What is an mra?

Magnetic Resonance Angiogram

It is used, typically with a slightly radioactive contrast agent injected into the arm, to visualize blood vessels, often in the brain.

Actually the contrast agent isn't radioactive at all (you're thinking of nuclear medicine procedures). It's paramagnetic and can be used to visualise any blood vessel. There are ways of imaging blood vessels without any contrast agent at all, useful if the patient has any problems with their kidney function.