Segmentation is not straightforward

Today i realized that it's not at all easy to segment the left-atrium, or be it any cardiac structure. I asked my PhD colleague, who's working on tongue segmentation, if it was easy to segment the tongue. From his experiences its neither easier to segment the tongue.

I attempted to draw out using ms-paint, an illustration of why it is difficult. The top two reasons are these:

• Left atrium is connected to other cardiac structures by blood-vessels. The segmentation algorithm needs to be able to differentiate between these.
• Sometimes, blood vessels (not connected to the LA) might pass over, or under the Left atrium. In an attempt to define a region-of-interest box, one might try to avoid the vessel, lets say, which passes on top of a LA vein. This would cause the box to define a region which is not sufficient and crops remaining part of the LA vein, which would have emerged out in further slices.
  

Here's my illustration:

There may be other segmentation methods (apart frm region-growing) which may be worth trying. There are deformable models such as Active contours or snakes, which ITK-Snap uses to segment its MRIs. However, all methods will require a great-deal of user-interaction, which is why I am doing my PhD.