Figure 2

Cumulative voxel distribution histogram showing derivation of voxel index and percentile point parameters. Voxel index (VI) below 950 Hounsfield Units (-950HU) is defined as the proportion of lung voxels of low density below a threshold of -950HU and increases with worsening emphysema. The 15^th percentile point (Perc15) is defined as the cut-off value in HU below which 15% of all voxels are distributed and, as a true measure of density, this parameter consequently decreases with worsening emphysema.