In Vivo Computed Tomography as a Research Tool to Investigate Asthma and COPD: Where Do We Stand?

Figure 4

Theoric models of voxel attenuation frequencies in a normal subject (green curve) and emphysematous patient (red curve). The density mask technique is defined as the percentage of total lung volume that contains voxels lower than a predefined voxel index, usually −950 HU to assess emphysema (purple arrows). The percentile method is based on predefined percentages at which voxels have lower attenuation values. Blue arrows indicate the crossing points of green and red curves with the 15th percentile.