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ORIGINAL ARTICLE
Year : 2019  |  Volume : 22  |  Issue : 8  |  Page : 1091-1098

Accuracy of CBCT images in the volumetric assessment of residual root canal filling material: Effect of voxel size


1 Department of Endodontics, Faculty of Dentistry, Ankara University, Ankara, Turkey
2 Department of Dentomaxillofacial Radiology, Faculty of Dentistry, Ankara University, Ankara, Turkey
3 Department of Endodontics, Faculty of Dentistry, Başkent University, Ankara, Turkey
4 Department of Anatomy, Faculty of Medicine, Hacettepe University, Ankara, Turkey

Date of Acceptance04-Apr-2019
Date of Web Publication14-Aug-2019

Correspondence Address:
Dr. F Yilmaz
Department of Endodontics, Faculty of Dentistry, Ankara University, Ankara
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_678_18

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   Abstract 


Aims: Our aim was to compare three different voxel sizes of CBCT images for the determination of residual filling material volume in root canals when compared with micro CT. Material and Methods: Forty-two root canals of 14 extracted human maxillary molar teeth were retreated by using ProFile® instruments. Images were obtained after retreatment by using ProMax 3D Max CBCT at 3 different voxel sizes (1) High resolution (0.1 mm); (2) High definition (0.15 mm); and (3) Normal resolution (0.2 mm). Two observers measured volumes of residual filling materials in exported CBCT images by means of 3D Doctor Software. Micro CT measurements were served as gold standard. Mann-Whitney U test and Wilcoxon Test were used for the comparison of CBCT and micro CT measurements. Statistical significance was set at P < 0.05. Results: No statistically differences were found between the two observers for all measurements (P > 0.05). There were no significant differences among different CBCT voxel sizes used (0.1 mm, 0.15 mm, and 0.2 mm) (P > 0.05). The Spearman correlation coefficients between CBCT at different voxel sizes significantly highly correlated with micro CT measurements for each observer (P < 0.05). Furthermore, no significant differences were found between the measurements obtained by the two observers in consideration to root canal location (P > 0.05). Conclusion: CBCT images may provide useful information in the volumetric assessment of the amount of residual filling material in root canals for retreatment procedures.

Keywords: CBCT, Micro CT, retreatment, voxel size


How to cite this article:
Yilmaz F, Sonmez G, Kamburoglu K, Koc C, Ocak M, Celik H H. Accuracy of CBCT images in the volumetric assessment of residual root canal filling material: Effect of voxel size. Niger J Clin Pract 2019;22:1091-8

How to cite this URL:
Yilmaz F, Sonmez G, Kamburoglu K, Koc C, Ocak M, Celik H H. Accuracy of CBCT images in the volumetric assessment of residual root canal filling material: Effect of voxel size. Niger J Clin Pract [serial online] 2019 [cited 2019 Oct 15];22:1091-8. Available from: http://www.njcponline.com/text.asp?2019/22/8/1091/264424




   Introduction Top


Root canal retreatment is indicated in order to eliminate or reduce intracanal and periradicular microbial infection. This treatment is usually considered to be the first treatment option in case of endodontic failure. The retreatment procedure involves complete removal of the previous filling material from the canal system to allow effective cleaning, shaping, and refilling of the root canal system.[1] Several techniques have been proposed for retreatment procedures such as; hand files, NiTi rotary files, solvents, heat, ultrasonics, and lasers. However, none of those techniques were found to be successful for completely removing the whole root canal filling material.[1],[2],[3],[4] Various methods have been used in endodontics in an attempt to evaluate the efficacy of root filling material removal including intraoral periapical radiography, stereomicroscopy and digital camera. However, none of those methods are able to provide quantitative volumetric information before and after conducting retreatment procedures.[5],[6],[7],[8] Assessment of residual filling material can be difficult with periapical radiography due to anatomic noise from surrounding structures, superimpositions, and projection geometry issues. Cone beam computed tomography (CBCT) has been specifically designed to reproduce undistorted 3D information of the maxillofacial skeleton, including the teeth and their surrounding tissues, with a significantly lower effective radiation dose compared with medical computed tomography.[9] A significant limitation of CBCT imaging is the presence of metallic artifacts, which are caused by metal and amalgam restorations and, to a lesser extent, root canal filling material, and implants.[10],[11] More recently, the use of CBCT in endodontic research has enabled 3D assessment of treatments performed within the root canal system.[12],[13],[14],[15],[16] Recent guidelines published AAE/AAOMR Joint Position Statement 2015/2016 Update recommended that limited FOV CBCT should be the imaging modality of choice when evaluating the nonhealing of previous endodontic treatment to help determine the need for further treatment, such as nonsurgical, surgical or extraction. Moreover, limited FOV CBCT was suggested for nonsurgical retreatment to assess endodontic treatment complications, such as overextended root canal obturation material, separated endodontic instruments, and localization of perforations.[17]

