Curtain sign | Radiology Signs

Answer:

The curtain sign is the normal appearance at the lung base in which aerated lung sweeps down during inspiration and obscures the liver or spleen, so that the subdiaphragmatic organ disappears behind an advancing curtain of air artefact and reappears on expiration. It confirms that lung reaches the costophrenic recess at that point and therefore argues against fluid or consolidation there. Its absence, so that the diaphragm and the upper abdominal organ remain continuously visible with the thorax above them, indicates that something other than aerated lung occupies the recess, most often a pleural effusion or basal consolidation. The related findings that identify an effusion are the quad sign, the roughly rectangular anechoic space bounded by the pleural line, the two rib shadows and the lung line on the B-mode image, and the sinusoid sign on M-mode, in which the lung line moves toward the pleural line during inspiration in a sinusoidal fashion, indicating that the collection is freely mobile and therefore likely non-septated fluid. Together these allow bedside detection of effusions as small as about 20 mL, far below the threshold of the supine radiograph, and they guide safe thoracentesis by identifying the largest pocket, the depth to the pleura and the position of the diaphragm and adjacent organs.

Why is it called so?

Aerated lung descends across the image like a curtain being drawn, hiding the organ behind it.

Pathophysiology

Ultrasound cannot pass through air, so lung that contains air produces a complete reflection at its surface and generates only artefact beyond it. During inspiration, the lung expands into the costophrenic recess and interposes air between the transducer and the liver or spleen, so the subdiaphragmatic organ is no longer imaged; on expiration the lung retracts and the acoustic window reopens. Any process that displaces aerated lung from the recess, whether fluid, consolidated lung or a mass, keeps the acoustic window open throughout the respiratory cycle and abolishes the sign. The same physics underlies the ability of ultrasound to detect effusion and consolidation, both of which conduct sound and become visible, while aerated lung remains invisible.

Alternative names: Curtain sign of the lung base

Other associated named signs: Quad sign and sinusoid sign of pleural effusion, and the spine sign of consolidation or effusion

References

1. Lichtenstein DA. Lung ultrasound in the critically ill. Ann Intensive Care. 2014;4(1):1. PMID: 24401163.