Quantitative imaging of alveolar recruitment with hyperpolarized gas MRI during mechanical ventilation

Abstract

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.

Ventilator-induced Lung Injury

“…3 In vivo preclinical models using diffusion-weighted hyperpolarized gas MRI have found the aerated baby lung may experience regional overdistension. 87,88 An ideal lung-protective strategy might scale tidal volumes to functional baby lung size rather than predicted healthy lung size (i.e. mL/kg PBW).…”

Hyperpolarized 3He diffusion MRI and histology of secreted frizzled related protein-1 (SFRP1) deficient lungs in a Murine model

“…However, small airways were included in the ADC analysis. As such, the ADC values do not exclude airways completely, but are a combined measurement of the acinar airway and alveoli size [19]. …”

Lung imaging: how to get better look inside the lung

“…Within the same individual, changes in lung volume will cause equal changes in proton density and, accordingly, equal changes in MR signal intensity of the lung parenchyma (69). Hyperpolarized gas MRI was used to evaluate alveolar recruitment and atelectasis-induced overdistension during mechanical ventilation in ARDS (70). Helium-3 and Xenon-129 (71) can be used as contrast agents due to their capacity of diffusing rapidly into airspaces and enabling detection of ventilated lung tissue (72).…”

“…Hyperpolarized Xenon-129 follows the same pathway as oxygen, it spreads across the alveolocapillary membrane allowing the calculation of gas exchange parameters, including alveolar surface area, septal thickness and vascular transit time (72,73). MRI enables calculation of alveolar size based on the MRI apparent diffusion coefficient (73), and this has a potential application to determine the consequences of atelectasis and recruitment manoeuvres (70). Gases with fast spin-rotation relaxation at thermal equilibrium polarization have been used to study differences in ventilation-perfusion ratio (74) and time constants for fast and slow filling lung compartments, giving information about ventilation-perfusion heterogeneity, pulmonary endcapillary diffusion of oxygen and lung microstructure in ARDS (75).…”