"Fusion" is one of those generic words like "overlay" that seem to be used with a degree of specificity that is matched by an inability or reluctance to produce a specific definition. Some examples are nothing more than a simple sum or arithmetic mean. I've made slightly less absurd guesses, but I know there are still intended meanings other than what I've shown. I'll leave it to you to decide what "fusion" means.
This is a simple example of various ways to combine PET scan and MRI images using basic image composition/blending.
These are an arbitrary selection of FEX submissions demonstrating vastly different concepts of "fusion" within similar application contexts. You may be able to find others with a bit of browsing.
