I am the queen of MRIs for medical reasons I won't bore you with. The radiology dept said they would not use a contrast dye MRI while I was still nursing unless I stopped nursing for 24 hrs. Try explaining that to a 2 year old, ha. Don't know if this is actually true or just CYA, just a heads-up for you. They might assume that a 17 month old would be weaned by now. I had to actually ask about it, they were just going to go ahead and use it and were really, really annoyed when I said I was not going to stop nursing for 24 hrs.
Anyway, I was also in a situation, for a previous MRI, where I could potentially have been pregnant. The neurologist and radiologist said I should take a pregnancy test, abstain for two weeks, take another one, and then they'd do the MRI if it was negative. MRIs don't have radiation, but they do use very powerful magnets to align electrons in your body's water. I don't think anyone knows what this could do to a developing embryo or fetus, and a study on this wouldn't exactly get approved by most ethics committees.