K. Subramaniam, A. R. Clark, E. A. Hoffman, M. H. Tawhai

Altered parenchymal microstructure and complexity have been observed in older age. How to distinguish between healthy, expected changes and early signs of pathology remains poorly understood. An objective quantitative analysis of computed tomography (CT) imaging was conducted to compare mean lung density (MLD), tissue density distributions, and tissue heterogeneity in 16 subjects - 8 aged >60 years who were gender- and BMI-matched with 8 subjects aged <30 years. Subjects were never-smoking, with no prior respiratory disease, and no radiologically-identified abnormalities on CT. Volume-controlled breath-hold imaging acquired at 80% vital capacity ('end inspiration') and 55% vital capacity ('end expiration') were used for analysis. MLD was not different between the age groups at end inspiration (p=0.806), but was larger in the younger group at end expiration (0.26 {plus minus} 0.033 vs. 0.22 {plus minus} 0.026, p=0.008), as is expected due to increased air trapping in the older population. However, gravitational gradients of tissue density did not differ with age; the only difference in distribution of tissue density between the two age groups demonstrated a lower density in the apices of the older group at end expiration. The heterogeneity of the lung tissue assessed using two metrics showed significant differences between end inspiration and end expiration, no dependence on age, and a significant relationship with BMI at both lung volumes when heterogeneity was calculated using Quadtree Decomposition, but only at end expiration when using a Fractal Dimension.