The Effect of Changes in the Angular Position of Implants on Metal Artifact Reduction in Cone-Beam Computed Tomography Images: A Scoping Review

Objective Dental implant artifacts can compromise the quality of cone-beam computed tomography (CBCT) scans and challenge radiographic detection in surrounding regions. This literature review was conducted to examine the impact of implant angle modification on reducing metal artifacts in CBCT scans. Materials and Methods A scoping review of literature was carried out in PubMed, Embase, Scopus, and Cochrane databases. Results Different spatial planes, including alpha, beta, gamma, and phi, along with 0°, 5.2°, 9.8°, 14.5°, 15°, 30°, 45°, 60°, 75°, and 90° angles were studied. Changes in the angular position of implants may reduce metal artifacts and improve the quality of CBCT scans. Conclusions Rotating implants within the alpha plane and angling them at 90° in the alpha plane enables reducing dental implant artifacts.

With the developing use of dental implants as the most common treatment form replacing lost teeth, CBCT has become the standard radiography for thorough pre-and postoperative evaluation of implants and surrounding anatomic structures. However, presence of metal artifacts degrades image quality and hinders assessment of adjacent structures [2]. Several studies have been conducted to surmount creation of artifacts or at least reduce their impact on image quality, focusing on the impact of diferent factors, including application of metal artifact reduction algorithm (MAR), using diferent implant materials, voltage, voxel size, tube current, the feld of view (FOV), or spatial location of implants, etc. . Tese factors have been observed to have minimal impacts on complete elimination of metal artifacts.
Given the scarce conclusive data regarding the plausible impact of changes in the angular position of implant while scanning on reducing metal artifacts in CBCT scans, this review study was carried out and an efort was made to answer the question whether changing the angular position of implant leads to metal artifact reduction or not.
As mentioned, metal artifacts decrease CBCT image quality and prevent evaluation of adjacent structures [2,33,34], and various factors may afect it . One of those factors is changing the angle of implants [2,33,34].
Due to the importance of the issue, we decided to review the articles written on this topic.

Research Question.
Te scoping review question was, "How does the modifcation of the angular position of implants can reduce metal artifact in CBCT scans?" 2.2. Search Strategy. Tis scoping review is an evaluation of current literature, aiming to assess the evidence and detect existing gaps. A literature search was independently conducted by two individuals on PubMed, Embase, Scopus, and Cochrane databases from October 2022 until June 2023. Te search strategy was ("dental implant" AND "cone beam computed tomography" AND "artifact") with no additional flters. It should be noted that employed keywords were selected based on the MeSH terminology. Te competency criteria were collected based on our research question.

Inclusion and Exclusion
Criteria. Inclusion criteria entailed (1) availability of article full text and (2) investigation of the impact of modifed angular position of implants on artifact reduction in CBCT scans. We excluded the studies based on the following exclusion criteria: (1) if they were failed to investigate the efect of angle change on reducing the artifacts caused by implants and (2) publication solely in non-English language.

Data Selection and Charting
Process. Te papers were gathered by performing an advanced search on the databases. To fulfl this goal, the keywords were separated with the Boolean operator "AND." Te search was completed using a manual search method. Te gathered articles were screened three times. In the frst screening, the databases were checked for copies of each paper. In the second screening, the papers were investigated based on their title and abstract. In this case, the papers that were not related to the topic were removed. In the third screening, the full texts of papers were reviewed and the screening process was carried out based on the exclusion criteria. After that, the chosen papers were synthesized according to their qualities. Ten, the data of each chosen paper were extracted, including author's name, publication year, implant placement environment, number of implants, width of implants, length of implants, center-to-center distance between implants, anatomical region of the implants, material and brand of the implants, angles, exposure time, voltage, beam currents, study analysis, results, and conclusions. Tematic analysis was employed to analyze the collected data. Tis analysis is usually used to determine the themes of diferent topics, analysis, exegesis, description, and conclusion in a systematic review [35]. Article search process was shown in Figure 1. Te characteristics and research results of the articles included in the study were given in Table 1. Tere were three articles to be studied in detail.

