Nuclear medicine is a independent specialty with it's own training requirements (two years after an internship) and boards exam, but usually it's run within a radiology department. However, radiology residents get at least six months of training in nuclear medicine and their boards examinations include sections on nuclear medicine. For the majority of nuclear medicine studies, general radiologists do them too. One major exception is therapeutic administration of radioisotopes. Radiologists can also do a one-year fellowship in "nuclear radiology" and get board certified by an ABR CAQ examination, which in that case they can administer therapeutic radionuclides too.
I would also add that there are some combined radiology/nukes programs (6 years) that allow you to be double boarded.
So the 4 options are:
NM residency --> Boarded by the ABNM
Rads residency alone (can do all diagnostic NM) -->ABR
Rads residency + fellowship (alternate to ABNM) --> ABR + CAQ
Combined residency --->ABR + ABNM
I think you need to be careful about doing the "straight nucs" (NM residency-->boarded by the ABNM) option. Judging from several posts on this board and from talking to some people,
I think this renders you virtually unemployable by most private practice groups. I think straight nucs residencies are very easy to get for this reason. With the competitiveness of diagnostic radiology, many people seem to be thinking of nucs as a back door into rads, although it dosen't seem easy to switch.