CBCT units offer multiple field of views (FOVs) and voxels that can better address a variety of specific tasks. Voxel size is of paramount importance in terms of image quality and scanning and reconstruction times of CBCT images. A “voxel” describes the smallest distinguishable box-shaped part of a 3-dimensional (3D) image. In CBCT imaging, voxels are usually isotropic and range from 0.4 mm to as small as 0.075 mm.[18]

High-resolution micro CT scanner enables three-dimensional evaluation of the root canal system after root canal filling and it is considered as gold standard for volumetric measurements. Micro CT is an innovative, non-destructive, and reproducible device that produces very thin sections and a true three-dimensional (3D) reconstruction of the object. Technically, it is possible to produce ultra-high resolutions of 1 μm ex vivo, using microfocal spot X-ray sources and high-resolution detectors, with a high radiation dose that is incompatible with human organism.[14],[19]

In view of the importance of accurate determination of the exact location and volume of the residual filling material during retreatment procedures, the use of CBCT images may provide useful information for the clinician in the volumetric assessment of the amount of residual filling material in root canals. To our knowledge, no previous study assessed the effectiveness of CBCT images for the volumetric measurement of residual filling material when compared to Micro CT in root canals. Our objective was therefore, to compare three different voxel sizes of CBCT with Micro-CT images to determine residual filling material volume in root canals after retreatment procedure.


   Materials and Methods Top


Specimen selection and preparation

The study protocol was reviewed and approved by the local ethical committee. Forty-two roots of 14 extracted human maxillary molars were used for the present study. All the samples were radiographed in a bucco-lingual direction by using intraoral photo stimulable phosphor plate and teeth with fracture, cracks, or any other defect were excluded. Debris and soft tissue remnants were cleaned and teeth were kept in 0.1% thymol solution for a week for disinfection; 24 hours before the experiment,

teeth were retrieved from thymol solution, rinsed, and stored in distilled water. After preparation of the access cavity, working length was determined by inserting a size 10 K-file (Dentsply, Maillefer, OK US). All root canals were then instrumented by using ProTaper rotary instruments (Dentsply, Maillefer, OK, US) in a crown-down manner. As the following sequence: SX file was used at 1/2 of the working length, whereas, S1, S2, F1, F2 files were used to the full working length. During shaping, each canal was irrigated between instruments with 2 mL of 2.5% sodium hypochlorite (NaOCl). Finally, the root canals were dried with paper points and obturated with a size F2 ProTaper single cone and AH Plus Sealer (Dentsply, DeTrey, Konstanz, Germany). The quality and apical extent of root canal fillings were confirmed radiographically. Teeth were stored at 37°C in 100% humidity for 2 weeks, to allow complete setting of the sealer.

Removal of root filling material

After setting of the sealer was completed, the coronal third of the root filling was removed with Gates-Glidden drills and the filling material was removed using 0.06 taper ProFile ® (Dentsply Maillefer, Baillagues, Switzerland) instruments sizes 40, 35, 30, 25, and 20 in a crown-down sequence at a speed of 600 rpm and a torque of 2.4 Ncm. After retreatment process, root canals were irrigated once again with 2 mL of 17% EDTA and flushed with 2 mL of 2.5% NaOCl. Each instrument was used to prepare a maximum of four root canals. No additional instruments and solvents were applied. A ten year experienced endodontist conducted all treatment and retreatment procedures.

