How do i create a button with a custom shape (for example a circle)?

Hi there,
i´ve got a simple question and i hope it´s simple to answer too! How do i create a button with a custom shape, like for a pie menu. I guessed using an image with an alpha channel as the button, but i´m pretty sure the engine doesn´t care about an alpha channel for creating a button. Any ideas/suggestions?

It’s an old question, but in case somebody will be searching to solve same problem: https:///
Download link should be in description under the video.