How to export bones of a dicom set (for example in .obj) in AMI.js?

### Description
I want to load some DICOM files and export the bones of that files or at least the volume. I already load the DICOM files and get the quadview and the volume Rendering, but I want to export the bones of that files.

When I export to .obj I get the box which is the content of the volume but I want to export the content inside.

#### Steps to Reproduce

1. Load DICOM data

2. I Load the volume using vrHelper = new AMI.VolumeRenderingHelper(stack);

3. I export it using OBJExporter and passing the scene which contains the volume, i also tried to pass the mesh ( let exporter = new THREE.OBJExporter(); let obj = exporter.parse(mesh);)

#### Expected Results
What I would really like is to export the bones and then slice it get only a part of that, for example get only the clavicle.
I would like to export this:

#### Actual Results
But I am only be able to export an empty box

This is the mesh that I am exporting which has 12 faces and 8 vertices and it has geometry: BoxGeometry, I Know that this is wrong but I don´t Know how to get the real mesh

I tried to create a labelmap using threshold and get only bones and then create a mesh from that labelmap but I wasn't able to do that.

Thank you very much.

### Browsers Affected
- [X] All
- [ ] Chrome
- [ ] Firefox
- [ ] Edge
- [ ] Safari 9
- [ ] Safari 8
- [ ] IE 11

### Versions
- AMI: v.0.32.0
- THREEJS: v.1.0.7