Fig. 2

Different degrees of expiratory central airway collapse perceived by cine CT at different airway levels. Free-breathing cine CT images obtained at end-inspiratory and end-expiratory phases revealed class 3 (≥ 70–99%) expiratory airway collapse at thoracic inlet level (a, d), class 2 (≥ 50–70%) airway collapse at carina level (b, e) and class 1 (< 50%) airway collapse at bronchus intermedius level (c, f). At bronchus intermedius level, mild airway collapse is diagnosed by flattening of its posterior wall during expiratory phase (arrow)