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Objective: the aim of this study is to compare the reliability and reproducibility of landmarks representing the labial alveolar bone crest of maxillary and mandibular teeth using two different CBCT machines with different image resolutions and voxel size and accordingly different radiation dose. Material & Methods: 340 landmarks representing the alveolar bone crest opposite each tooth from the central incisor up to the second molar on sixteen CBCT data sets were identified. The landmarks were located by the principle operator, and relocated by the same principle operator to evaluate the intra-observer reproducibility, then located by the second observer to evaluate the interobserver reliability error. For testing the intra-observer reproducibility and inter-observer reliability of the landmark identification within each scanner data, the Mean Absolute error (MAE) and the Dahlberg error (DE) and Intra-class correlation coefficient tests with 95% confidence intervals were used. Independent samples t-test was used to compare the intra-observer and inter-observer error between the two scanners. Results: The Mean Absolute Error (MAE), the Dahlberg Error (DE) and the Intra-Class Coefficient (ICC) tests showed excellent intra-observer reproducibility values in the x, y and z-axis for the Galileos scanner, with the least reproducibility in the z-axis. The same concordance was found for the Planmeca CBCT scanner, with excellent reliability in the x, y and z-axis, and the least reproducibility in the z-axis. For the inter-observer reliability testing, the same pattern was found for each CBCT scanner. Very good inter-observer reliability was found in the x, and y dimensions, while the least was found in the vertical z-dimension. Conclusion: The Galileos and the Planmeca CBCT scanners produced comparable reliability and reproducibility for identification of alveolar bone crest landmarks. The vertical axes showed the least accurate coordinates for landmarks identification. (1216) Nora Saif and Amr Ragab El-Beialy E.D.J. Vol. 64, No. 2 INTRODUCTION The perpetual alliance between the teeth and the embracing alveolar bone renders the evaluation of the alveolar bone an everlasting research point each time a new diagnostic tool is invented. Currently, although the pendulum of dentofacial diagnosis is swinging towards 3D CBCT, [1] the argument between the proponents of the ALARA (As Low As Reasonably Achievable) philosophy and the eagerness of the maxillofacial community for this new technology is not settled so far. Concerning the use of CBCT to analyze the alveolar bone, some evidence should be considered. Firstly, there is a consensus of evidence that a mutual relationship exists between the teeth and the supporting alveolar bone. The presence of buccal alveolar bone defects decreases the bony support for the teeth. [2] The lack of bony support during orthodontic movement can be detrimental to the health of the periodontium and consequently the teeth,[2-5] a potential factor for orthodontic relapse, [6-8] and gingival recession. [4, 9-11] On the other hand orthodontic tooth movement can create alveolar bone defects. [12-14] The correlation between the labial tipping of teeth and the associated changes in the labial alveolar plate is factual in some studies. [11, 15, 16] Posteriorly, the 5mm of buccal bone loss following rapid palatal expansion is striking. [17] The capability to accurately evaluate the alveolar bone defects 3-dimensionally via CBCT is a leap in orthodontic diagnosis [18-22] with a direct impact on the treatment decision. The main advantage of CBCT is its lower acquisition time and patient dose when compared to medical CT scanning [23-26]. However, CBCT devices and different imaging protocols should not be addressed as a single entity. The image quality of CBCT scans and accordingly the ability of CBCT to display anatomic features or pathology depends on acquisition parameters such as milliamperage, kilovoltage and also spatial resolution defined by the voxel size [27]. The ability to view periodontal structures such as the periodontal ligament space, cortical bone, alveolar crest and alveolar cortical plate requires images