Great analogyJust as the issue is with CTA and cardiologists only looking at the heart and having the rest of the visualized structures overread by radiologists, any neuroimager who looks at films of the spine, be they plain, CT, or MRI, will need to comment on all of the visualized structures. This will include diagnoses relating to things that are not "neuro." If you love the intellectual challenge of looking at head and back films and making diagnoses, then you will probably enjoy diagnosing pathology in other structures, too.
And to echo the comments above, in PP you will likely not be doing only neuro stuff. Most of the neuro fellows at my program were lamenting how poor the job market is for them right now, and said that they were doing their fellowship in neuro to boost their application for private practice jobs rather than to remain in academia doing neuro only. However there were a few ex-neurosurgeons who were planning on staying in academia doing neuro only, so I guess that is always an option and is indeed pursued by some.
Bottom line: If you want to be a neuro imager, you need to and should also have a solid foundation in imaging of all areas of the body, just like a cardiologist or nephrologist needs a solid foundation in internal medicine to practice their specialty properly. The best and currently only route for this is via radiology residency with fellowship, although I do understand that neurology offers imaging "fellowships," the weight of which I do not know.