Retrocardiac sail sign

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The retrocardiac sail sign represents the characteristic and highly specific appearance of left lower lobe collapse on a frontal chest x-ray.

Radiographic appearance

The collapsed, medially displaced left lower lobe is represented by a triangular area of increased density with sharp margins, superimposed on the heart shadow.  The heart contours remain preserved (no silhouette sign), while the contour of the left hemidiaphragm is at least partially effaced .