Including air as part of mesh

Hi there.

I’m looking to run a FEM model of light propagation across biological tissue (including free space / pockets of air), and am using Slicer to create a volumetric mesh. I need to include air as part of the mesh. Does anyone know how to include air / free space as part of the mesh?

If you use “Cleaver” mesher in SegmentMesher extension then it can add such background material. You can also add a background segment on Segment Editor by using any editing tool (e.g., Paint or Scissors) with choosing “outside all segments” as editable region.