Increased transparency in thin-slice computed tomography of lung parenchyma

Areas with reduced density are a common finding with thin-slice computed tomography (CT) examinations of lung parenchyma. Diffuse and localized patterns can be differentiated. The most frequent cause for a diffuse decrease in density is pulmonary emphysema which can be classified into centrilobular, panlobular and paraseptal emphysema based on the distinct anatomical features. A mosaic pattern (due to mosaic perfusion or air trapping) is also characterized by a diffuse decrease in density. Focal cystic changes can be differentiated from emphysematous changes and mosaic patterns which do not reveal wall structures. Langerhans cell histiocytosis and lymphangioleiomyomatosis are the most frequent cystic diseases of the lung parenchyma. Based on the geometry and anatomical distribution of the cysts a differentiation is mostly possible. Rare diseases leading to a reduced lung density include the Birt-Hogg-Dube syndrome, lymphoid interstitial pneumonia as well as congenital diseases or diseases which are acquired during early childhood, such as cystic adenomatoid malformation or Swyer-James syndrome.