Reproducibility of Maxillofacial Anatomic Landmarks on 3-Dimensional Computed Tomographic Images Determined with the 95% Confidence Ellipse Method

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1 Original Article Reproducibility of Maxillofacial Anatomic Landmarks on 3-Dimensional Computed Tomographic Images Determined with the 95% Confidence Ellipse Method Atsushi Muramatsu a ; Hiroyuki Nawa b ; Momoko Kimura a ; Kazuhito Yoshida c ; Masahito Maeda d ; Akitoshi Katsumata e ; Eiichiro Ariji f ; Shigemi Goto g ABSTRACT Objective: To assess the plotting reproducibility of landmarks on 3-dimensional computed tomography (3D-CT) images through use of the 95% confidence ellipse in order to propose sufficiently stable coordinate systems for 3D-CT measurements. Materials and Methods: Six dentists plotted 19 landmarks twice on 3D-CT images. Scatterplots and the 95% ellipses were produced 3-dimensionally, and the areas of the ellipses were calculated for evaluating the reproducibility of landmarks. Results: The plotting reproducibility of each landmark showed characteristic features. Among five landmarks (the sella [S], nasion [N], basion [Ba], orbitale [Or], and true porion [Po]) that are frequently used as reference points on cephalograms, Ba showed the smallest areas for all three coordinate axes, indicating high reproducibility. The coronoid process (CP) and the tooth-related landmarks showed relatively high reproducibility. Conclusion: Sufficiently stable coordinate axes could be proposed for different treatments and studies. KEY WORDS: Orthodontics; 3-Dimensional; Cephalometry; Tomography; X-ray computed; Reproducibility of results INTRODUCTION Computed tomography (CT) has been widely used for clinical diagnosis, and it has also been applied to a Graduate PhD student, Department of Orthodontics, Aichi- Gakuin University School of Dentistry, Nagoya, Japan. b Assistant Professor, Department of Orthodontics, Aichi-Gakuin University School of Dentistry, Nagoya, Japan. c Research Fellow, Department of Oral and Maxillofacial Radiology, Aichi-Gakuin University School of Dentistry, Nagoya, Japan. d Graduate PhD student, Department of Oral and Maxillofacial Surgery, Aichi-Gakuin University School of Dentistry, Nagoya, Japan. e Associate Professor, Department of Oral and Maxillofacial Radiology, Asahi University School of Dentistry, Hozumi, Japan. f Professor, Department of Oral and Maxillofacial Radiology, Aichi-Gakuin University School of Dentistry, Nagoya, Japan. g Professor, Department of Orthodontics, Aichi-Gakuin University School of Dentistry, Nagoya, Japan. Corresponding author: Dr Eiichiro Ariji, Department of Oral and Maxillofacial Radiology, Aichi-Gakuin University School of Dentistry, 2-11 Suemori-dori, Chikusa-Ku, Nagoya, Japan. ariji@dpc.agu.ac.jp Accepted: May Submitted: April by The EH Angle Education and Research Foundation, Inc. orthodontic treatment and orthognathic surgery. 1 8 It allows us to evaluate 3-dimensional (3D) morphology of the maxillofacial skeleton, which is difficult to identify with cephalometric radiography. 1 3,6,7,9 12 On the other hand, use of CT increases radiation exposure, and this should not be ignored. Several 3D reconstructed CT (3D-CT) systems have been proposed for maxillofacial skeletal measurements. 3,9,11,13 18 We have also proposed a 3D-CT system for evaluating the facial asymmetry, in which landmarks are plotted on axial slice images. 14,15 Measurement accuracy depends on the image resolution of individual machines and is generally reported to be sufficient for linear measurements with recently available CT apparatuses. 9,10 Although reproducibility has been evaluated for 3D-CT, most of the evaluations dealt with linear measurement of the length between two landmarks. When reproducibility in the length measurement is evaluated using relative indices, such as coefficient of variance, it appears higher as longer length is analyzed. Moreover, when the reproducibility of the length measurement is not sufficient, it cannot be determined which of the two landmarks that creates the line is unstable. 396 DOI: /

