There would be little scattered radiation from an ankle x-ray. Most of the radiation is specifically directed towards the x-ray film, and very little escapes from the beam path or gets re-directed from scattering effect. The dentist would want to shield the thyroid gland, which is extra sensitive to x-ray damage. Modern x-ray equipment (film & intensifying screen) requires far less exposure, compared with the exposure levels of, say, 30 years ago.