||A central problem in cranio-maxillo-facial (CMF) surgery is to restore the normal anatomy of the facial skeleton after defects, i.e., malformations, tumours, and trauma to the face. Careful pre-operative planning can significantly improve the precision and predictability and reduce morbidity of the craniofacial reconstruction. In addition, time in the operating room can be reduced and thereby also cost. Of particular interest in CMF surgery planning is to measure the shape and volume of the orbit (eye socket), comparing an intact side with an injured side. These properties can be measured in 3D CT images of the skull, but this requires the orbit to be extracted from the rest of the image, a process called segmentation.
Some of the bone structures in the orbit are only a few tenth of a millimeter in thickness. In CT images, the boundaries between these thin bone structures and the surrounding soft tissue are blurred due to the partial volume effect. This, together with anatomical shape variations, makes automatic segmentation of the orbit a difficult task. Today, orbit segmentation is usually performed by manual tracing of the orbit in a large number of slices of the CT image. Manual segmentation is, however, time consuming, tedious, and sensitive to operator errors.
The outcome of this master thesis project is a prototype of a semi-automatic system for segmenting the orbit in CT images. The system utilizes a deformable surface model, driven by external forces computed from the input CT image, to perform interactive segmentation of the orbit. A 3D user interface with haptic feedback allows the user to guide the deformable model during the deformation and place landmarks on the orbital rim. The landmarks are used to create a barrier that defines the orbital opening and prevents the deformable model from leaving the orbit.
To evaluate the performance of the segmentation system, we performed a case study where three users were given the task to segment the orbits in 7 CT images by using the segmentation system. To assess segmentation accuracy, crisp and fuzzy ground truth segmentations were constructed from manual orbit segmentations performed by the three users. The preliminary results of the case study indicate that the segmentation system can be used to obtain fast and accurate orbit segmentations, with high intra- and inter-operator precision.