When they ask if you have any surgical clips or staples most of us answer no. As I've found, depending on your surgery, you might well have numerous that you weren't told about.
I haven't had an MRI since my last surgery, but a PET showed a spray of shrapnel I had no idea was in there. Everyone was a bit cross because, even though it made no difference for that procedure, I hadn't filled the paper work in properly. I was bloody furious because I had no bloody idea at all, none of the reports I'd been given mentioned them.