@ omar. The type of segmentation algorithm that you are describing is possible, but this program was not designed for it. I suggest you read about the gradient vector flow (GVF) on page 15 of "Biomedical Image Analysis: Segmentation" by Acton and Ray http://www.morganclaypool.com/doi/abs/10.2200/S00133ED1V01Y200807IVM009?prevSearch=allfield%253A%2528segmentation%2529&searchHistoryKey=
Additionally, I don't believe that the algorithm used in this program tries to minimize the area of the contour. Notice how the snake will expand to fill an area bounded by an edge.