You need to know the name of the contrast they are using, if they are using one! However, I just flicked through the appendix on radiocontrast agents in the 2008 edition of Hale's "Medication and Mothers' Milk" and they are all L1, L2 and L3. So assuming your baby is healthy and not brand new anything they throw at you should be ok, in my brief flick through I've not even spotted any of the L3 ones have any cautions, seems like they are L3 because of less data.
If you decide to find out the contrast agent, don't tell them why unless forced, I needed an MRI in an emergency situation a couple of months ago, I'd already had a whole host of other drugs, but it was only the MRI they were bothered about, they told me not to breastfeed for 48 hours and I just smiled and nodded and planned to check later. I called my husband and had him bring my copy of Hale's and asked the nurseh to find out what they had used. I breastfed DD when they visited and also pumped and asked the nurse to store the milk, by which stage it must have been a different nurse because they did, but the next morning when it was the nurse who'd taken me to the MRI and we were leaving, I asked her to get the milk for me and she didn't want to give it to me and said something like if we knew you were going to breastfeed we wouldn't have given you the scan, though I knew that if I'd initally said no to their order not to breastfeed, it wouldn't have changed the fact I needed the scan and would likely just have meant calling the doctor for him to tell me not to breastfeed and pressurise me to say I wouldn't. So basically I would use your own judgement but I wouldn't take the risk of having the appointment cancelled because you were saying you were ignoring their advice.
