Won't they get some metal detector over your first? Who knows you might just, let's say, have a magnet in your little finger! But seriously, there are more metal things under your skin possible, so if you can't make them 100% sure nothing is going to happen when they turn it on, I guess they will find out by themselves beforehand.
My dad's a radiologist and I've never seen anyone get metal detected prior to an MRI. Maybe they have one for unconscious ER patients, but I've never heard of it.
