Fig. 4

Long-term change in emphysema-related CT indexes in current and former smokers. a and b show long-term (at least 5 years) longitudinal changes in total lung volumes measured on CT, and LAA%(− 950) defined as percent low attenuation area less than − 950 HU in current and former smokers with COPD (n = 17 and 42, respectively). c D950, (d) D’15, (e) D’25, and (f) D’35 are exponents that characterize a power law that governs the cumulative frequency size distribution of LAA clusters that are identified using a threshold of − 950 HU and the 15th, 25th, and 35th percentiles of a CT density histogram, respectively. D950, D’15, D’25 and D’35, respectively. Base = baseline, Follow = follow-up