Regional Scoring of Emphysema
A density mask is a subregion of the lung parenchyma containing a particular tissue density range.
To calculate a density mask, the computer vision system performs automatic segmentation of the lung parenchyma in CT images. The desired tissue density range is translated into an X-ray attenuation (Hounsfield) range and voxels within this range are identified. Thus the system automatically identifies lung regions with different air/tissue contents and computes the percentage of lung within a particular density range.
By performing density mask analysis, we can quantitate the amount of lung destruction due to emphysema [Mao02a]. Areas of lung destruction are identified as regions of very low density: > 10 ml of gas per gram of tissue. This density is equivalent to voxels of X-ray attenuation < –910HU in thick-section CT images.
The image below shows density masks, with voxels below –910HU for an emphysematous subject. The highlighted pixels indicate regions of destroyed lung, and the % of pixels involved is computed. The % is computed for the entire lung and for dependent zones to measure regional variations in the amount of emphysema within the parenchyma.
