The reasoning: pneumothorax
- Erect CXR: sharp visceral pleural line, absent markings peripherally; size measured at the hilum (BTS: ≥ 2 cm interpleural distance = large) or apex (ACCP: ≥ 3 cm). Expiratory films are no longer routinely recommended.
- Supine: anterior and subpulmonic air: deep sulcus sign, hyperlucent upper abdomen, sharp cardiac border and diaphragm, double diaphragm sign.
- CT: most sensitive (occult pneumothorax in trauma, bullae vs pneumothorax).
- Mimics: skin folds (extend beyond the chest wall, no pleural line), bullae (concave inner margins), scapular edge.