new UI/Sprite scratch/remove using GetPixel

hello what i am trying to do is make a scratch effect using Get Pixel but for the new UI system that came in with 4.6+ , i'v created that effect for a 3d mesh quad that has a box collider and with material , so wt i was doing is getting the material main texture and apply it to a texture 2d then taking the texture cords and then setting the pixels and applying them to the texture and the material same as painting but instead i was scratching and painting the (alpha of that material) but now what i am trying to do is scratch a sprite (UI image) , is that possible ? can someone explain to me how i can do that ?!

thank you