Per Wikipedia, it "is a colorless, odorless, non-flammable, and non-toxic gas."

When used as a contrast agent for ultrasound, it "has been used to examine the vascularity of tumours" -- which would be similar to its use in the OP. Then "[i]t remains visible in the blood for 3 to 8 minutes, and is exhaled by the lungs."

So -- not collected and excreted by the liver, as I at first thought.