Pulmonary acinar biomechanics imaged with synchrotron phase contrast microtomography in live rats

Introduction: Little is known about the in vivo micromechanics of pulmonary alveoli due to insufficient temporal and spatial resolution of current imaging techniques. Here we describe a time-resolved synchrotron radiation phase-contrast CT technique which allows mapping the dynamics of acinar deformation in vivo down to the alveolar length scale. Methods: The experiment was performed in 3 anesthetized, muscle-relaxed and ventilated adult rats. Images were acquired using a fast camera with a pixel size of 6 µm and 10 ms time resolution. 3D images of a lung region were reconstructed at 78 time points within a breath. Acinar structures were segmented at each time point and registration-based voxel-by-voxel displacement was computed with reference to the start of inspiration and displayed as a function of time. Results: Figure 1A shows a quantitative map of the local acinar strain, between the minimal and maximal pressure points during the breathing cycle. Figure 1B shows the position of regions of interest where local strain within pulmonary acini was displayed as a graph (1C). Regional strain within the lung peripheral airspaces took average ± SD values of 0.09 ± 0.02. Fitting the expression Surface = constant x Volumen (S=kVn) yielded n = 0.82±0.03, suggesting predominant alveolar expansion rather than ductal expansion or alveolar recruitment. Conclusions: Our results demonstrate the feasibility of mapping the regional acinar strain in in vivo rat lungs at 6 µm resolution and suggest predominant alveolar expansion as a mechanism of acinar configuration change during a positive-pressure breath.