Our overall approach is to define a new static external force field, which we call the gradient vector flow (GVF) field. Using a force balance condition as a starting point (rather than a variational formulation) we then let the GVF field replace the potential field in (7), defining a new snake, which we call the GVF snake. The GVF field points toward the object boundary when very near to the boundary, but varies smoothly over homogeneous image regions, extending to the image border. The main advantages of the GVF field are that it can capture a snake from a long range -- from either side of the object boundary -- and can force it into concave regions.