Hilum overlay sign
The hilum overlay sign is an appearance on chest radiographs whereby the margins of the hilar vessels can be seen at the level of a mass in the lung or mediastinum. In order to be able to see hilar structures, the mass cannot be located in the middle mediastinum.
Therefore the presence of the hilum overlay sign tells us that the mass is arising from the anterior or posterior mediastinum. The ability to see the heart border separate from the mass is seen with posterior mediastinal masses and obscuration of the heart border places the lesion in the anterior mediastinum.