Cephalometric image analysis and measurement for orthognathic surgery

Automatic identification of landmarks in cephalometry is very important and useful for orthognathic surgery. A computerised automatic cephalometric an alysis system (CACAS), based on image processing, is presented. For an orig inal X-ray image, median filtering and histogram equalisation are used to i mprove image quality. The edge of an X-ray image is detected by a wavelet t ransform and Canny filter. Seventeen landmarks in cephalometry are successf ully identified by knowledge-based edge tracing and changeable templates. S eventy-three measurements based on distances, angles and ratios between lan dmarks are computed automatically. The reliability of the landmarks and the validity of the measurements are compared for automatic and manual operati on. The values of measurements obtained by CACAS are more precise and relia ble: the mean error for linear measurements is less than 0.9mm; the mean er ror for angular measurements is less than 1.2 degrees, The rate of validity is over 80%, even if the image quality is poor. For an image with a high s ignal-to-noise ratio, the rate of validity of landmarking and measurements using the CACAS system is over 90%.