If you have a CT image as input then then a workflow like this should work:
- Import the CT
- Create a copy of the CT (by going to right-click and click Clone)
- Mirror the CT around the LR axis by applying a transform in Transforms module that has
-1in the top-left corner and then click the harden button to harden that transform on the volume
- If the original and mirrored CTs are not aligned then harden the needed, align the original and the cloned image using registration method described on this page
- Select the original CT as background volume (by clicking on the eye icon in Data module)
- Go to Segment Editor module
- Create a new segment, segment the skull using Threshold effect
- Click “Show 3D” button to see the bone in the 3D view
- Select the mirrored CT as master volume
- Create a new segment
- Use Threshold effect to specify editable intensity range (to limit painting to bone region)
- Select Scissors effect
- Choose Fill inside mode (so that region that are circled will be filled in the segment)
- Choose Masking / Editable area → Outside all segments (to prevent the new segment overwriting the original bone shape)
- Trace the boundary of the defect using Scissors in the 3D view
