Output only the calculated area (not the bounding box)

Hi, Developer,
I have been looking into aneurysm case and wonder if I only want to produce a VTK 3D results on/in the geometry, how it should be done?
What I can see is that the whole bounding box of the voxel mesh is produced, and if the flag can be used to produce only the shape of the geometry as it is represented in stl, no position shift and 1:1 (scaling factor = 1), how can it be done?
Any example of codes?