Will a small permanent retainer (2 front teeth) interfere with a head mri image?

It may: Depending on how much iron it has in it. If it can be removed you should.

Likely not: The metal will make imaging of structures close to the retainer hard to see. However, most of the brain should be adequately evaluated.

Doubtful: Many adults who have mris have dental restorations with metals in them. They are not removed during imaging as it is for soft tissues.