Which radiographic view is most useful to distinguish between a pneumothorax and a pneumomediastinum?

• A. Anteroposterior (AP)
• B. Decubitus
• C. Cross-table lateral
• D. Lateral decubitus

Answer: (B)

Gravity-driven layering in a decubitus radiograph makes this view especially informative. When air collects in a pneumothorax, it rises and separates the lung from the chest wall on the side that is dependent in the decubitus position, producing a clear pleural edge and absence of markings beyond it. In contrast, air in pneumomediastinum stays within the mediastinal spaces and does not create that distinct visceral pleural line or a layered edge. So a decubitus view can reveal a visible pleural line for a pneumothorax while a pneumomediastinum will not produce that pleural-edge layering, helping to distinguish between the two conditions.