There exist several approaches to extract the boundary of a 3D image. Most of them represent the extracted boundary as a collection of a large number of little triangular or quadrangular faces whereas few approaches give general orthogonal faces (with any number of edges and with possible holes). One of these approaches is based on a secondary model EVM and focusses mainly on the process to obtain the orientation of the output primitives (edges and faces). Actually, this algorithm obtains, for each plane, a set of oriented edges that have to be rearranged as contours and these contours have to be classified in order to have the corresponding inclusion relationships. These last two processes are performed in a simple brute force way. In this paper, we present an improved algorithm that processes all the edges of a plane and, following a plane-sweep based method, obtains the contours and the inclusion relationships.