1. A method for altering image values of a plurality of voxels, each of the voxels having a current image value, the method comprising: for each voxel, determining if the voxel represents material tagged with a contrast agent; identifying an interface between material tagged with a contrast a contrast agent and a gas; and for each voxel that represents material tagged with a contrast agent, but that is not within the identified interface, setting the image value of the voxel to a value lower than the current image value of the voxel.