after a cardiology fellowship, what are the major further fellowship trainings that can exist

I know interventional and electrophysio?

What is noninvasive? How many years is that?

Is that the same thing as a cardiac imaging fellowship? Is that going to exist in the future?

From what I understand the only two recognized sub-specialties are interventional and electro-physiology. "Non-invasive" is just general cardiology with the standard 3 years and no sub-fellowship.
I've seen professors at medical schools and hospitals say that they specialize in Heart Failure and Nuclear Cardiology, but I think those are just areas that they concentrate in or in which they received extra instruction. In other words, there aren't board certification exams in those last two.
If anyone knows better, and many likely do, please correct me. :D