1. A method for segmenting structures from three-dimensional (3D) images, the method comprising the steps of: receiving 3D image data; visualizing the 3D image data on a display device; selecting a structure in the 3D image data by placing a seed in the structure; initializing a plurality of rays from the seed to form a surface; determining a speed function of each of the plurality of rays; evolving the surface by propagating the plurality of rays based on the speed function of each of the plurality of rays; converging the plurality of rays on a boundary of the structure; and segmenting the structure when all of the plurality of rays have converged on the structure's boundary.