Cone Beam Computed Tomography (CBCT) scanning procedures and evaluation

For CBCT scanning procedures, each tooth was placed in the appropriately prepared maxillary first molar socket of a dry skull with a natural second premolar and second molar teeth. The dry skull along with its maxilla was covered with 1.5-cm red wax as a soft tissue equivalent material. Images of each tooth inserted in the maxilla were obtained by using the ProMax 3D Max CBCT (Planmeca, Helsinki, Finland) with a flat panel sensor using low artifact reduction mode operating at 96 kVp, 1 mA, 55 × 50 mm FOV at 3 different voxel sizes 1) High resolution (0.1 mm) 2) High definition (0.15 mm) and 3) Normal resolution (0.2 mm) voxel size [Figure 1]. The exposure time and dose area product (DAP, mGy*cm 2) showed variations among different voxel sizes. The exposure time ranged between 11.948 and 12.154 seconds for 0.1-mm voxel size, 14.916 and 15.142s for 0.15-mm voxel size, and 12.042 and 12.133 seconds for 0.2-mm voxel sizes. The DAP values were 67 mGy*cm 2, 83 mGy*cm 2, and 67mGy*cm 2 for 0.1-mm, 0.15-mm, and 0.2-mm voxel sizes, respectively. Exposure time was determined by consensus. Pulpal root canal, dentin, and enamel visibility were used as indicators of optimal image quality. After CBCT data was exported as DICOM files for all the samples, volumetric measurements of residual filling material inside the canal was conducted by using 3D-DOCTOR (Able Software Corp., Lexington, MA) [Figure 2], and measurements were then compared with micro-CT image measurements. 3D-DOCTOR (Able Software Corp., Lexington, MA), is volumetric rendering software capable of vector-based segmentation technology. This software allowed segmentation of residual filling material on consecutive axial slices enabling visualization at each level apicocoronally. This ensured detailed slice-by-slice manual segmentation of the residual filling material borders using a mouse with turquoise colored delineation via trace boundary tool.
Figure 1: a: CBCT images of residual filling materials obtained by using 0.1 mm voxel size in coronal, sagittal, and axial view. b: CBCT images of residual filling materials obtained using 0.15 mm voxel size in coronal, sagittal, and axial view. c: CBCT images of residual filling materials obtained using 0.2 mm voxel size in coronal, sagittal, and axial view

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Figure 2: Volumetric measurement of residual filling material analyzed by using 3D-DOCTOR (Able Software Corp., Lexington, MA)

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Micro-CT scanning procedures and evaluation

Teeth were scanned postoperatively by using Skyscan 1174 micro-CT (Skyscan, Kontich, Belgium) by using following parameters: 800 (μA), 50 (kVp) and 21 μm projections within 180° rotation and 4000 (ms) scanning time for each specimen. Scan time was 60 min in duration. Three-dimensional reconstruction data was obtained with the aid of dedicated software (NRecon version 1.6.9.4, Skyscan, Kontich, Belgium) using post threshold-based segmentation and viewed by using Dataviewer Program (version 1.5.2.4, Bruker micro CT). Scanning was then transferred into CTan software (version 1.13.5.1) for image analysis. The inflection point of the micro CT absorption histogram between dentine and filling material compartments was used as segmentation threshold. Volumes of residual filling materials were calculated in mm, and the mean values for each specimen were determined. The mean percentage of residual filling material was color coded and calculated [Figure 3].
Figure 3: Postoperative micro-CT images of residual filling materials obtained by using Skyscan 1174 micro-CT (Skyscan, Kontich, Belgium)

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Two observers (a three years experienced radiologist, a 10-year experienced endodontist) assessed the images obtained from 2 techniques in random order. The observers were allowed to use enhancement facilities, such as: g-curve, contrast, brightness, and zoom function if necessary. All images were evaluated separately on a 22” NEC MD213MG LCD monitor (NEC, Tokyo, Japan), at a screen resolution of 2048 × 1536 pixel and 32-bit color depth. Image sets were viewed at 1-week intervals, and repetitions performed on half of the images one month after the initial viewings. No time restriction was placed on the observers.

Statistical analysis

Mann-Whitney U test was used to compare differences between observers. Wilcoxon Test was used for the comparison of CBCT and micro CT measurements. The Spearman correlation coefficient was used to display the relationship between volume measurements at different voxel sizes of CBCT and micro CT. The significance level was set at P<.05.


