What radiographic features distinguish interstitial diseases from airspace diseases?
Two primary characteristics radiographically distinguish interstitial diseases from airspace diseases. First, interstitial diseases displace little of the air within the lung, whereas airspace diseases displace large amounts of air. Interstitial diseases change the overall opacity of the lung very little, whereas airspace diseases in most cases dramatically increase the opacity (whiteness) of the lung on chest radiography. Second, interstitial diseases appear as increases in small nodules (generally <5 mm in diameter) or thin lines (<5 mm in width) (or both) within the lung, whereas airspace diseases appear as indistinctly marginated patches of opacity. See Figure 19-1 for the normal appearance of the lungs on frontal chest radiography.