1. A method for spinal canal segmentation in a 3D medical image, comprising: detecting an initial set of spinal canal voxels in the 3D medical image using a trained classifier; refining a spinal canal topology defined by a current set of spinal canal voxels based on an estimated medial line of the spinal canal; sampling seed points based on the refined spinal canal topology; and detecting an updated set of spinal canal voxels in the 3D medical image using random walks segmentation based on the sampled seed points.