Azygos vein
Vein with variable origin which ascends in the posterior mediastinum along the right side of the vertebral bodies and at the level of the 4th thoracic vertebrae - arches forwards above the root of the right lung and joins the superior vena cava before the latter enters the pericardium.
Source: The Anatomical Atlas of CardiacMRI, Editor Mohan Sivananthan - Leeds General Infirmary – used with permission