Just the senior docs get that much. It doesn't take long in some practices to get 13 or more though. With radiologists in the US, it doesn't have anything to do with exposure as far as I know, because they don't get exposed to very much radiation at all. They generally just read the resulting images once they're captured, usually in a dark room with huge computer monitors. They rarely even see patients anymore. The peons (techs) do have hard-and-fast exposure limits and their schedule rotations are built around that fact. Most of the time, techs also wear radiation badges, which turn black as they're exposed to more and more radiation.