Why are the superior vena cava and right subclavian vein also hyperdense?
This was a contrast study and this is just the contrast enhancing in the vessels.
Why is barium used for swallow studies?
It is a heavy metal that shows up particularly well on x-rays and can be easily swallowed by patients. In cases where oesophageal perforation is a concern, water soluble contrast agents are preferred as barium and the peritoneum do not agree with each other and can lead to a chemical peritonitis.
This is an older CT scan which also demonstrates the distribution of the barium - on bone window the aspirate is hyperdense in keeping with metal.