I think it's the x-rays they can't do.

I had my appendix out at 17 weeks and was under a general so I think anaesthetic would be ok...I know that they gave me a different type through a mask instead of the needle in the arm because they said something about administering via gas is easier to control the level you are under than administering via the needle in the arm...anyway bub and I recovered fine after having the general.

But with anything like this best to talk to your doctors/dentist about what's involved.