Solid body vs. sheet body

When I export an STL file of the ribcage I segmented from 3D Slicer CTACardio CT scan, I obtain a solid body in CAD software for further processing of the body. However, when I export an STL file of this ribcage after performing an ROI cut, I obtain a sheet body in CAD software. Is there a reason why no solid body is obtained after performing an ROI cut? Is there a solution for this?

Thank you in advance!

Kind regards

a motivated student and admirer of 3D slicer

Normal meshes are 3D surfaces, like the one you obtain with Dynamic Modeler ROI cut tool. If you want a tetrahedral mesh you’ll need something like GitHub - lassoan/SlicerSegmentMesher: Create volumetric mesh from segmentation using Cleaver2 or TetGen