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Cone beam computed tomography (CBCT) has been used for preoperative treatment planning for dental implants, one of the major advantages is its accurate linear measurements. But when it comes to automatic detection of voxel intensity values as in automatic nerve detection, little evidence in the literature was found. Thus; in order to examine the accuracy of this automatic process, this study was performed in vitro. Methods: A dry edentulous mandible was imaged using i-CAT next generation (Imaging sciences international, Hatfield, PA, USA) using gutta percha markers at certain areas of interest, the reconstructed panoramic image was evaluated and nerve tracing was done for both sides, then linear measurements was performed from the edge of the inferior alveolar nerve canal (IANC) to the inferior border of mandible, buccal, lingual and crest of the ridge. These measurements were compared to actual physical measurements performed by using a digital caliber after sectioning of the mandible at the gutta percha sites. Results: Intra-observer agreement was good to very good regarding all measurements for both observers, while inter observer agreement was weak to very good regarding all measurements for both observers. There was no statistically significant difference between the CBCT scans (0.2 mm, 0.3 mm, and 0.4 mm voxel size) regarding all measurements except one measurement at the 0.4 mm voxel size CBCT scan showed a statistically significant high mean error. Conclusion: The choice of FOV and voxel size should be made by clinicians based on the clinical task at hand, keeping in mind that their choices not only affect the diagnostic quality of images but also the amount of radiation exposure that their patients receive. (2404) Mostafa S. Ashmawy, et al. E.D.J. Vol. 63, No. 3 While the number of dental implants have increased, the numbers of neurosensory disturbances and haemorrhages have increased even in locations which was considered safe before as the anterior mandible.[5,6] Therefore, preoperative planning must be done to identify the inferior alveolar nerve canal (IANC) to prevent damage to the neurovascular bundle that passes inside the mandibular canal, the course of which varies within the mandibular body as well as the shape and dimensions of the bone.[7] Several authors investigated the mandibular canal position using cone beam computed tomography (CBCT), IANC proved to run near the lingual cortical side from its entrance in the ramus to the mental foramen, and that it is positioned about 1 cm above the inferior border of the mandible. [8-10] On the other hand other researches found that IANC position may vary from one patient to the other according to the degree of alveolar ridge resorption.[10,11] Although CBCT has gained increased popularity in many indications, its main indication is dental implant procedures [12], specially providing a superior display of the IANC and its variations which is critical to perform a more expected surgical procedures, avoiding possible sensory disturbances and complications.[13] Viewing CBCT images using the software viewer is considered essential during implant planning either by the oral radiologist or the oral surgeon. All CBCT systems have their viewing software supplied on a compact disk (CD); each software is different in its task specific reconstruction capabilities provided by the manufacturer. Although these features may include panoramic reconstructions, implant planning reconstructions with 2-dimensional (2D) and 3dimensional (3D) windows, temporomandibular joint reconstructions, airway reconstructions. Almost any CBCT software has a nerve tracing application. This allows for the identification and colour coding of IANC, thereby assisting its recognition. [4,15] Up to our knowledge, few researches were focusing on the accuracy of the nerve tracing tool in order to provide sufficient scientific information for the dental community, so the aim of the current study is to determine the accuracy of using IANC tracing technique on CBCT images in precisely locating the IANC using different voxel sizes. MATERIALS AND METHODS For the purpose of our study, one edentulous mandible, randomly selected, was borrowed from the Anatomy Department, Faculty of Medicine, Ain Shams University. The study design was expedited from review by the Faculty’s Research Ethics Committee. The mandible was anonymous and not identified by age, gender, or ethnic group. Mandible preparation: The alveolar crest of the mandible in both sides at the molar, premolar areas was plateaued using a diamond bur mounted on straight hand piece. The buccal and lingual cortical plates came to be nearly on the same horizontal level in order to facilitate reference points determination. Creating reference points: In order to create reference markers in the mandible, small pieces of gutta percha were fixed on the mandible with glue in a five predefined locations to yield 5 dentoalveolar specimens (Fig. 1). The position of