Alveolar proteinosis and exogenous lipid pneumonia are airspace diseases. In alveolar proteinosis, airspaces are filled with a phospholipoproteinaceous material. On CT, the filling of the alveoli is responsible for the ground-glass appearance. When the airspaces adjacent to the inter-and intralobular septa and to the alveolar walls fill, the periacinar pattern becomes visible (Fig. 2c) [9] [10] [11] .