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Fig. 4. —33-year-old-man with giant bullous emphysema and left pneumothorax. Chest CT scan shows asymmetric upper lobe bullae. Double-wall sign on left confirms presence of extrapleural air. Note comparison with right side where air is contained only within walls of bullae. Recognition of this sign in patients with giant bullae, shortness of breath, and suspicion of pneumothorax is invaluable in accurately directing appropriate chest tube placement.