While some nighthawk services operate abroad, studies must be read by board certified radiologists who are licensed to practice medicine in the states where the studies are performed.
One sneaky route around this is to have an unlicensed physician elsewhere (e.g. India) issue preliminary reads on overnight studies that are approved by an American radiologist in the morning, but this is rare and legally nebulous. After all, what if a critical finding is missed? Who is liable? If there are agencies that still do this, their future is in serious jeopardy.