Novel spatial measurements of inspiratory expansion and expiratory deflation rates in ILD and COPD

<bold>Background:</bold> The processes of inspiration (an active process) and expiration (a passive process) are affected by the respiratory system mechanics (compliance/elastic recoil). Respiratory elastic recoil is increased in fibrotic ILDs and reduced in COPD. X-ray Velocimetry (XV, 4DMedical Limited, Australia) enables dynamic imaging of the lung to quantify spatially mapped lung expansion and deflation during inspiration and expiration. <bold>Aim:</bold> This study aims to compare spatially mapped inspiratory expansion and expiratory deflation rates in COPD and ILD. <bold>Methods:</bold> In this preliminary cross-sectional study, subjects underwent spirometry, gas transfer and XV imaging. Spatial measurements of the ratio between inspiratory expansion and expiratory deflation rates (Ir:Er) were made via XV in each voxel. <bold>Results:</bold> Subjects with ILD (n=7) displayed a restricted pattern on spirometry (FVC = 50.7±15.8 %pred), and COPD subjects (n=8) displayed an obstructive pattern (FEV₁/FVC = 0.3±0.04). Across the whole lung, COPD subjects demonstrated a greater ratio of voxel inspiratory expansion to expiratory deflation rates compared with ILD subjects (Ir:Er = 2.00±0.37 vs 1.57±0.03, p=0.03). This is consistent with reduced elastic recoil in COPD and increased elastic recoil in ILD. In COPD, the emphysema score from CT (HU<-950) was related to Er:Ir (R² =0.21, p<0.01). <bold>Conclusion:</bold> This study introduces a novel technique for 3-dimensional analysis of lung mechanics. XV has several advantages over other lung function measurements in visualising and quantifying dynamic lung motion and lung mechanics. This method can potentially spatially identify the mechanical changes that occur with fibrosis and COPD.