Why would neurologists/neurosurgeons want to do a neuroimaging fellowship?
How would someone like this fit into the medical system? It is not a practical option.
There would be few opportunities for employment outside of academia for neuroimagers as the great majority of private practice jobs require general radiology work.
I doubt these clinicians would be happy doing only neuroimaging work at an academic center.
Perhaps it would allow them to get priveleges to read and bill for studies in a private hospital setting. I think the return on the investment is not worth it. You would charge only the professional fee on relatively few studies performed on your own patients.