iPhone如何在圆圈内剪辑圆圈？

I have two circles, one bigger and one smaller. I want to clip smaller circle from bigger one, and then use that new shape (big circle with a hole in it) to apply it to arbitrary image. I have played a little bit with quartz, but could not find solution to this. Is there any easy way to do this?I have two circles, one bigger and one smaller.