Setting of initial contours is a critical problem of active contour models. Although gradient vector flow (GVF) has simplified them greatly compared to the traditional snake, additional work is still necessary. We propose a specific setting method based on vector field analysis. Some critical points in the force field are used to estimate curve deformation. These points seem like sources whose neighbor vectors emanate from them, therefore they can be regarded as inflated centers of deformable contours in GVF snakes. The proposed method detects these points and sets initial contours to contain all the ones in the object area and none of the others. It can prevent the contours from stabilizing on false boundaries in homogeneous regions or real boundaries of other objects. Several experimental results demonstrate that this method is robust and effective.