Air Bronchogram on CT
An air bronchogram on CT represents the visualization of air-filled bronchi surrounded by consolidated or opacified lung parenchyma, appearing as branching lucent (dark) tubular structures against a background of increased density. 1
Radiologic Definition and Appearance
The air bronchogram sign occurs when normally air-filled bronchi become visible because the surrounding alveolar spaces are filled with fluid, cells, or other material that increases tissue density, creating contrast between the patent airways and the consolidated lung tissue. 2 On CT imaging, these appear as:
- Branching tubular lucencies (air-filled bronchi) within areas of consolidation 1
- Linear or arborescent patterns that follow the anatomical distribution of airways 1
- Can be visualized in tissue-like patterns where lung aeration is completely lost 1
Dynamic vs. Static Air Bronchograms
Dynamic air bronchograms move synchronously with tidal ventilation and indicate patent main airways, highly specific for pneumonia (community-acquired or ventilator-associated depending on context). 1 This pattern suggests the airway remains open and functional.
Static air bronchograms do not move with respiration and may indicate airway obstruction or different pathologic processes. 1
Clinical Significance and Differential Diagnosis
Malignant Lesions
Air bronchograms are significantly more common in malignant than benign solitary pulmonary lesions (28.7% vs. 5.9%, p<0.05). 3 They occur in all histologic types of lung cancer including:
- Bronchioloalveolar carcinoma (most common) 4, 3
- Adenocarcinoma 3
- Squamous cell carcinoma 3
- Small cell and non-small cell carcinoma 3
The morphology of bronchi within malignant lesions varies: normal, tortuous, ectatic, or cut-off patterns, whereas benign lesions typically show normal bronchial morphology. 3
Infectious and Inflammatory Processes
- Organizing pneumonia: CT typically shows patchy consolidation with air bronchograms in subpleural locations 1
- Pneumonia: Dynamic air bronchograms are characteristic 1
- Tuberculosis: Can demonstrate air bronchograms with specific morphologic features 5
Post-Obstructive Consolidation
In bronchial obstruction (from cancer or broncholith), CT fluid bronchograms may be present when bronchi are filled with fluid instead of air, particularly in total bronchial obstruction where traditional air bronchograms may be absent. 6
Interstitial Disease
Air bronchograms can occur in interstitial diseases through compressive atelectasis causing tissue crowding around open airways, or when disease processes encroach on distal airways producing obstructive pneumonia (as in sarcoidosis and lymphoma). 2
Diagnostic Utility
The shape and lumen characteristics of bronchi with air bronchogram sign can distinguish between lung cancer, tuberculosis, and pneumonia. 5 The length of involved bronchus and the ratio between bronchus length and lesion length help differentiate lung cancer from tuberculosis and pneumonia. 5
Important Caveats
- Air bronchograms can be seen in lesions of all sizes, including those <2 cm in diameter 3
- The presence of an air bronchogram does not exclude malignancy—in fact, it increases suspicion for lung cancer in solitary pulmonary lesions 3
- Optimal CT evaluation requires thin-section imaging (≤1.5 mm slice thickness) to avoid partial volume averaging that can obscure airway anatomy 1, 7
- Adequate breath-holding at inspiration is essential, as respiratory motion can create erroneous impressions 1