Adding (to FW's post) my rudimentry knowledge of the Indian Medical system...
The 3 year MD program in radiodiagnosis? is considered "equivalent" to radiology residency in the U.S. I believe there is also a 2 year "diploma", which allows one to practice radiology in India, but it is not considered formal radiology residency (since one doesnt get board certified after the completion of the above).
In addition, if you have been a board certified radiologist (in good standing) in your home country for at least 5 years, you may be able to obtain a special license, which allows one to join an academic medical center as an instructor. It is obviously a difficult process, as you would need to demonstrate an excellent track record in academia (lots of research, publications in U.S./European journals etc). For further details, contact the ABR.