Figure 1.

Estimation of the normal-density ratio of expiratory to inspiratory mean lung density in a representative subject. First, the lung parenchyma was delineated into regions with disease and normal regions on the basis of traditional fixed-density thresholds for mild emphysema (−910 Hounsfield units [HU]) and gas trapping (−856 HU), as shown by the bold vertical lines. All voxels with density greater than −910 HU on inspiratory scans were classified as normal lung parenchyma at inspiration, and all voxels greater than −856 HU on expiratory scans were classified as normal at expiration. We then separately calculated the mean lung density of these normal regions at expiration (E) and inspiration (I), as shown by the red dashed lines. The ratio of E over I was referred to as ND-E/I, where ND represents the normal-density regions in the lung. CT = computed tomography.