Actually, because of the engine, the collision detection uses squares, not circles <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->.
I don't really know if it would be easy to implement what you are asking, anchored collisions, that would be <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->.