Hi Sara....(By the way, I love your name!)
As to your question, dyes used with MRIs ...usually a gadolinium substance....are relatively harmless with very few side effects. If you do have concerns, discuss this with your urologist.
Personally, I feel that ANYTHING that can help in the diagnosis of bladder cancer issues is well-worth doing. The potential for issues with bladder cancer is so much more serious than any minor "side effect" of a contrast medium. My urologist insists on a CT-Urogram every five years, with and without contrast medium. I am much more concerned about the radiation from a CT scan and he and I have discussed this; he has convinced me that this is necessary to be sure that my cancer has not returned or spread to other parts of the system such as the kidneys. (Note that MRIs do not have the radiation issues that CT scans do.)
Best of luck to you
Sara Anne
!!