Create and save mesh inside of another mesh in .ply format

I want to create and save a mesh inside of another mesh. I just create and render it as below using VTK, but how I can save this as to a single .ply file? If so how can I do that? On the other hand, I can give opacity to the cylinder through the vtkActor object. Is this possible to save it with the same configuration as including the opacity to .ply file ?
One more thing, is that possible to add Hounsfield Unit values to each of these meshes? For example, if I assume that the sphere which is inside the cylinder as a liver and the cylinder as the air or something, how can I assign Hounsfield values to each of vertex? Is this possible ?


This is more of a VTK question, fit for the VTK forum.