On a chest radiograph, the silhouette sign most commonly indicates involvement of which lung region?

• A. Right lower lobe pneumonia
• B. Left lower lobe pneumonia
• C. Right upper lobe pneumonia
• D. Right middle lobe pneumonia

Answer: (A)

The silhouette sign refers to the loss of a sharp border on a chest radiograph where a dense process abuts an adjacent air-filled structure, revealing which border is involved. This helps localize the pneumonia to the lobe that lies next to the obscured border. When the right diaphragmatic margin becomes indistinct, it points to involvement of the right lower lobe because that lobe sits directly above the right hemidiaphragm and presses into that border. If the lesion were in the right middle lobe, you’d expect the border that blurs to be the right heart border; a left upper lobe/lingula process would blur the left heart border, and left lower lobe disease would blur the left diaphragmatic margin. Hence, the silhouette sign most commonly indicates right lower lobe pneumonia.