Transparent Areas block raycasts

Hi,

I have a very exotic UI. All UI elements are circles and free form curves and so things with a lot of transparent space. I want that the not transparent areas block the raycasts and the transparent areas let them through.

In my case it is not possible to split the ui in small pieces that are rectangular.

Her is an example. The green area is UI and shall block the raycasts. The red are is transparent and shall not block the raycasts.

View attachment 203187

How can I accomplish...

Transparent Areas block raycasts