the reference markers were at the level of the crestal bone between the sockets of the premolar and molar teeth. Other reference markers were positioned along a perpendicular line drawn to the inferior border of the mandible from the previous reference markers at each location. This was done to ensure reproducible measurements. Individual sections were assigned a numeric value of 1 through 5, starting anteriorly in a posterior direction. R and L letters were used to discriminate the two sides. EFFECT OF VOXEL SIZE ON THE ACCURACY OF NERVE TRACING (2405) Radiographic Scanning: The mandible was wrapped with thin plastic wrapping (in order not to disturb the dryness of the mandible) and placed in a thin, clear plastic container which was filled with water to simulate soft tissue attenuation. Then, the mandible was scanned using the i-CAT next generation (Imaging Sciences International, Hatfield, PA, USA) at the Oral Radiology Department, Faculty of dentistry, Ain shams university. (Fig. 2) The assembly was adjusted on the machine’s platform in a central position which was checked with the three laser beam. Images were obtained at 3 different voxel sizes 0.2 mm, 0.3 mm and 0.4 mm. All the three scans were performed at 120 kVp, 5 mA. The field of view (FOV) was adjusted in order to cover the sigmoid notch superiorly and the mandibular inferior border inferiorly in order to easily locate the mandibular foramen and inferior mandibular border respectively on the resultant image. Nerve tracing and radiographic measurements After volume acquisition, images were examined by two oral and maxillofacial (OMF) radiologists with ten and fourteen years of experience. Images were examined using i-CAT Vision® software (Imaging Sciences International, Hatfield, PA, USA). The examiners separately reviewed the volumes in ideal dimly lit viewing conditions using the same computer monitor (15.6 inch HD LED) at full size, 1:1. They were allowed to handle the volumes freely with the software and use all available tools to identify the IANC. They were also allowed to change contrast and brightness. IANC tracing and measurements: On the implant screen, panoramic reconstruction was centrally adjusted along an intermediate level between the crest of the ridge and mandibular inferior border. Manual IANC tracing was performed using the “estimate nerve canal” tool provided by the software; right and left IANCs were traced separately based on four points starting from the mandibular foramen down to the mental foramen as displayed on cross-sectional images. The border of the color coded tracing was used as the starting measurement point as shown in (Fig. 3) Then cross sectional images were reoriented in order to pass through gutta percha markers in order to precisely standardize measurements’ locations. On each section passing through gutta percha markings, four linear measurements were performed using the “distance” tool. The distance from IANC superior, inferior, buccal and lingual edges to the alveolar crest, inferior, buccal and lingual mandibular borders respectively were measured at each section using a perpendicular line as shown in (Fig. 3). Each observer performed the measurements twice separated by two weeks interval using the same protocol. Fig. (1) Gutta percha markers fixed on the dry mandible Fig. (2) Positioning of the mandible on the platform during CBCT acquisition (2406) Mostafa S. Ashmawy, et al. E.D.J. Vol. 63, No. 3 Physical Measurements After scanning, the dry mandible was sectioned using a Stryker saw (Stryker Model 810 Autopsy Saw, Azusa, CA) at the 5 predefined locations along the lines joining the reference markers from the alveolar crest to the inferior boarder of the mandible on each side as shown in (Fig. 4). For each section, the distance from the IANC border to the alveolar crest, inferior, buccal and lingual mandibular borders was measured by the caliber using the gutta percha markers as a guide for measurement. These measurements were recorded for further comparison with CBCT measurements. Statistical Analysis Numerical data were explored for normality by checking the distribution of data and using tests of normality (Kolmogorov-Smirnov and ShapiroWilk tests). Measurement error data showed nonparametric distribution. Data were presented as mean, median, standard deviation (SD), minimum, maximum and 95% Confidence Interval (95% CI) values. Intraand inter-observer agreement was assessed using Cronbach’s alpha reliability coefficient. Friedman’s test was used to compare between measurement error with the three CBCT scans (0.2, 0.3, and 0.4 mm voxel size). Wilcoxon signed-rank test with Bonferroni’s adjustment was used for pair-wise comparisons when Friedman’s test is significant. The significance level was set at P ≤ 0.05. Statistical analysis was performed with IBM SPSS Statistics Version 20 for Windows (SPSS, Inc., an IBM Company, NY, USA). Fig. (3) Implant screen on i-CAT VisionR

Effect of Voxel Size On The Accuracy of Nerve Tracing Module of Cone Beam Computed Tomography Images