How do I extract properties of shell from a 3D stack of Images (665x600x500) from XCT

I have started by binarising the 2D images and I have identified the outer boundary (N.B the images are typically irregular). I am a bit confused on how to make shells of a few voxels, starting from the surface until I get to the centroid, then extract the volumes representing 0 and 1's from the 3D structure. Does anyone have an idea on how to achieve this or any code that is suitable?

I have attached a sample 2D solution of what I am trying to achieve in 3D.

Thanks in advance