1. A computer-assisted method of using volumetric image data to construct a representation of a virtual organ, the method comprising: determining, using one or more computers, a set of one or more candidate segments in the volumetric image data; identifying, using the one or more computers, a set of one or more landmark segments from the set of candidate segments; computing, using the one or more computers, a characteristic path for each segment in the set of candidate segments; and creating, using the one or more computers, a first combined path by connecting the characteristic paths, wherein the connecting is performed using a cost network, wherein using the cost network comprises: bounding a local cost in the cost network; bounding a total cost in the cost network; and using the local cost and the total cost to limit a search space in the cost network.