exporting mesh from image data

[John Hunter]

I am trying to generate a mesh for a finite volume solver (gambit,
fluent) from 3D image data (CT, MRI).  To generate the fluent msh
file, you need not only a list of vertices and polygons, much like
what is available in the vtk file format, but also the volume elements
in the mesh that the polygons abut.  Eg for a given triangle in the
mesh, you would have a line like

  3 3 2 1 11 0

which is 

  numfaces vert0 vert1 vert2 vol1 vol2

where vol1 and vol2 are indices that indicate the volume in the mesh
that the triangle belongs to (vol2 is 0 for triangles on the surface).

The specific problem at hand involves building a mesh for ventricles
in the brain.  I have no trouble building the isosurface that
surrounds the ventricles using the marching cubes and connectivity
filters in VTK, but now need to be able to generate a mesh over the
interior and assign volumes to faces.

Does such capability exist in VTK? and if so I would be thankful for
pointers to class docs or examples.  Are there other algorithms in the
world of python 3D libraries that can provide 3D meshes for 2D surface
isocontours, assigning volume elements from the mesh to the surfaces
that surround the volume.

Thanks!
JDH