Keeping the head straight Standardizing reference plane orientation in CBCT scans for accurate assessment

This thesis investigates how to improve the accuracy and reliability of three-dimensional (3D) cephalometric analysis by developing a new reference plane for orienting CBCT scans. Across five studies, the research evaluates anatomical landmarks in the external acoustic meatus (EAM), examines their symmetry, and introduces the ACTA plane as a novel alternative to the traditional Frankfurt Horizontal (FH) plane.
The first studies focused on identifying consistent landmarks within the EAM. Among the evaluated points, the most superior point (MSP) proved to be the most reliable, while the most inferior point (MIP) and the epitympanic notch (EN) showed greater variability due to their complex anatomy. Automated segmentation improved visualisation but also highlighted limitations in distinguishing subtle structures.
Next, the symmetry of the EAM was assessed. The EAM was found to be largely symmetric, supporting the MSP as a potential reference for 3D imaging. A new coordinate system was introduced to reduce variability caused by CBCT scanner positioning, enabling more accurate comparison between left and right cranial structures.
The ACTA plane was then developed using internal midsagittal references and external orbital points, avoiding reliance on the Porion—an inconsistent landmark in 3D imaging. Reorienting scans with the ACTA plane improved examiner reliability, particularly for landmarks traditionally difficult to locate.
Finally, the ACTA plane was compared to the FH plane. Both produced similar angular orientations, but the FH plane was less stable due to challenges in identifying Porion3D. The ACTA plane therefore offers a more reproducible alternative for 3D skull orientation.
Overall, this thesis supports the ACTA plane as a promising new standard for CBCT orientation in 3D cephalometry.