In addition CBCT machine with FPD was used

In addition, CBCT machine with FPD was used owing to the fact that these detectors are superior to IIT/CCD in terms of their increased dynamic range, AMN-107 and spatial resolution, decreased pixel noise and image artifacts as mentioned by Ref. [19] and then by Ref. [18]. In addition, FPDs have an improved performance and image quality, a view which is supported by Ref. [31]. Furthermore, as mentioned by Seeram 2001 CBCT machines with FPDs have higher quantum detection efficiency than those with an IIT, i.e. they are more efficient at recording signals from photons reaching the detector. This view is also supported by Refs. [3,6,11]. Moreover; FPDs are not associated with geometric distortion unlike IITs, as confirmed by Refs. [30,31]. In this study, the selected CBCT machine was a FPD-based system with an adjustable FOV as it is considered as one of the CBCT machines with a significant superiority due to the influence of different FOV selection during the scan as recommended by Ref. [16]. CBCT scanning was done with a small FOV of 7.5 cm × 14.5 cm × 14.5 cm. This FOV was comfortably large enough to scan the teeth bearing area. Using a limited FOV was recommended by Ref. [19] who reported that smaller FOV selection provides better resolution and contrast in comparison with large FOV, and this improves the clarity and visibility of CBCT images. In contrast, increasing the tube current only reduced the artifact area under some conditions. This finding is consistent with previous studies that also found a significant reduction in metallic artifacts with increased tube voltage, but not with increased tube current [37]. Other studies have also demonstrated no reduction in metallic artifacts with increased tube current when scanning body regions or phantoms with metallic prostheses [14]. It has been reported by Ref. [10] that there may be discrepancies in grey levels owing to inherent deficiencies in the FPD used in some dental CBCT machines. In addition to this problem, there are the effects of scatter and beam hardening. The CBCT devices employ an area detector such as a FPD or IIT which capture more scattered photons than a linear array detector. It was found by many authors that by reducing the voxel size will increase the spatial resolution [9,22] Many CBCT machines that offer small voxel sizes are being advertised as providing the best image quality for diagnostic purposes based on an assumption that a smaller voxel size will increase the image quality. An important aspect of image quality in CBCT other than spatial resolution is contrast resolution. Contrast resolution is referred to as the ability of an imaging modality to distinguish between various contrast levels in an acquired image [23]. A study done by Ref. [24] to assess artifacts induced by metallic restorations in three-dimensional (3D) dental surface models derived by CBCT concluded that metallic restorations induce considerable artifacts in 3D dental surface models. Artifact reduction should be taken into consideration for a proper diagnosis and treatment planning when using 3D surface model derived by CBCT in dentofacial deformity patients. In addition, voxel size can influence the characteristics of the final image in several ways. It may influence noise in the orthogonal sections of an image: the smaller the voxel size, the greater the noise, but of course, the higher the spatial resolution [2]. Depending on the voxel size, radiopaque structures can become invisible. This can be caused by the partial volume averaging effect, which is a common computed tomography artifact and occurs when a voxel lies on the borders of two objects of different densities. This voxel will then reflect the average density of both objects rather than the true value of either object [3]. This “invisibility” of some structures could also be caused by the limitations in contrast resolution related to CBCT units, which determines the ability to distinguish two objects of similar densities and in close proximity [7,20].