the total DP signal continued to follow an empirically verified conversion equation over the entire investigated parameter range, which yields the equivalent TR of a hypothetical 90° measurement for any experimental flip angle–TR combination. Conclusion We have introduced a method for converting the flip angle and TR of a given HXe DP measurement to a standardized metric based on the virtual quantity, TR90°,equiv, using their equivalent RF relaxation rates. This conversion permits the comparison of measurements obtained with different pulse sequence types or by different research groups using various acquisition parameters.

Background Mechanical ventilation worsens acute respiratory distress syndrome, but this secondary “ventilator-associated” injury is variable and difficult to predict. The authors aimed to visualize the propagation of such ventilator-induced injury, in the presence (and absence) of a primary underlying lung injury, and to determine the predictors of propagation. Methods Anesthetized rats (n = 20) received acid aspiration (hydrochloric acid) followed by ventilation with moderate tidal volume (VT). In animals surviving ventilation for at least 2 h, propagation of injury was quantified by using serial computed tomography. Baseline lung status was assessed by oxygenation, lung weight, and lung strain (VT/expiratory lung volume). Separate groups of rats without hydrochloric acid aspiration were ventilated with large (n = 10) or moderate (n = 6) VT. Results In 15 rats surviving longer than 2 h, computed tomography opacities spread outward from the initial site of injury. Propagation was associated with higher baseline strain (propagation vs. no propagation [mean ± SD]: 1.52 ± 0.13 vs. 1.16 ± 0.20, P < 0.01) but similar oxygenation and lung weight. Propagation did not occur where baseline strain was less than 1.29. In healthy animals, large VT caused injury that was propagated inward from the lung periphery; in the absence of preexisting injury, propagation did not occur where strain was less than 2.0. Conclusions Compared with healthy lungs, underlying injury causes propagation to occur at a lower strain threshold and it originates at the site of injury; this suggests that tissue around the primary lesion is more sensitive. Understanding how injury is propagated may ultimately facilitate a more individualized monitoring or management.