  Purpose: The facial recess is a delicate structure that must be protected in
minimally invasive cochlear implant surgery. Current research estimates the
drill trajectory by using endoscopy of the unique mastoid patterns. However,
missing depth information limits available features for a registration to
preoperative CT data. Therefore, this paper evaluates OCT for enhanced imaging
of drill holes in mastoid bone and compares OCT data to original endoscopic
images.
  Methods: A catheter-based OCT probe is inserted into a drill trajectory of a
mastoid phantom in a translation-rotation manner to acquire the inner surface
state. The images are undistorted and stitched to create volumentric data of
the drill hole. The mastoid cell pattern is segmented automatically and
compared to ground truth.
  Results: The mastoid pattern segmented on images acquired with OCT show a
similarity of J = 73.6 % to ground truth based on endoscopic images and
measured with the Jaccard metric. Leveraged by additional depth information,
automated segmentation tends to be more robust and fail-safe compared to
endoscopic images.
  Conclusion: The feasibility of using a clinically approved OCT probe for
imaging the drill hole in cochlear implantation is shown. The resulting
volumentric images provide additional information on the shape of caveties in
the bone structure, which will be useful for image-to-patient registration and
to estimate the drill trajectory. This will be another step towards safe
minimally invasive cochlear implantation.
