Standardization of 3-D CT measurements for length and angles by matrix transformation in the 3-D coordinate system

Objective: We attempted to establish a new 3-D cephalometry with helical co mputed tomography (CT) by introducing the matrix transformation of the 3-D coordinate system. Subjects and Methods: Three-dimensional cephalometric landmarks on the cran iofacial bones were expressed as 3-D vectors originating from the center of the sella, These vectors were standardized by the matrix transformation so that the midsagittal plane and cranial base line coincided with the XZ pla ne and X axis of the 3-D coordinate system, respectively. We also applied t his new method to trace the normal growth of the craniofacial bones in 44 p atients with head and neck cancer (age range, 5 to 26 years; 19 women and 2 5 men). Results: The accuracy for length measurements was less than 3% of standard error of means with a slice thickness of 1 or 3 mm for 28 out of the 29 len gth measurements recorded. The precision errors by repeated measurements we re 0.8% to 1.0% coefficients of variation for intra- and interobserver vari ability. Standardization of 3-D vectors representing the cephalometric land marks allowed us to assess successfully the age-related transition of these landmarks of the patients' craniofacial bones. Conclusion: A new assessment method for 3-D CT cephalometry has been develo ped by standardizing cephalometric landmarks using a matrix transformation of the 3-D coordinate system. This new assessment method may offer potentia l in planning plastic and reconstructive surgery.