Scimitar sign. Frontal radiograph of the chest (A) shows a curvilinear opacity paralleling the right heart border representing an anomalous pulmonary vein. Graphic depiction of the curvilinear anomalous vein and its usual common entry into the inferior vena cava (B). A follow-up unenhanced CT (C, D) and cavagram (E) for central line placement confirmed a supradiaphragmatic insertion of the anomalous vein into the inferior vena cava just prior to its entry into the right atrium. Used with permission, Canadian Association of Radiologists Journal, April 2005.
Source
Signs in Thoracic Imaging
Journal of Thoracic Imaging 21(1):76-90, March 2006.

On PA chest radiography, a characteristic broad, curved shadow extends inferiorly toward the diaphragm along the right side of the heart (Figs. 18A, B).53 Its appearance resembles a Turkish sword and signifies a partial anomalous pulmonary venous return, most commonly to the infradiaphragmatic inferior vena cava (Figs. 18C?E). It is part of the congenital hypogenetic lung (scimitar) syndrome.4,53