with a higher definition and smaller voxel size, resulting in increased milliamperage and kilovoltage values. [28-30] Images acquired in smaller voxel sizes will increase the radiation dose to the patient but might provide the same diagnostic outcome as lower resolution image [31] . Thus, it is important to consider that the comparison of CBCT examinations with various voxel settings is relevant to understand the impact of the inherent image quality on the reliability and reproducibility of a specific diagnostic outcome [21]. Accordingly, the aim of this study is to compare the reproducibility and reliability of landmarks representing the labial alveolar bone crest of maxillary and mandibular teeth using two different CBCT machines with a different image resolution and accordingly a different radiation dose. MATERIAL AND METHODS In the current study, Cone Beam CT volumes were utilized to evaluate the accuracy of identification of the alveolar bone crest landmarks on CBCT data from two different CBCT scanners. The inclusion criteria for enrollment of the patient CBCT in this study is the presence of erupted teeth, and good quality CBCT data with absence of motion of the patient during the image capturing procedure. The scanners used were the Galileos* and the Planmeca Promax3D Mid** CBCT scanners. The parameters used during imaging using the Galileos scanner (CBCT scanner A) and the Planmeca scanner (CBCT scanner B) are shown in Table 1. * Sirona Dental Systems GmbH-Operating Instructions GALILEOS ** ProMax 3D by Planmeca Oy, Helsinki, Finland-Operating Instructions ProMax 3D® THE EFFECT OF VOXEL SIZE ON THE RELIABILITY AND REPRODUCIBILITY (1217) Three hundred and forty landmarks representing the alveolar bone crest opposite each tooth from the central incisor up to the second molar on sixteen CBCT data were identified on each CBCT volume. The orthogonal cuts were used to accurately identify the location of the alveolar bone crest landmarks at the depth of the alveolar bone in the midline section of each tooth (Fig.1) The x, y, and z-coordinates for each landmark were recorded by the primary observer. For intra-observer reliability testing, the same landmarks were re-identified by the same observer 2 weeks after the first observation. The second observer identified the same landmarks and recorded the x,y, and z-coordinates of each landmark. Statistical analysis was performed using SPPS/ SPSS® v. 17 (IBM corporation, Armonk, NY). For testing the intra-observer reliability of the landmark identification within each scanner data, the Mean Absolute error (MAE) and the Dahlberg error (DE) and Intra-class Correlation coefficient tests with 95% confidence intervals were used. Independent samples t-test was used to compare the intraobserver error between the two scanners, as well as the inter-observer error between the two scanners. Fig. (1) a & b: depicting the landmark localization on the CBCT images TABLE (I) Technical parameters and settings of the Galileos and ProMax 3D CBCT scanners* Technical Parameter Galileos Scanner ProMax 3D Scanner X-ray source voltage 85 kVp 90 kVp X-ray source current 7mA 8 mA Scanning time Approx. 14 s 13.542 s Radiation Source Pulsed Pulsed Detector type Image Intensifier-CCD Flat panel Voxel size (mm) 0.3 x 0.3 x 0.3 mm3 0.4 x 0.4 x 0.4 mm3 Scanned volume dimensions (Field of view) 15 cm x 15cm x 15cm 20.0 x 20.0x20.0 cm Patient positioning Standing or seated with flat occlusal plane Standing or seated with flat occlusal plane Data output DICOM DICOM Effective dose value (Deff) ICRP 2007=54 mSv* 23 mSv‡ Focal spot size 0.5 mm 0.5 mm, fixed anode Primary reconstruction time Approx. 2.5 minutes 55 s * “Comparative dosimetry of GALILEOS Dental CBCT imaging,” Prof. Dr. John B. Ludlow, Department of Diagnostic Sciences and General Dentistry, University of North Carolina School of Dentistry, Chapel Hill, North Carolina, USA ‡ Rottke, D et al. “Effective Dose Span of Ten Different Cone Beam CT Devices.” Dentomaxillofacial Radiology 42.7 (2013): 20120417. PMC. Web. 5 Feb. 2018 (1218) Nora Saif and Amr Ragab El-Beialy E.D.J. Vol. 64, No. 2

The Effect of Voxel Size on the Reliability and Reproducibility of Alveolar Bone Crest Identification on Cone Beam CT Scans