2 REPRODUCIBILITY OF LANDMARKS ON 3D-CT 397 Table 1. Definitions of Anatomic Landmarks for 3-Dimensional Computed Tomography Landmark Abbreviation a Definitions on CT image Sella S Center of the pituitary fossa Nasion N Nasofrontal structure at the midline Basion Ba Anterior-inferior margin of the framen magnum Orbitale Or (Or-L, Or-R) The midpoint of the infraorbital margin Porion Po (Po-L, Po-R) Center of the narrowest segment of the external auditory meatus Anterior nasal spine ANS The most anterior point of the anterior nasal spine Condyle Cd (Cd-L, Cd-R) The most superior point of the condyle Coronoid process CP (CP-L, CP-R) The most superior point of the coronoid process Gonion Go (Go-L, Go-R) The most inferior and posterior point of the mandibular angle Menton Me The lowest border of the mid-mandibular suture Upper first molar U6 (U6-L, U6-R) The most superior point of the mesial buccal cusp of the maxillary first molar Upper incisor U1 The mid-point between the mesial angles of maxillary central incisors Lower incisor L1 The mid-point between the mesial angles of mandibular central incisors a L indicates left; R, right. Although various coordinate systems have been proposed for 3D-CT based on the anatomic landmarks that are traditionally used for cephalometric measurements to identify the location of skeletal indices and to clarify their changes after treatment, there are few studies regarding the reproducibility of the coordinates of a certain landmark. The plotting reproducibility of landmarks that are referred to for creating a coordinate system should be verified before the system is set up. A method using scatterplots and the 95% confidence ellipse is an alternative for evaluating the coordinate reproducibility 19 and has been used to evaluate plotting reproducibility of cephalometric landmarks. 20 The method can be modified for 3D evaluations. 21 The purposes of the present study were to assess the plotting reproducibility of landmarks on 3D-CT images through use of the 95% confidence ellipse and to propose sufficiently stable coordinate systems for 3D-CT measurements. MATERIALS AND METHODS CT Data Acquisition and Image Processing An acrylic head phantom with an embedded human dried skull (Kyoto Kagaku, Kyoto, Japan) was scanned parallel to the Frankfort horizontal plane at the dorsal position using a CT machine (HiSpeed NX/i Pro, GE Yokogawa Medical Systems, Tokyo, Japan). Referring to the scout view, scans were performed from 1 cm superior to the nasion (N) to 1 cm inferior to the menton (Me) at 120 kv and 200 ma. The sequential axial slice data were acquired at 1 mm, 2 mm, and 3 mm slice thickness. The image matrix was , field of view was 170 mm, and the size of 1 pixel was approximately 0.33 mm. Nineteen landmarks were selected and defined for 3D-CT images with references to landmarks in cephalometric measurements (Table 1). Sice data were transferred to a notebook PC (VAIO PCG-NV99M/BP, Sony, Tokyo, Japan) in DICOM (digital imaging communications in medicine) format via a portable hard disc (HD-D120U2, Buffalo, Nagoya, Japan) and were converted to 8-bit images in tagged image file format (TIFF) using image analysis software VG Studio Max 1.1 (Networking Visualization Science, Tokyo, Japan). The TIFF images were moved to image-processing software (Adobe Photoshop 6.0, Adobe Systems, San Jose, Calif), and a landmark was plotted as a circle of 7-pixel diameter on the axial slices images (Figure 1). After the landmarks were plotted, the TIFF images were imported again to VG Studio Max 1.1, and a 3D image was created using a volume rendering method. The plotted landmarks were also displayed on the created 3D-CT image (Figure 2). Using this software, a corner of a cubic 3D matrix was automatically set as the origin (0, 0, 0), and the X-axis was determined roughly parallel to the right-left direction of the phantom s maxillofacial skeleton. The Y-axis and Z-axis corresponded to the posterior-anterior and superior-inferior directions, respectively. The coordinates (x, y, z) could be determined for each voxel (Figure 2) and were expressed as the actual size that was calculated by converting the number of voxels to millimeters. Evaluation of Reproducibility Six dentists (four orthodontists, a radiologist, and an oral surgeon) plotted the 19 landmarks twice for each slice thickness. There was at least 1 week between the two sets of plotting. A total of 12 plots were obtained for each landmark and were 3-dimensionally displayed as scatterplots for determining the 95% confidence ellipse. The area of the 95% confidence ellipse was calculated for each landmark on three planes; the axial plane (the XY plane), the coronal plane (the XZ plane), and the sagittal plane (the YZ plane).

