Complexity of terminal airspace geometry assessed by lung computed tomography in normal subjects and patients with chronic obstructive pulmonary disease

Increases in the low attenuation areas (LAA) of chest x-ray computed tomogr aphy images in patients with chronic obstructive pulmonary disease (COPD) h ave been reported to reflect the development of pathological emphysema, we examined the statistical properties of LAA clusters in COPD patients and in healthy subjects. In COPD patients, the percentage of the lung field occup ied by LAAs (LAA%) ranged from 2.6 to 67.6. In contrast, LAA% was always <3 0% in healthy subjects. The cumulative size distribution of the WA clusters followed a power law characterized by an exponent D. We show that D is a m easure of the complexity of the terminal airspace geometry. The COPD patien ts with normal LAA% had significantly smaller D values than the healthy sub jects, and the D values did not correlate with pulmonary function tests exc ept for the diffusing capacity of the lung. We interpret these results by u sing a large elastic spring network model and find that the neighboring sma ller LAA clusters tend to coalesce and form larger clusters as the weak ela stic fibers separating them break under tension. This process leaves LAA% u nchanged whereas it decreases the number of small clusters and increases th e number of large clusters, which results in a reduction in D similar to th at observed in early emphysema patients. These findings suggest that D is a sensitive and powerful parameter for the detection of the terminal airspac e enlargement that occurs in early emphysema.