Regional lung function and heterogeneity of specific gas volume in healthy and emphysematous subjects

The aim of our study was to study regional lung function by standard computed tomography (CT) and characterise regional variations of density and specific gas volume (SVg) between different lung volumes. We studied 10 healthy and 10 severely emphysematous subjects. Corresponding CT images taken at high and low lung volumes were registered by optical flow to obtain two-dimensional maps of pixel-by-pixel differences of density (Delta HU) and SVg (Delta SVg) at slice levels near the aortic arch, carina and top diaphragm. In healthy subjects, Delta HU was higher at all levels (p<0.001) with higher variability expressed as interquartile range (p<0.001), largely due to its differences between dorsal and ventral regions. In patients, median Delta SVg values were 3.2 times lower than healthy volunteers (p<0.001), while heterogeneity of Delta SVg maps, expressed as quartile coefficient of variation, was 5.4 times higher (p<0.001). In all patients, there were areas with negative values of Delta SVg. In conclusion, Delta SVg is uniform in healthy lungs and minimally influenced by gravity. The significant Delta SVg heterogeneity observed in emphysema allows identification of areas of alveolar destruction and gas trapping and suggests that Delta SVg maps provide useful information for evaluation and planning of emerging treatments that target trapped gas for removal.