I'm getting an mri/mra with contrast today. can i drink grape fruit juice and prune juice right after?

Yes, water is better: Unless you have some underlying issue with these drinks, it should be fine. However, you should drink water. Mri contrast ("dye") is not directly toxic to your kidneys, but if you have kidney failure, you can be susceptible to complications. Drink plenty of water, and you'll be fine. You should complete a history (with allergies) prior to getting contrast.