Results and Discussion
Te purpose of this study was to review the impacts of changes in implant angular position on reducing metal artifacts in CBCT images. Reviewed articles were selected from the following databases: PubMed, Embase, Scopus, and Cochrane. Te PRISMA-ScR fowchart showing the scoping review process is shown in Figure 1. Table 1 shows a summary of included articles, including author's name, year, implant placement environment, number of implants, width of implants, length of implants, center-to-center distance between implants, anatomical region of the implants, material and brand of the implants, angles, exposure time, voltage, beam currents, study analysis, results, and conclusions. Out of 143 articles, only three studies were included. Hence, more research is warranted to make clinical conclusions.
Evaluation of the efect of implant material on the number of artifacts has revealed that artifacts resulting from zirconium and titanium implant to hold the highest and least number of artifacts, respectively. Tere was a moderate chance of the creation of artifacts associated with titanium-zirconium implants [19,29,30,32]. Kuusisto et al. reported that titanium implant artifacts were more common than glass fber-reinforced composite implant artifacts [16].
Considering the voltage factor, all studies stated that an increase in kVp limited metal artifacts [11,19,29,32].
Various studies have been conducted on implant position. Machado et al. postulated that artifacts appear more frequently in the mandible than maxilla, and artifacts appear more in the anterior areas than that of the posterior areas [12]. According to [31], artifacts most frequently occur around the incisors [31]. Moreover, Kocasarac et al. concluded that implants led to greater artifact expression in the exomass than inside the FOV [32].
Regarding the voxel size, previous studies have shown that smaller voxel size caused increased artifact production, while larger voxel size decreased the number of artifacts [11,31]. Te tube current (mA) factor had no impact on the number of artifacts according to Shokri [31], smaller FOV increased artifacts in CBCT images [18,31]. Several pertaining factors that can afect artifacts have also been examined. For example, according to Cortes et al., edited 3D-CBCT follow-up images of multiple implants minimized the number of artifacts [36].
Based on [30] the quantitative study, it was found that BAR (blooming artifact reduction) flter was the most effective among the four investigated flters (BAR and multi-CDT NR (noise reduction) flter in e-Vol DX Software and Filter 1x and Filter 2x in OnDemand software) in reducing artifacts and thus improving the quality of images [30]. Emami et al. revealed that the artifact removal (low-medium) option can reduce the number of artifacts caused by delicate structures like lamina dura, but it does not afect the number of artifacts caused by large anatomical structures and linear bone measurements [37]. According to [38], system automated motion artifact correction is useful in reducing artifacts [38]. Based on [19], resolution changes had no efect on the number of artifacts [19]. According to [20], adaptive image noise optimizer (AINO) optimization flter was efective in reducing artifacts [20]. In Cardarelli et al.'s study in 2021, they investigated a new protocol called "low-dose CBCT imaging protocol" which was useful in reducing artifacts [39]. Based on [27], increasing the number of basis images was efective in reducing artifacts [27].
In previous studies, the efect of "changes in the angular position of implants" on the reduction of metal artifacts was examined [2,33,34], so we decided to summarize this issue by conducting a review study. Te hypothesis that diferent implant angles in patients with multiple implants can reduce metal artifacts can be justifed by the beam hardening efect. Since by preventing the overlapping of these high radiation absorbing materials, beam hardening is reduced, improvement in image quality will occur [2,33].
In Min and Kim's study in 2021, two dental implants were placed in a homogeneous dental impression material block. Te block was scanned with a CBCT scanner at seven diferent angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°) along three diferent planes. Rotation in the frontal plane was called alpha rotation. Te second rotation in the sagittal plane was called beta rotation. Te simultaneous rotation in the frontal and sagittal planes was called gamma rotation. Tirteen volumes of interest (VOI) were selected from each axial reconstruction. Gray values (GVs) of each VOIs were measured. Mean diferences in GV between the control area and VOIs were calculated. Tese ΔGVs from diferent spatial angle were compared and analyzed by Welch's analysis of variance and linear regression. According to the research, increases in alpha angle increase ΔGVs of groups A and B but fail to signifcantly change group C. Furthermore, any notable changes in ΔGVs were observed for gamma rotation in all VOIs. Te study concluded that an increase in alpha angle can reduce interimplant metal artifacts in CBCT images. Terefore, modifcation of the angle of the patient's head may lead to the reduction of metal artifacts [2].
Te authors in [34] performed a study on alpha rotation, the rotation in the frontal plane. In this research, polyurethane synthetic bone blocks were frst scanned without implants by CBCT and micro-CT (a gold standard for CBCT). Ten, two dental implants were placed in the bone blocks and these blocks were scanned by CBCT at diferent alpha angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°). Six microstructural parameters including bone volume per total volume, bone surface per total volume, trabecular spacing, trabecular thickness, fractal dimension, and connectivity were measured. Spearman correlation coefcients for each parameter from CBCT and micro-CT were calculated and compared using Steiger's Z test. Results manifested that for VOI1 and VOI2, as the alpha angle increases, the correlation coefcient increases; however, for VOI3, the correlation coefcient decreases with increasing alpha angle. Terefore, modifying the alpha angle can modify CBCT image quality [34].   In Luckow et al.'s study in 2011, two dental implants were placed in the mandible of a pig, and CBCT was performed based on varying the starting rotation angle of the mandible in the source-detector plane, beam current, accelerating voltage, and angles of the jaw with respect to the source-detector plane. In this research, rotations were performed in three diferent planes. Tese rotations were called alpha and beta rotations, respectively, which were performed in the frontal and sagittal planes. Simultaneous rotation in the frontal and sagittal planes was called phi rotation. In alpha and beta rotations, 0°, 5.2°, 9.8°, and 14.5°angles, and phi rotation, 0°, 30°, 45°, and 90°angles were examined. Te diferent datasets were automatically registered with respect to micro-CT data to extract the common volume and the deviance to the predefned standard that characterizes the image quality. Te research showed that image quality can be improved by increasing the α angle. Since the overlap of highly X-ray absorbing implants may reduce by tilting the jaw, tilting the lower jaw with dental implants can reduce artifacts in CBCT images [33].

