Why isn't Model Definition an Automated Process?

Graphical user interface of the workstation.

The threshold value for contour (isodensity line) finding is predefined on the workstation. Normally a threshold value of 180 (scale from -1024 to +3071, 12bit DICOM images) will sufficiently separate bone from soft tissue; sometimes adjustment is required.

Contour finding: Note the effect of different threshold values.

Once a threshold value is chosen, the workstation finds the corresponding contours automatically, but the result is, for various reasons, not suitable for model production:

Surface rendering of contours defined automatically; threshold value 180.

• Rather than showing the exact anatomy, the model should focus on the region to be operated: E.g. in shown severe case of overbite which asks for a Le Fort Osteotomy, the skull base and cervical spine (shown in surface rendering) are not needed at all, thus, they are removed in contour definition, and that process cannot be automated.
• Furthermore there are other limiting factors, in this case the metal artifacts (caused by dental inlays), requiring manual correction of the automatically chosen contours.

Surface rendering of manually corrected contours. This is an exact rendering of the model-to-be.

The modeling volume can be split.

In this specific case, we produced a synopsis (combination of maxilla and mandibula) along with separate models of maxilla and mandibula (rendering after contour definition), the former intended to provide the surgeon with an overview of the pathology, and the later for surgery planning, as to be seen with the mandibula model pictured (Not same patient).

Mandibula (other patient). Separation of model parts facilitates surgery planning.

Thus, along with choosing the appropiate parameters for CT scanning, the volume definition is a critical process, supervised by the radiologist.
Data conversion is automated, and model production is also non-critical: once the modeling volume data are converted, the modeling system is set up, and the process can be left on its own.

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