3 398 MURAMATSU, NAWA, KIMURA, YOSHIDA, MAEDA, KATSUMATA, ARIJI, GOTO Figure 1. Landmarks for 3-dimensional computed tomography (3D-CT) are plotted on sequential axial CT images. N indicates nasion; Po-R, porion right; Ba, basion; S, sella; Po-L, porion left; Or-L, orbitale left. Figure 2. Automatically determined 3D coordinates axes identified with the software used. The X-axis is set at the right-left direction of the scanned head phantom. The Y- and Z-axes correspond to the anterior-posterior and superior-inferior directions, respectively. The landmarks plotted on the axial images are visible as black points on the 3-dimensional computed tomography (3D-CT) image. RESULTS The areas of 95% confidence ellipses on three planes (the XY, YZ, and ZX planes) and in images with three-slice thicknesses were calculated for each landmark evaluated (Table 2). Consequently, nine confidence ellipses were produced for each landmark. Generally, the areas increased as the slice thickness increased. The basion (Ba) and the bilateral coronoid processes (CPs) showed small areas (under 2 mm 2 ) in all nine ellipses. The tooth-related landmarks had relatively small areas, indicating high reproducibility. For five landmarks (the sella [S], N, Ba, orbitale [Or], and true porion [Po]) that are frequently used as reference points on cephalograms, the ellipses were displayed on three planes (Figure 3). The plotting reproducibility showed characteristic features for each landmark. Among the five landmarks, the Ba showed the smallest ellipses in all planes on 3D-CT images, regardless of the slice thickness. As for the S and N, the ellipses for the image of 1-mm slice thickness were smaller than those for other slice thicknesses on all three planes (the XY, YZ, and ZX planes). The ellipses for 2-mm and 3-mm slice images were expanded

4 REPRODUCIBILITY OF LANDMARKS ON 3D-CT Table 2. Landmark Area of 95% Confidence Ellipse for 19 Landmarks (mm 2 ) a Slice thickness Plane XY YZ ZX S 1 mm mm mm N 1 mm mm mm Ba 1 mm mm mm Or-L 1 mm mm mm Or-R 1 mm mm mm Po-L 1 mm mm mm Po-R 1 mm mm mm ANS 1 mm mm mm Cd-L 1 mm mm mm Cd-R 1 mm mm mm CP-L 1 mm mm mm CP-R 1 mm mm mm Go-L 1 mm mm mm Go-R 1 mm mm mm Me 1 mm mm mm U6-L 1 mm mm mm U6-R 1 mm mm mm U-1 1 mm mm mm L-1 1 mm mm mm a S indicates sella; N, nasion; Ba, basion; Or-L, orbitale left; Or-R, orbitale right; Po-L, porion left; Po-R, porion right; ANS, anterior nasal spine, Cd-L, condyle left; Cd-R, condyle right; CP-L, coronoid left; CP-R, coronoid right; Go-L, gonion left; Go-R, gonion right; Me, menton; U6-L, upper first molar left; U6-R, upper first molar right; U- 1, upper incisor; L-1, lower incisor. 399 along the Z axis (the superior-inferior direction); no enlargements were found along the X- and Y-axes compared with those of the 1-mm slice image. These findings indicated relatively higher reproducibility in the right-left (X-axis) and anterior-posterior (Y-axis) directions. The ellipses of Or in 1-mm and 3-mm thickness images expanded along the right-left direction (the X-axis) with small variation along the Y- and Z-axes. In the 2-mm slice image, enlargement was also seen in the superior-inferior direction (Z-axis). These results indicated that the Or showed good reproducibility in the superior-inferior direction (Z-axis) on 1-mm or 3-mm thickness images. Regarding the Po, the reproducibility for the 1-mm slice was good in the superior-inferior (Z-axis) and anterior-posterior (Y-axis) directions. For 2-mm and 3-mm slices, the reproducibility was still stable along the Y-axis but became worse along the Z-axis. DISCUSSION Various coordinate systems for 3D-CT measurements have been proposed. 3,7,9,11 Many include at least one of three landmarks, namely the S, N, and Ba, as the points that are referred to when the coordinate axes are determined. Based on the results of the present study, use of these landmarks is considered to be reasonable for 3D-CT because of their high plotting reproducibility. Moreover, orthodontists are familiar with these landmarks because they are well accepted as reference points on cephalograms. We have previously proposed a 3D coordinate system with the S, N, and Ba as references for evaluating facial asymmetry. 