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Background
Large airway evaluation may be challenging even for the most skilled radiologists. This exhibit presents case-based examples to improve diagnosis and avoid common pitfalls of large airway imaging which encompass the trachea to the second order bronchi.

Educational Goals / Teaching Points
Illustrate large airway anatomy and imaging techniques. Discuss common pitfalls of image interpretation and pearls to help characterize important pathology.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Airway lesions can often be missed on chest radiography (CXR). Inclusion of the trachea and central bronchi in the standard search pattern for CXR can increase the likelihood of detecting important airway abnormalities. Airway masses may present as subtle filling defects in the large airway columns. Tracheal and bronchial narrowing or thickening may cause an abnormal appearance of classic CXR lines and stripes, clues that may be missed without thorough interrogation. Computed tomography (CT) accurately assesses large airway abnormalities and provides characterization of the distribution and morphology of tracheal and bronchial pathology but is also subject to technical and interpretive pitfalls. A systematic approach to evaluating the airways on CT should include a search for focal tracheal or bronchial masses and nodules, focal, segmental, or diffuse thickening, narrowing, or dilatation. Knowledge of certain uncommon diseases with large airway findings can help prevent missed diagnoses. A variety of conditions can cause various combinations of airway thickening, bronchiectasis, and mosaic attenuation. Conditions involving the airways that may be overlooked include adult-onset cystic fibrosis, allergic bronchopulmonary aspergillosis, bronchiolitis obliterans, and systemic conditions such as IGG4-related disease, inflammatory bowel disease, and amyloidosis. Expiratory central airway collapse (ECAC) is an underappreciated source of chronic cough and can be assessed on CT. Evaluation of ECAC can be complicated by confusing terminology and technical pitfalls. Excessive dynamic airway collapse (EDAC), an abnormal collapse of the posterior tracheal membrane without significant deformity of the cartilaginous trachea, should be distinguished from tracheobronchomalacia (TBM), characterized by luminal collapse due to deformation of the cartilaginous trachea. Certain morphologies of the trachea at inspiration are associated with tracheomalacia, such as a lunate shape, and can be missed at routine inspiratory CT performed for other indications. Technical pitfalls of ECAC imaging include inadequate expiratory effort and lower sensitivity of end-expiratory CT than dynamic cine CT imaging during forced expiration. Interpretative pitfalls include failure to distinguish normal tracheal or bronchial expiratory change from true ECAC, lack of appreciation of focal or segmental collapse, and lack of appreciation of combinations of airway stenosis and malacia.

Conclusion
Awareness of key pitfalls and pearls is essential in accurate diagnosis of diseases involving the large airways.