The portal vein is formed by the union of the SMV (superior mesenteric vein) with the splenic vein. 

The final image is a wedge portal venogram obtained during a TIPS (transjugular intrahepatic portosytemic shunt) procedure, using carbon dioxide as a contrast agent. The venogram demonstrates a rather prominent left gastric vein (a.k.a. coronary vein) draining into the SMV-splenic venous confluence. This patient was known to have portal hypertension which probably accounts for the size of this vein.