14,15 In that system, the sagittal plane (YZ plane) is initially determined with reference to these three landmarks. Kitaura et al 22 have also proposed a coordinate system using the S as the origin and the N and Ba as references. They have applied it to patients of different ages and have 3-dimensionally determined the age-related changes in various landmarks of the maxillofacial skeleton. They examined the accuracy and reproducibility of 29 types of length measurements between two landmarks. They concluded that the reproducibility of length measurement decreased when the slice thickness increased. The present study verified that the coordinate axes proposed in these two studies were established with sufficiently stable landmarks, although they did not examine the reproducibility of the S, N, and Ba measures. Park et al 3 plotted 19 landmarks five times on 3D-CT images and determined the standard deviations (SDs) of the coordinates of the landmarks. The SD was determined for the XYZ axis components for each landmark. Although their method of determining

5 400 MURAMATSU, NAWA, KIMURA, YOSHIDA, MAEDA, KATSUMATA, ARIJI, GOTO Figure 3. Scatterplots and 95% confidence ellipses of five landmarks are displayed in the three planes (the XY, YZ, and ZX planes) for each slice thickness. S indicates sella; N, nasion; Ba, basion; Or-L, orbitale left; Po-L, porion left. the coordinate axes was not described in detail, the coordinate axes were established with the N as the origin and the anterior, superior, and left directions as positive for the Y-axis, Z-axis, and X-axis, respectively. Therefore, the directions of these axes roughly matched those in the present study. According to their results, the Po on the 2-mm thickness image had larger SD in the Z-axis direction than in the directions of other axes. As for the Or, the SD in the Z-axis direction was not increased, indicating only a small variability in the superior-inferior direction. Although they did not propose new coordinate axes, their results are consistent with the results of the present study. Generally, the reproducibility became higher as a thinner slice thickness was used for 3D reconstruction. However, for some landmarks, such as the Or, the reproducibility in the Z-axis was poorer for the 2-mm slice thickness image than for the 3-mm image. This discrepancy may be due to the difference in morphology surrounding the landmark between the adjacent slices. The difference may be more easily identified on the 3-mm thickness image than on the 2-mm thickness image. Therefore, one could determine the landmarks on axial CT images with much confidence. Among several methods for identifying landmarks on 3D-CT images, we have chosen the method of plotting them on axial slice CT images and then creating 3D images by means of a volume rendering procedure. 14,15 Because CT data are traditionally stored in the form of sequential slice data, the method is considered to be useful for establishing a system independent of the model of CT machine and software used. Recently, however, it has become easy to obtain and manipulate volume data with the development of multi-slice CT or cone-beam CT. 6,23 Therefore, it should be possible to modify and improve the method used in the present study in the near future. Eventually, we might obtain better reproducibility than that in the present study. It is noteworthy that the reproducibility was determined here in three directions (X-, Y- and Z-axes) by means of the 95% confidence ellipse. For the Or on the 1-mm and 3-mm images, the confidence ellipse showed small variability in the Z-axis, indicating high reproducibility in this direction. This means the Or can be used to determine the axial plane (XY plane). On the 1-mm thickness image, the Po was stable in the directions of the Y- and Z-axes but was unstable in the X axis. Therefore, the Po is suitable for determining the axial plane (XY plane) and the coronal plane (ZX planes) on 1-mm images. Based on these results, we propose a new coordinate system using the Ba, Po, and Or (Figure 4). For example, using a 1-mm thickness image, a plane is

