Ignore transparent areas of sprites in uGUI

I need the mouse events (over/highlight/press) to only trigger if I'm in the colored area of a sprite, and ignore the transparency.

To give you an idea of what I'm trying to achieve/why I need this: Imagine a map of the US. Each state is a separate sprite which I want to be intractable. If you look at states that are next to each other like Oklahoma and Texas there's an issue with overlap if each uses a rectangular sprite that doesn't ignore transparent areas.