What does kidney function have to do with an eye scan?

I’m to have a CAT scan of the area around my eye. First the doctor wants blood work to check kidney function. The lab results will determine “contrast” or “no contrast.” What does this mean, and what’s the connection?

I asked her several questions but didn’t get to this one.

 
Tags as I wrote them: eyes, CAT scan, blood test, kidney function, health, medical