Superior vena cava is not identifiable on this scan, instead a rich collateral network of smaller veins is demonstrated. The majority of blood flow is seen to be diverted into the azygos-hemiazygos-accessory hemiazygos system (these veins are densely opacified), left renal vein (which is also significantly dilated and measures around 25mm) and finally into inferior vena cava. Chest wall, mediastinal, diaphragmatic and vertebral venous plexuses are also opacified as a sign of SVC obstruction.