How to deal with transparent button [New UI 4.6b20]

I have a circle button like below. (circle area is colored, square area is transparent)
How to make a button that can only clickable on the circle area.

 

Better late than never...

Code (CSharp):

1. Image.eventAlphaThreshold

should do just that.
http://docs.unity3d.com/460/Documentation/ScriptReference/UI.Image-eventAlphaThreshold.html

But as it says, it doesn't work for atlassed Sprites.