This young patient was involved in a trauma. A supine thoracic spine film demonstrates a rounded opacity in the right mediastinum, just superior to the right main bronchus (click image for arrow). This opacity is a distended azygous vein, and it collapses when the patient is erect (erect chest xray).
The azygous vein is a normal structure which may be distended in the supine position, right heart failure, inferior vena cava obstruction, and portosystemic shunting. The normal size of the azygous vein is <7mm (erect CXR).
Reference: Dähnert W. Radiology Review Manual 5th edition. Lippincott, Williams & Wilkins 2003Credit: Dr Laughlin Dawes