Successful dental surgery needs complete information about regional anatomy. Some of the surgical procedures including insertion of dental implants, mandibular third molar surgery, osteotomy, and other surgeries close to inferior alveolar canal (IAC) require assessment of IAC to prevent injuries to neurovascular bundles. ¹ Mandibular canal is one of the most important anatomic structure due to its content and risk of damage during surgery. This is more critical in atrophic ridges in premolar and molar area. In conventional radiographic images, IAC could be seen as a radiolucent line with two thin cortical borders. In some osteoporotic mandibles detection of IAC may be difficult due to its lack of well-defined cortical boundaries. Panoramic is a widely used technique for tracing IAC and estimating its position in vertical dimension. ² , ³ In the late 1990s, cone-beam computed tomography (CBCT) was introduced to dentistry which provided more sharpness. ² Linear measurements in CBCT are accurate enough for implant surgery. ³ , ⁴ CBCT images are useful for evaluating IAC and its position three-dimensionally, considering tooth roots, and alveolar crest. Furthermore, abnormal mandibular canals like branched canals can be identified by CBCT. This technology facilitates the evaluation of IAC due to its cross-sectional images which overcome the weak points of conventional images. ⁵ Various parameters influence the characteristics of CBCT image, such as field of view, slice interval, and tube current. One of the parameters which may affect the visibility of anatomic structures like IAC is the slice thickness. There is a common belief in multi-detector computed tomography (MDCT) that thin slices make better images. ⁶ It is known that thinner slices provide less partial volume averaging and better image quality with more detailed visibility. ⁷ The aim of this study is to evaluate the effect of slice thickness on visibility of IAC in CBCT images.

We used CBCT archive in the Oral and Maxillofacial Department of Radiology, Tehran University of Medical Sciences, Tehran, Iran. CBCT images of 30 patients (15 male and 15 female) with age range between 40 and 60 years old were used. Imaging was performed by Alphard VEGA (Asahi Roentgen, Japan). Peak kilovoltage and mA was 80 and 4, respectively for all samples. The images were processed using Neo 3D software Version 2.01 (Asahi Roentgen, Japan) and cross-sectional images were prepared by 0.5, 1, and 2 mm slice thicknesses and 2 mm interval. Default thickness of cross-sectional images in the software was 1 mm, and we selected half and double thicknesses of default. All of the 90 images were coded and saved in joint photographic experts format Figure 1, Figure 2, Figure 3. Two oral radiologists with at least 5 years of experience observed images randomly and blindly using 19˝ LCD monitor (Samsung, SyncMaster P23700). They rated the images based on the visibility of IAC in a 4-score classification (highly visible, visible, nearly visible, nearly invisible). For the second time, all images were rated 10 days later with different codes. Figure 1

Cross sectional image, 0.5 mm thickness.

Visibility of IAC in different slice thicknesses of both raters had no significant difference (P = 0.20) Table 4.{Table 4}

Our result demonstrates that 53.3% of images in females were highly visible, compared with 80% of images in males. Samples were not adequate to compare males and females.

Image quality is a subjective interpretation of visual data and does not have an exact definition. An image should have enough information to obtain a clinical decision. The aim of our study was to evaluate the effect of slice thickness of CBCT images on the visibility of IAC. IAC is one of the most important anatomical structure of mandible due to its content, including inferior alveolar nerve, artery, and vein. It is critical to consider the location and variations of IAC, especially in atrophic ridges before implant installation. ⁸

CBCT machine provides perfect images with low dose and cost, compared to MDCT. ⁹ There is a wide variety of specification of CBCT machines which are used with different acceptable images and exposure doses. ¹⁰ One of the parameters which can be altered in CBCT to affect the image quality is slice thickness. ¹¹ There have been few researches about the efficacy of slice thickness of CBCT and its effect on visibility of anatomical structure.

In our study, there was no significant difference between visibility of IAC in different slice thicknesses and any attempt to change slice thickness did not cause better visualization of IAC in cross-sectional images.

Any change in slice thickness either increasing or decreasing may affect subject's contrast and visibility. In the present research, the subject contrast of IAC was not affected by the thickness of cross-sectional images. It may be due to the favorable contrast in well-corticated IACs so that good visibility of IAC was not influenced by slice thickness. On the other hand, ill-defined IAC has low contrast and may be mistaken with surrounding bone marrow space, to an extent that altering the thickness is of no benefit to its visibility.

There are some other researches with different methods which concluded positive effect of slice thickness. Sezgin et al. evaluated the effect of slice thickness on volumetric calculation of CBCT, showing that estimated volumes of intrabony lesions did not change with different slice thicknesses. ¹² Jia et al. studied the effect of slice thickness of CBCT on the precision of image-guided radiotherapy. They showed that slice thickness has no significant effect on positioning precision. ¹³

Similar research was done in MDCT with different methods and results. Mühler et al. studied the accuracy and reliability of age estimation, using clavicular epiphyseal plate in forensic medicine. In their study, the best result was obtained with slice thickness of 1 mm. ¹⁴ Dogan evaluated the effect of heart rate and slice thickness on volumetric analysis of CT images of cardiac phantom and concluded that slice thickness of 2 and 8 mm have the same diagnostic value. ¹⁵ Guggenbuhl et al. studied microstructure of tibia in CT and concluded that increasing the slice thickness will result in lower accuracy of tissue analysis parameters. ¹⁶ In another research, Rubinstein et al. noticed that the diagnostic accuracy of fractures in odontoid process will increase with 1 mm section thickness. ¹⁷ Jung et al. studied the multiplanar reformation using 1.25, 2.5, 3.75, and 5 mm slice thicknesses for quantitative measurement in CT and demonstrated that thinner slice thicknesses result in better image quality. ¹⁸ Sirin studied diagnostic accuracy of reconstructed CBCT images, using different slice thicknesses, in detection of simulated mandibular condylar fracture. They concluded that the diagnosis will be more accurate using slice thickness of 0.2 and 1 mm, compared to 2 and 3 mm. ¹⁹

Our study showed that slice thickness has no effect on visibility of IAC in cross-sectional images. Tracing ill-defined IACs can be achieved by other multiplanar images, such as panoramic or axial view; also nerve tracing tools in software's used for CBCT might be useful. Further studies with different methodology may be useful to achieve more results.

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The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non.financial in this article.