Radiology Research and Practice
In the three mentioned articles, the materials of the implants were the same and they were made of titanium. Also, quantifcation of artifacts was carried out in three articles and the method of measuring artifacts was quantitative [2,33,34]. As a result, comparing these articles together, the material of implants and method of measuring the artifacts were not considered interfering factors. Te anatomical region of the implants in Luckow et al.'s study was in the mesial and distal of the canine [33], but in the other two articles, a bone block was used [2,34], and due to the placement of the implants in the bone block, no equivalent can be considered for the exact location of the implant in the jaw. So, since this factor was not same in the articles, this factor can be considered an interfering factor in our study, and maybe the comparison of the results of the studies has a little problem due to this diferent index. We took this point into consideration, but unfortunately, there is no solution to solve it.

Conclusion
Based on the conclusion of three articles included in this study, among all the introduced plans (alpha, beta, gamma, and phi), alpha plane rotation was the most efective in artifact reduction and an increased alpha angle reduced the number of metal artifacts [2,33,34]. In summary, it seems that modifcation of implants' angle can reduce metal artifacts in CBCT images, improving the image quality consequent to head and jaw tilting which can reduce the overlap of implants and hard tissues with high x-ray absorption [2].
Given the small number of papers, as well as the laboratory nature of the studies, it is not possible to highly extend the conclusions to clinical works and more studies in this feld are required; besides, more evidence must be collected and clinical research should be carried out.

Data Availability
Te study's fndings were supported by previously reported data in the studies and cited datasets.