In your second screenshot you cannot edit because there is no anatomical volume selected. That’s why Segment Editor is asking you to “Select master volume to enable editing”, as shown in your screenshot as well.
In the third case, I can see a master volume. If that “6: CHEST…” image occupies the same space in 3D as your imported STL, then you should be able to edit it. You can verify that by going to Volume Rendering and showing the CT in 3D.
If “chaos breaks loose” means what you described next that the vessels have shrunk, then it’s because of smoothing. When you import an STL into segmentation and edit it, then in the background the model is converted to a labelmap. Then when you show it in 3D again, then default smoothing is performed, hence the data loss.
You can oversample the labelmap:
- Use 4.10.1 or latest nightly
- Create new segmentation node in Segmentations module
- In Representations section make Closed surface as master (because you want to have control over the geometry of the labelmap afterwards)
- Import model in Import/Export section
- In Representations section long-click Create and choose Advanced create
- Click the only path in the top part
- Click Specify geometry button in the bottom part
- Select your CT volume
- Specify an oversampling factor (2 should be fine, 3 even higher resolution, but 4 may occupy too much memory)
- Click OK
- Go to Segment Editor and do your thing
You can also disable smoothing to prevent any shrinking (but after oversampling it might not be necessary):