Segmentation according to a chosen midsagittal plane

hi everyone,

I’m working on surgical planning in facial bone repositioning. I’m focusing actually on broken zygomatic bones.
To plane the repositioning I use the mirroring of the normal side. (considering that the patient is symmetric…)

To dot it I first readjust the CT scan to obtain a symmetry between the image of left and right side of the patient (ACPC transform)

Then I’d like to segment the normal side according to the midsagital plane to mirror this segment on the broken side ans obtain the “perfect” projection of the repositioning.
For now i don’t know how to segment according to a specific plane???

I use a tricky method with the rectangular scissors but as I can’t follow perfectly the midsagittal plane my mirroring is not perfect…

Is anyone can help me?

thanks for helping me.

You can segment on any plane. It is not necessary to use a plane that you’ll later use for mirroring.

Instead of ACPC transform, you can mirror your volume (apply a transformation matrix of diagonal(-1, 1, 1, 1)) and register it to the original volume to get an accurate mirroring transform. You can either use automatic intensity-based registration (General registration (BRAINS) module) or you can use landmark-based registration (Fiducial registration module in SlicerIGT extension) to find the mirroring transform.

Once you have your segmentation completed on one side, clone the segmentation node, apply the mirroring transform, harden the transform, and copy the mirrored segment to the original segmentation.

If you get stuck at any point then let us know.

Thanks a lot for this very detailed answer. I’ll try and let you know if I get stuck.