1. Imaging system Hybrid CBCT apparatus (Veraviewepocs 3Df, J Morita Mfg. Co., Kyoto, Japan) ・ tube voltage ： 90 kV ・ tube current ： 1 mA ・ scan time ： 9.4 s (rotation angle, 180 ゜ ) ・ FOV ： φ 40 mm × H 80 mm Evaluation of cone angle and artifacts in hybrid cone beam computed tomography: a study of the subtraction method Kohinata K 1, Ejima K 1, Arai Y 2, and Honda K 1 1: Department of Oral and Maxillofacial Radiology, Nihon University School of Dentistry, Tokyo, Japan 2: Nihon University School of Dentistry, Tokyo, Japan The objective of the present study was to use the digital subtraction method to evaluate the relationships between the production of artifacts and the vertical positioning and the cone angle (CA) in hybrid cone beam computed tomography and panoramic radiography (hybrid CBCT). Positioning of the phantom The phantom was positioned at an axis of rotation and height at which the incident angle of x-ray beam was horizontally projected on top of the TMJ model, which was defined as 0˚ CA (relative height, 0 mm; Fig. 2). The position of the hybrid CBCT was lowered to 20 mm (CA = 3.5 ゜ ), 40 mm (CA = 7.0 ゜ ), and 60 mm (CA = 10.5 ゜ ), and CBCT scans were done 6 times at each position. Subtraction process The image analysis was performed with original software that was developed with Visual Studio 2010 C# (Microsoft Corporation, Redmond, WA, USA). Each median sagittal image as 8-bit gray-scale that was referred to the TMJ model was used in the analysis. The regions of interest (ROI: 100 [height] × 200 [width] in pixels) for the images were set and clipped. Then, the object images (CA = 3.5 ゜, 7.0 ゜, or 10.5 ゜ ) were positioned with the reference image (CA = 0 ゜ ) by the least square method, and the subtraction images were created (Fig. 3). The standard deviation (SD) of the image density in the whole subtraction image was computed (Fig. 4). Thus, the variations in the images that were obtained at other CAs by comparison with the reference image (CA = 0 ゜ ) were analyzed. Phantom An aluminum condyle and glenoid fossa [Temporomandibular joint (TMJ) model; J. Morita Mfg. Co., Kyoto, Japan] 0˚ 3.5˚ 7.5˚ 10.5˚ 0˚ Subtraction ANOVA and multiple comparison tests revealed significant differences in each of the CAs (Fig. 5; P < 0.01). The linear regression analysis clearly showed a significant relationship between the CA of the hybrid CBCT and the SD of the density in subtraction images. Image analysis Fig. 1: Phantom used in the present study a: TMJ model embedded in acrylic resin b: Schematic showing a cross section of the TMJ model (1-mm-thick aluminum; height, 23 mm; diameter, 20 mm) a b Fig. 2: Relationship between CA and positioning of the hybrid CBCT Fig. 4: Subtraction images used to calculate SD The image for the subtraction of the 0 ゜ image from the reference image is completely gray. In contrast, the images for the subtractions between the objective images that were obtained at CAs of 3.5 ゜, 7.0 ゜, or 10.5 ゜ and the reference image showed distortion of the outline of the TMJ model. The background gray level of the subtraction images was adjusted to 128 and the SD of the subtraction images was computed and used in the statistical analysis. Statistical analysis Analysis of variance (ANOVA) and linear regressions were performed on the SD data that were obtained from six scanned images at each of the CAs. Fig. 3: Example of the creation of the subtraction images between the reference image (CA = 0 ゜ ) and an image with a 3.5 ゜ CA The least square method was used for position adjustment of objective image relative to the reference image. Fig. 5: Regression analysis of the relationship between the SD and each angle. † (P < 0.01, multiple comparison with Tukey’s test) These results showed that the methodology of analyzing the SD values of the subtraction images seemed to be appropriate for evaluating image quality, and increasing CAs were correlated with the SDs of the subtraction images. The increasing SDs of the subtraction images reflected increased artifacts and reductions in the quality of images. A future study will investigate the dimensional stability of the use of the subtraction method. : Subtraction image : the ROI (100 [height] ×200 [width] in pixels) on the image is shown for reference (CA = 0 ゜ ) : the ROI on the image with a 3.5 ゜ CA Results Objectives Materials and Methods Image acquisition Phantom Conclusion Regression analysis y = 1.573x + 8.125 SEE: 0.994 r = 0.979 R 2 = 0.958 Angle SD of the subtraction image Axis of rotation FOD 350 mm Focus 0 mm 20 mm 40 mm 60 mm CA = 3.5 ˚ CA = 0 ˚ CA = 10.5 ˚ CA = 7 ˚ † †† Acknowledgments This study was supported by the Sato Fund of the Nihon University School of Dentistry. The authors thank Drs. Shoji Kawashima and Kunihito Matsumoto of the Nihon University School of Dentistry for the statistical analysis and data analysis interpretation.