Three-dimensional cephalometry using helical computer tomography: measurement error caused by head inclination.

We performed a study of three-dimensional (3-D) linear measurements in the maxillofacial region using helical computer tomography (CT). The high accuracy of the linear measurements showed errors of less than 5% from the actual measures. But, it is possible that the accuracy was influenced by inaccurate head positions. In this study, we evaluated the errors when the head positions were tilted using the 3-D measurement system. Helical CT was used to scan a dry skull, and the data were reconstructed into a 3-D image. A total of 18 points were plotted on the 3-D images, and the distance between two points was calculated when the points were expressed as coordinates. A dry skull was tilted by 10 degrees from the reference position in the horizontal, sagittal, and frontal planes and was then tilted in a combination of directions. Scanning was performed with slice thicknesses of 1 mm, 3 mm, 5 mm, and 7 mm. The length between two points measured by 3-D cephalometry was compared with the actual length determined using an antenna meter and a caliper and expressed as percentage errors of the actual length. In all head positions, errors in all linear measurements on the images and the actual length measured on the skull were less than 5% when a slice thickness of 1 mm or 3 mm was used. But, on using a slice thickness of 5 mm or 7 mm, some linear measurements showed larger measurement errors. Therefore, a thickness of less than 3 mm was thought to be clinically appropriate because the accuracy of the measurements was not influenced by head rotation.