   Results Top


Mann-Whitney U test showed no statistically significant differences between the two observers for all measurements (P > 0.05). According to Wilcoxon Test, there were no statistically significant differences among different CBCT voxel sizes (0.1 mm, 0.15 mm, and 0.2 mm) in comparison to micro CT measurements for each observer (P > 0.05) [Table 1]. The Spearman correlation coefficients between CBCT at different voxel sizes significantly highly correlated with micro CT measurements (P < 0.05) [Table 2]. Furthermore, no significant differences were found between the measurements obtained by the two observers in consideration to different root canal location (P > 0.05) [Table 3]. According to micro CT measurements the average volume of residual filling material in mesiobuccal (MB), distobuccal (DB) and palatinal (P) root canals were found to be 1.456 mm 3, 1.126 mm 3, and 1.905 mm,3 respectively. Average volume of residual filling material calculated at different CBCT voxel sizes (mm) for different root canal locations and for both observers was shown in [Table 4].
Table 1: Median, mean, and standard deviation values of the volumes of residual filling materials at different CBCT voxel sizes and micro CT measurements for each observer

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Table 2: Spearman Correlations Coefficient for each voxel size and micro CT measurement for each observer

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Table 3: Median, mean, and standard deviation values of the volumes of residual filling materials at different CBCT voxel sizes for each observer at different root canal locations

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Table 4: Average volumes of filling materials

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   Discussion Top


The success of root canal retreatment procedures depends on complete removal of obturation materials from the root canal system. Moreover, the presence of residual root canal filling material was identified as a prognostic factor related to retreatment failure that enables the effective action of instruments and irrigating solutions on debris and microorganisms responsible for the apical infection.[19],[20] Various methods were used to evaluate the amount of residual root filling material in vitro. One of them was splitting the roots longitudinally in order to evaluate the canal walls microscopically. Radiographs were another way to evaluate residual root filling material both in vitro and in vivo.[5],[8],[16],[21] In a previous study, residual filling material was first evaluated radiographically and then by microscopy. Although, computerized morphometric analysis was regarded as an objective analysis tool, the projection of the root canal surface in either the radiographs or the digital photographs did not accurately represent the canal surface. Periapical radiography is the most common method that can be used for the evaluation of residual root canal filling material clinically.[8] However, periapical radiographic image is the compression of three-dimensional (3-D) structures into a two-dimensional view. Therefore, they are insufficient to locate and assess residual root canal filling materials specifically in the maxillary molar region due to superimpositions of adjacent teeth and anatomical structures.[22],[23] The main advantage of using CBCT for endodontic applications is that it can provide three-dimensional views which intraoral radiography cannot provide with considerably higher radiation doses than 2 dimensional techniques.[14],[15],[22] CBCT scan is an important tool for detecting intraoperative endodontic complications or difficulties, such as; complex anatomy, root resorptions, perforations, and root fractures in cases in which the diagnosis or treatment was compromised by the limitations of radiographs.[16],[23] We used maxillary first molar teeth in the appropriately prepared maxillary first molar socket of a dry skull with a natural second premolar and second molar teeth in order to create a more realistic clinical situation. In addition, we preferred using ProFile system for retreatment procedures. The validity and accuracy of this methodological approach has been previously demonstrated. In a recent study, authors found that by using ProFile instruments, the clinician left significantly less residual filling material when compared with ProTaper and Mtwo retreatment files.[24]

The clinical use of different Field of View (FOVs) and voxel sizes may affect image quality along with patient radiation doses. Currently, voxel sizes between 0.1 mm and 0.2 mm were suggested and regarded as optimal for various endodontic purposes. In addition, the smallest FOV of the CBCT unit is recommended in endodontic applications due to reduced radiation dose with smaller FOV.[16],[18] We used the smallest FOV (55 × 50 mm) offered by the CBCT unit that is suggested for endodontic use with the advantages of lower patient exposure, shorter scan times, and smaller voxel sizes available in comparison with larger FOVs. It is possible that the use of CBCT units with a different FOV and a voxel size than the one we preferred would reveal different observer performance.

CBCT artifacts include streaks around materials as well as dark zones that affect the overall quality of the image and may cause discrepancies between the reconstructed image and the scanned object. CBCT manufacturers are now actively developing artifact-reducing algorithms to be used during image reconstruction.[10],[11] In the present study, we employed artifact reduction filters for both CBCT and micro CT used, however; comparison of different artifact reduction filters offered by different CBCT units was out of our scope. We preferred using low mode since endodontic filling materials cause less artifacts when compared to metal and amalgam restorations. Moreover, in applying our results to clinical situations, it should be made clear that patient motion was not a factor in this experimental in vitro set up.

