I haven't worked from abroad myself, but the answer to your last question is 'yes'.
Payments from CMS require the service to have originated from within the United States. For an imaging study's professional fee, that is wherever the radiologist is located, which is part of the reason locating radiologists in Hawai'i is increasingly popular. To separate out CMS patients is an administrative burden and it may not be financially tenable, depending on one's payer mix. Even some private insurance companies put stipulations on location. For example, in my state, final reads will only be reimbursed if performed in state, so virtually all overnight reads are preliminary.
Changes in reimbursement, mostly straight up paying less per exam, has really changed the calculus. I don't know enough to say that there are no viable abroad options, but I will say that the majority of teleradiologists are U.S.-based. And, if they're working for a big firm, they're probably practicing in a high volume/low compensation per RVU environment.