Formulation of a reference coordinate system of three-dimensional (3D) head & neck images: Part I. Reproducibility of 3D cephalometric landmarks
Formulation of a reference coordinate system of three-dimensional (3D) head & neck images: Part I. Reproducibility of 3D cephalometric landmarks
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이 연구는 기존의 두부방사선사진에서 사용되었던 계측점들을 3차원으로 재구성한 CT자료에서 다시 정의하고, 그 점들의 재현도를 조사함으로써, 재현성이 높고 해부학적 특징을 잘 표현하는 점들을 제안하고자 시행하였다. 3차원 영상에서 상대적 좌표값을 구하기 위해 어느 4점도 같은 평면에 존재하지 않는 7개의 고정점을 선정하여 기준 좌표계를 설정하였다. V works 4.0 (Cybermed Inc., Seoul, Korea)을 이용하여 고정점과 I점 설정을 위한 volume model (voxel size = 4*4*2, threshold value = 639)을 형성한 후 계측점을 선택하였으며, 나머지 점들은 모두 volume mode의 MPR 창을 이용하여 선정하였다. 이렇게 선정한 계측점을 V surgery (Cybermed Inc., Seoul, Korea)로 이출하여 각 점의 상대적 공간 좌표값을 구하였다. 모든 자료는 2주 간격으로 2번 측정한 후 재현도를 계산하였다. 각 계측점의 재현도는 x축 0.13 ~ 1.24 mm, y축 0.23 ~ 1.53 mm, z축 0.15 ~ 1.81 mm로 유사한 범위를 보였다. 재현도가 우수한 것은 고정점을 이용한 I점이며 봉합에 의해 정의되는 J점의 재현도가 비교적 낮게 나타났다.
Abstract
The purpose of this study was to redefine the cephalometric landmarks in three-dimensional (3D) images, which are used in orthodontic cephalometric radiography, and to evaluate the reproducibility of each landmark for 3D cephalometric analysis. Eighteen CT scans were taken at the Department of Diagnostic Radiology at Seoul National University Dental Hospital and manipulated with V works 4.0 (Cybermed Inc., Seoul, Korea). The coordinate system was established using 7 reference points, with no more than 4 points on the same plane. These 7 points were generated as a volume model, the voxel size of which was 4 by 4 by 2 (threshold value = 639). The cephalometric landmarks were selected at the multiplanar reformation (MPR) window on the volume mode of V works 4.0. The selected landmarks were exported to V surgery (Cybermed Inc., Seoul, Korea) for the calculation of coordinate values. All the data were taken twice with a lapse of 2 weeks by one investigator. The reproducibility of each landmark was 0.17 ~ 1.21 mm in the x axis, 0.30 ~ 1.53 mm in the y axis, and 0.27 ~ 1.81 mm in the z axis. In all three axes, the range of error was similar. These error ranges were acceptable with regards to the pixel space and slice thickness. The most reproducible points were I points which were selected on the basis of the volume model. The least reproducible points were J points that were defined by sutures.
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