Abstract
Objectives:
To assess whether dimensional changes occur as shape distortion (unevenly), contraction or magnification (evenly) in cone beam computed tomography (CBCT) considering materials, anatomical regions and metal artefact reduction algorithms.
Methods:
Four cylinders of amalgam (Am), cobalt-chromium (Co-Cr), gutta-percha (Gu), titanium (Ti) and zirconium (Zi) were inserted inside a polymethylmethacrylate phantom in anterior and posterior regions for acquisitions in Picasso Trio and OP300 with MAR enabled and disabled. Two observers measured the dimensions of each cylinder in three axes: Y (height), Z (antero posterior diameter) and X (latero-lateral diameter). Repeated measures ANOVA with Tukey post-hoc test compared the data (α = 5%).
Results:
Shape distortion occurred for all materials in anterior region of Picasso Trio without MAR (p < 0.05). With MAR enabled, Gu and Ti contracted (p ≥ 0.05), while the others showed distortion (p < 0.05). In posterior region, all materials distorted in both MAR conditions (p < 0.05), except Gu, which magnified without MAR (p ≥ 0.05) and contracted unevenly with MAR (p < 0.05). In anterior region of OP300, all materials magnified without MAR, (p ≥ 0.05) and had shape distortion with MAR (p < 0.05). In posterior region, only Am showed magnification without MAR (p ≥ 0.05), while all materials presented shape distortion with MAR (p < 0.05).
Conclusion:
Dimensional changes of high-density materials in CBCT can be either a magnification, a contraction or a distortion; the last condition is the most prevalent. Furthermore, changes differ considering material, anatomical region and MAR condition.
Keywords: Artefacts, Cone-beam computed tomography, Dental materials
Introduction
Within the disadvantages of cone-beam computed tomography (CBCT), there are inherent artefacts related to high-density materials (e.g. gutta-percha, metal posts and titanium/zirconium implants) that may impair the image quality.1,2 Those artefacts include scatter streaks, photon starvation, and blooming or volumetric distortion.3–6
Blooming is defined as the type of artefact that causes volumetric changes of objects composed by high-density materials, not reliably magnifying its dimensions in the reconstructed images.6,7 However, this distortion can either underestimate or overestimate the physical dimensions of the scanned object in the CBCT volume.6 Clinically, the blooming artefact may impair the assessment of adjacent structures, for example, peri-implant bone thickness.8 The influence of different factors in blooming has been evaluated, such as CBCT device, type of material, field of view (FOV), position of the object within the FOV, and use of the metal artefact reduction (MAR) algorithm.4–6,9
A common aspect among such studies is that the measurement under investigation has been the objects’ total volume,4–6,9 which does not elucidate whether the volumetric changes occur evenly or unevenly in the three axes (x, y, and z). Such understanding may clarify whether algorithms can eventually correct volumetric changes as a distortion (unevenly) or a magnification (evenly) of the objects’ dimensions. Therefore, the aim of the present study was to assess how linear dimensional changes occur in two CBCT devices considering five high-density materials, two anatomical regions and two MAR conditions.
Methods and materials
Sample preparation
Four cylinders of five high-density materials were manufactured with similar dimensions: amalgam dental alloy (Southern Dental Industries Ltd., SDI, Australia); cobalt-chromium alloy (Co-Cr) (Scardua Laboratory, Vila Velha, Brazil); gutta-percha (Dentsply, Petrópolis, Rio de Janeiro, Brazil); titanium (S.I.N. Implantes, São Paulo, Brazil) and zirconium (Scardua Laboratory, Vila Velha, Brazil) (Figure 1a). The physical dimensions (height, anteroposterior diameter, and lateral-lateral diameter) of each cylinder were obtained using a digital caliper to set a reference standard (Tables 1–4); the measurements were repeated three times to guarantee accuracy. A polymethylmethacrylate (PMMA) phantom with a diameter of 100 mm and height of 43.5 mm containing perforations that simulated a lower dental arch was used for image acquisition of the cylinders (Figure 1b).10
Figure 1.
(a) Cylinders of five high-density materials (from left to right): amalgam, cobalt-chromium, gutta-percha, titanium and zirconium. (b) PMMA phantom used for acquisition. (c) Superior view of the phantom showing the perforations in which the cylinders were inserted: I, anterior region and right side; II, anterior region and left side; III, posterior region and right side; IV, posterior region and left side. (d) PMMA phantom positioned for acquisition in Picasso Trio, and (e) OP300 Maxio.
Table 1.
