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  1.    Atelectasis: When air is removed from the lung, the tissue collapses, resulting in volume loss on chest radiographs. If severe enough, the mediastinum and/or diaphragm are pulled toward the lesion. Air may still remain in larger bronchi, creating air bronchograms on radiograph. Collapse and re-expansion can occur quickly.
  2.    Infiltrate: A fluid (blood, pus, edema) that invades one of the compartments of the lung (bronchoalveolar air space or peribronchial interstitial space) is seen as a density on a radiograph. When alveolar air is displaced by fluid, but air remains in the bronchi, the classic pneumonic infiltrate with air bronchograms is seen. When infiltrate is interstitial, its borders are more vague, and bronchial walls may be thickened. Typically, infiltrates resolve in 2–6 weeks.