Up until now, the potential of CBCT images obtained at different voxel sizes to explore the root canal system for volumetric measurement of the residual root canal filling material has not been assessed in comparison to Micro CT as the gold standard. We found that there were no significant differences among voxel sizes used (0.1 mm, 0.15 mm, and 0.2 mm) compared with Micro CT measurements. Also, pairwise comparisons showed high correlation between Micro CT and CBCT for each voxel sizes and for two observers. Volumetric measurements may vary depending on the observer performance, CBCT unit and settings and software used along with mouse sensitivity. In this study, trained researchers in using measurement tools of the software acted as observers. Furthermore, it may be speculated that differences between tooth and root canal types and curvatures may cause discrepancies between volumetric measurements. However, we found no significant differences between the measurements in consideration to different root canal location.

In a previous regenerative endodontic treatment (RET) study, volumetric measurement of a newly formed hard tissue was conducted to develop a standardized quantitative method for RET outcome analysis based on cone-beam computed tomographic (CBCT) volumetric measurements. CBCT images were obtained in high resolution, 80 μm voxel size and 6 × 6 cm FOV. Similar to our study, volumetric measurements were conducted and compared. The results showed no statistical differences and the authors found that there was a strong correlation between CBCT and micro CT volumetric measurements. Thus, they suggested that CBCT images could be used for teeth treated with regenerative endodontic procedures to detect treatment outcome and patterns of hard tissue formation for clinical relevance.[25] Similarly, we found significant correlation between CBCT and micro-CT measurements suggesting that CBCT images have the strong potential to be used for volumetric measurement of residual root canal filling material that may be useful for retreatment procedures. However, it should be kept in mind that it is more difficult to segment unclear residual filling materials in comparison to well-defined regenerative bone tissue.

In a similar previous study, authors measured residual filling material volume by CBCT to compare the efficacy of different file systems. ProTaper retreatment files were used followed by either WaveOne reciprocating file or the Self-Adjusting File (SAF). The CBCT scans were obtained using ProMax 3D Mid (Planmeca OY, Helsinki, Finland). The samples were exposed to 90 kV and 8 mA with a FOV of 4.5 × 4.5 cm 2 and an isotropic resolution of 0.1 mm, with 12.28 s exposure time. According their results the mean postoperative volume of the root filling residue was 9.4 (±0.5) mm 3 in Group 1 and 2.6 (±0.4) mm 3 in Group 2. These residue volumes represented 26% and 7% of the original root filling volumes in Group 1 and Group 2, respectively. Analoguous to our findings, they reported that the use of two-dimensional (2D) radiographs failed to represent the real cleanliness of the canal. Therefore, CBCT with small FOV, which is readily available to more researchers than micro-CT, may be a valid tool for retreatment cases.[26]

Authors of a previous study suggested the use of CBCT images obtained at voxel sizes ranging between 0.1 mm to 0.2 mm as an adjunct to periapical radiography for the detection of various endodontic complications.[27] However, up until now there is no accepted protocol for the use of different voxel sizes for different endodontic tasks. Our study was the first to assess different voxel sizes for the volumetric measurement of residual filling materials in root canals. Radiation concerns regarding CBCT is a limitation of the system and its potential benefits should outweigh its hazards when prescribing CBCT. The effective dose for the CBCT unit utilized in the present study is in the range 28 to 122μSv.[28] This is higher than the effective doses from periapical radiography taken with PSP with rectangular collimation (0.1 to 2.6 μSv).[29] Prospective randomized clinical trials based on well-defined methodologies are needed to assess the potential benefits of CBCT in endodontic retreatment. Within the limitations of the present ex vivo study, CBCT images obtained at voxel sizes <0.2 mm appears to be an effective imaging technique for the determination of the location and volume of residual filling material in root canals for retreatment procedures.[24]


   Conclusion Top


All CBCT images obtained at different voxel sizes performed similarly in the detection of residual root canal filling material volume. In addition, measurements of the volumes from CBCT images highly correlated with micro-CT volume. CBCT images may provide useful information in the volumetric determination of the amount of residual filling material in root canals for retreatment procedures.

Acknowledgements

Authors declare no conflict of interest. No funding was obtained for this study. No financial support and sponsorship.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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