First of all, modern tattoos, while using some inks that contain metals, have such low levels of metal that it would be unlikely to be affected/affect medical equipment. Metal would have no impact on X-rays whatsoever, I believe your concern is more related to MRI (Magnetic Resonance Imaging) which employs strong magnets. Tattoos done at tattoo parlors should not be affected by these machines, but tattoos done crudely at home or in prison may contain high levels of metal that can cause serious harm.