Getting position where border of a 9-sliced sprite ends

Suppose I have a UIImage referencing a 9-sliced sprite in which is really a thick frame of a button/etc. I want to position a child object so that it precisely covers the centre rectangle of the 9-sliced sprite. How can I compute the extents of the centre sub-rect of the 9-sliced sprite in Canvas space so I can position the child element?