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Background
Lesions associated with abnormal pulmonary air are frequently encountered, have various etiologies, and may elicit a series of differential diagnoses. Understanding their underlying pathophysiology and recognizing specific patterns of abnormal pulmonary air enables accurate imaging diagnosis.

Educational Goals / Teaching Points
This exhibit will help the learner recognize the most common air-containing pulmonary lesions, understand their pathophysiology, list their imaging features and ancillary findings, and present a pragmatic algorithm for the assessment of air-containing pulmonary diseases to generate a focused differential diagnosis.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The etiologies of air-containing pulmonary diseases are protean, and include infection, malignancy, inflammation, vascular disease, environmental triggers, congenital abnormalities, and idiopathic processes. An algorithmic approach for evaluating air-containing lesions will assist the radiologist in formulating a logical and clinically useful differential diagnosis. Air-containing lesions can be separated into one of 4 major categories - cysts and cystic lung disease, cyst-like lesions, and lesions with air and solid components. Some abnormalities may mimic air-containing lesions and present diagnostic pitfalls. Cysts and cystic lung diseases are characterized as discrete air attenuation foci with well-defined walls. The differential for cystic lung disease includes pulmonary Langerhans cell histiocytosis (PLCH), lymphangioleiomatosis (LAM), lymphoid interstitial pneumonia (LIP), Birt-Hogg-Dubé syndrome, and light chain deposition disease. Cyst-like lesions are characterized by the absence of a cyst wall with an epithelial lining and include pneumatoceles, honeycomb cysts, bullae/blebs, and congenital pulmonary airway malformations. Lesions with air and solid components include cavitary lesions (e.g., abscess, necrotizing pneumonia, cavitary malignancy) and cystic malignancies (e.g. adenocarcinoma, pleuropulmonary blastoma). “Pitfall” lesions may also occur and must be distinguished from the above mentioned air-containing abnormalities. These include emphysema (centrilobular emphysema lacks a perceptible wall and exhibits the central dot sign), bronchiectasis (which may appear cystic on axial imaging but exhibits a tubular morphology on multiplanar imaging), and extra pulmonary lesions, such as loculated hydropneumothorax.

Conclusion
Air-containing pulmonary lesions include common and uncommon abnormalities and may pose a diagnostic challenge for the radiologist. Recognition of the characteristic imaging features of such lesions, understanding their pathophysiology, and avoiding misdiagnosing pitfall lesions enables the radiologist to formulate an appropriate differential diagnosis to guide management, impact clinical outcomes, and elevate the quality of patient care.