Voxel frequency distribution histogram indicating the appearance in normal lung and in emphysema, and the derivation of the densitometric indices that were used in the current study. 15th percentile point is defined as the cut-off value, in Hounsfield Units (HU), below which are distributed the 15% of voxels with the lowest density. (This index may be converted to the 15th percentile lung density (PD15) in g/L by the addition of 1000.) The voxel index at a threshold of -950HU (VI-950) is shown and is defined as the percentage of voxels with a value less than -950HU. The mean lung density is defined as the mean value (in g/L) of all voxels distributed within the lung histogram.