Table 5.

Key imaging findings and imaging signs in chronic airspace disease

Imaging finding/sign	Cause
Imaging signs	Halo sign: Fungal infection including angio-invasive pulmonary aspergillosis, pulmonary tuberculosis, lung adenocarcinoma, organizing pneumonia, granulomatosis with polyangiitis
	Reverse halo sign: Fungal infection including angio-invasive aspergillosis, organizing pneumonia, granulomatosis with polyangiitis, pulmonary tuberculosis, alveolar sarcoidosis
	1-2-3 sign (right paratracheal and bilateral hilar lymphadenopathy): Alveolar sarcoidosis
	Crazy paving: Pulmonary alveolar proteinosis, pulmonary hemorrhage, chronic eosinophilic pneumonia, organizing pneumonia, alveolar sarcoidosis
	Pseudocavitation: Lung adenocarcinoma
	Fried egg sign: Lung adenocarcinoma
	Fissural retraction: Lung adenocarcinoma
Distribution	Upper lung: Sarcoid, chronic eosinophilic pneumonia (mid and upper), tuberculosis
	Lower lung: Lipoid pneumonia, organizing pneumonia (mid and lower)
Ground glass vs consolidation	Ground-glass opacity: Adenocarcinoma, pulmonary hemorrhage
	Mixed: Adenocarcinoma, organizing pneumonia, angio-invasive aspergillosis
	Consolidation: Invasive mucinous adenocarcinoma, lymphoma, PTLD
Presence of cavitary lesion	Pulmonary tuberculosis
	Granulomatosis with polyangiitis and other vasculitis
	Fungal infection
Peri-lymphatic nodules	Alveolar sarcoidosis
Minor or major change in the distribution and severity	Chronic eosinophilic pneumonia
	Organizing pneumonia
	Pulmonary hemorrhage
Arcade-like or polygonal opacities	Organizing pneumonia
Fat density	Lipoid pneumonia
Interlobular septal thickening and subpleural sparing	Organizing pneumonia
	Pulmonary hemorrhage
Involvement of other organs	Liver and bowel: PTLD
	Renal involvement: Granulomatosis with polyangiitis
	Involvement of upper respiratory tracts and paranasal sinuses: Granulomatosis with polyangiitis, eosinophilic granulomatosis with polyangiitis

PTLD: Post-transplant lymphoproliferative disorder.