There are 2 types of contrast that they use for CT, MRI, X-Ray, RAIU.
MRIs and CT scans do not use radioactive dyes, but a non-radioactive
contrast media, usually gadolinium. (Gadolinium has no iodine, so it
causes few allergies.)
While SestaMIBI, used for stress testing, thyroid scans and mammograms, is radioactive.
To have a computed tomography scan (CT scan) done, you may need to have an injection of "contrast" or "dye.