Who could've thought that injecting SF6 into blood vessels is actually safe enough to be FDA-approved... interesting.

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.

Contrast-enhanced ultrasound (CEUS) with microbubbles has been around clinically for 20+ years. There are many contrast-agent manufacturers, e.g. Bracco (SonoVue/Lumason) or GE Healthcare (Optison). Safety-wise it's probably better than CT iodinated contrast or gadolinium MRI agents, and it's pretty well-established at this point.