6 REPRODUCIBILITY OF LANDMARKS ON 3D-CT 401 Figure 4. An example of the proposed coordinate axes for 3-dimensional computed tomography (3D-CT). (a) A plane is initially determined using the left orbitale (Or-L), and the bilateral true porions (Pos). The XY plane (the axial plane) is defined as the plane parallel to this initially determined plane including the basion (Ba) as the origin. (b) Then the ZX plane (the coronal plane) is determined as the plane including the Ba and parallel to the plane that includes the bilateral Pos and perpendicular to the XY plane. (c) Finally, the YZ plane (the sagittal plane) is defined as the plane perpendicular to both the XY and ZX planes and including the Ba as the origin. initially determined with reference to the bilateral Po and the left Or, and then the XY plane is defined as the plane including the Ba as the origin of the coordinate axes and parallel to the initially determined plane. Next, the ZX plane is determined as the plane including the Ba and parallel to the plane that includes the bilateral Po measures and is perpendicular to the XY plane. Finally, the YZ plane is determined as the plane that is perpendicular to both the XY and ZX planes and includes the Ba. As shown here, if we establish a stable coordinate system with use of these three landmarks (Ba, Po, and Or), which are situated in a relatively low part of the maxillofacial skeleton, there is no need to scan the superior region where the N and S are located. This could significantly reduce the radiation exposure to patients, especially for the dose to the lens. CONCLUSION The methods of the present study could be used to evaluate the plotting reproducibility of the anatomic landmarks for 3D-CT and to provide a suitable coordinate system for different treatments or studies in accordance with their objectives and subjects. REFERENCES 1. Kawamata A, Ariji Y, Langlais RP. Three-dimensional computed tomography imaging in dentistry. Dent Clin North Am. 2000;44: Kawamata A, Ariji Y, Langlais RP. Three-dimensional imaging for orthognathic surgery and orthodontic treatment. Oral Maxillofac Surg Clin North Am. 2001;13: Park SH, Yu HS, Kim KD, Lee KJ, Baik HS. A proposal for a new analysis of craniofacial morphology by 3-dimensional computed tomography. Am J Orthod Dentofacial Orthop. 2006;129: Kawamata A, Fujishita M, Nagahara K, Kanematu N, Niwa K, Langlais RP. Three-dimensional computed tomography evaluation of postsurgical condylar displacement after mandibular osteotomy. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1998;85: Ariji Y, Kawamata A, Yoshida K, Sakuma S, Nawa H, Fujishita M, Ariji E. Three-dimensional morphology of the masseter muscle in patients with mandibular prognathism. Dentomaxillofac Radiol. 2000;29: Maki K, Inou N, Takanishi A, Miller AJ. Computer-assisted simulations in orthodontic diagnosis and the application of a new cone beam X-ray computed tomography. Orthod Craniofac Res. 2003;6(suppl 1): Ono I, Ohura T, Narumi E, et al. Three-dimensional analysis of craniofacial bones using three-dimensional computer tomography. J Craniomaxillofac Surg. 1992;20: Yeshwant K, Seldin EB, Gateno J, Everett P, White CL, Kikinis R, Kaban LB, Troulis MJ. Analysis of skeletal movements in mandibular distraction osteogenesis. J Oral Maxillofac Surg. 2005;63: Kwon TG, Park HS, Ryoo HM, Lee SH. A comparison of craniofacial morphology in patients with and without facial asymmetry a three-dimensional analysis with computed tomography. Int J Oral Maxillofac Surg. 2006;35: Togashi K, Kitaura H, Yonetsu K, Yoshida N, Nakamura T. Three-dimensional cephalometry using helical computer tomography: measurement error caused by head inclination. Angle Orthod. 2002;72: Hayashi I. Morphological relationship between the cranial base and dentofacial complex obtained by reconstructive computer tomographic images. Eur J Orthod. 2003;25: Xia J, Ip HH, Samman N, Wang D, Kot CS, Yeung RW, Tideman H. Computer-assisted three-dimensional surgical planning and simulation: 3D virtual osteotomy. Int J Oral Maxillofac Surg. 2000;29: Farman AG, Scarfe WC. Development of imaging selection criteria and procedures should precede cephalometric assessment with cone-beam computed tomography. Am J Orthod Dentofacial Orthop. 2006;130: Katsumata A, Fujishita M, Maeda M, Ariji Y, Ariji E, Langlais

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