Picasso Trio system, anterior region: tomographic measurements in millimeters, and difference between the measured and the physical values in percentage, according to materials and MAR condition
| Materials and axes | Physical measurements in mm | Without MAR | With MAR | ||
|---|---|---|---|---|---|
| Tomographic measurement in mm | Relative difference in % | Tomographic measurement in mm | Relative difference in % | ||
| Mean (SD) | Mean (SD) | Mean (SD) | Mean (SD) | ||
| Amalgam | |||||
| Y-axis | 5.50 | 5.82 (0.11) | 5.74 (1.85) A | 5.68 (0.17) | 3.24 (2.99) A |
| Z-axis | 5.35 | 5.81 (0.13) | 8.56 (2.66) B | 5.73 (0.15) | 7.02 (2.59) B |
| X-axis | 5.35 | 6.06 (0.32) | 13.26 (5.77) C | 5.96 (0.34) | 11.33 (6.06) C |
| Co-Cr | |||||
| Y-axis | 5.50 | 5.61 (0.14) | 2.03 (2.49) A | 5.57 (0.16) | 1.23 (2.94) A |
| Z-axis | 5.40 | 5.86 (0.30) | 8.43 (5.48) B | 5.70 (0.14) | 5.48 (2.65) B |
| X-axis | 5.40 | 6.02 (0.34) | 11.51 (6.37) B | 5.89 (0.33) | 9.12 (6.18) B |
| Gutta-percha | |||||
| Y-axis | 5.50 | 5.80 (0.24) | 5.47 (4.40) A | 5.46 (0.19) | −0.77 (3.49) A |
| Z-axis | 5.50 | 5.95 (0.14) | 8.24 (2.59) B | 5.45 (0.10) | −0.91 (1.73) A |
| X-axis | 5.50 | 6.20 (0.29) | 12.77 (5.35) C | 5.44 (0.15) | −1.04 (2.79) A |
| Titanium | |||||
| Y-axis | 5.50 | 5.64 (0.14) | 2.47 (2.62) A | 5.38 (0.19) | −2.20 (3.41) A |
| Z-axis | 5.50 | 5.80 (0.16) | 5.44 (2.90) B | 5.39 (0.12) | −2.00 (2.11) A |
| X-axis | 5.50 | 6.11 (0.28) | 11.14 (5.13) C | 5.48 (0.10) | −0.33 (1.81) A |
| Zirconium | |||||
| Y-axis | 5.50 | 5.57 (0.13) | 1.27 (2.34) A | 5.48 (0.18) | −0.38 (3.25) A |
| Z-axis | 5.35 | 5.79 (0.16) | 8.15 (3.01) B | 5.69 (0.15) | 6.37 (2.75) B |
| X-axis | 5.35 | 6.01 (0.27) | 12.34 (5.09) C | 6.04 (0.18) | 12.93 (5.32) C |
Different uppercase letters indicate statistically significant differences among the dimensions for each material and each MAR condition, according to ANOVA.
Table 2.
Picasso Trio system, posterior region: tomographic measurements in millimeters, and difference between the measured and the physical values in percentage, according to materials and MAR condition
| Materials and axes | Physical measurements in mm | Without MAR | With MAR | ||
|---|---|---|---|---|---|
| Tomographic measurement in mm | Relative difference in % | Tomographic measurement in mm | Relative difference in % | ||
| Mean (SD) | Mean (SD) | Mean (SD) | Mean (SD) | ||
| Amalgam | |||||
| Y-axis | 5.50 | 5.68 (0.10) | 3.22 (1.87) A | 5.65 (0.12) | 2.60 (2.17) A |
| Z-axis | 5.35 | 5.59 (0.13) | 4.39 (2.34) A | 5.54 (0.17) | 3.55 (3.24) AB |
| X-axis | 5.35 | 5.73 (0.09) | 7.18 (1.76) B | 5.67 (0.09) | 6.06 (1.73) B |
| Co-Cr | |||||
| Y-axis | 5.50 | 5.61 (0.16) | 2.08 (2.84) A | 5.59 (0.12) | 1.67 (2.25) A |
| Z-axis | 5.40 | 5.64 (0.14) | 4.47 (2.64) B | 5.48 (0.13) | 1.48 (2.42) A |
| X-axis | 5.40 | 5.74 (0.13) | 6.37 (2.46) B | 5.69 (0.15) | 5.28 (2.85) B |
| Gutta-percha | |||||
| Y-axis | 5.51 | 5.73 (0.09) | 3.91 (1.84) A | 5.44 (0.08) | −1.34 (1.65) AB |
| Z-axis | 5.48 | 5.81 (0.15) | 6.00 (2.86) A | 5.27 (0.15) | −3.96 (2.90) A |
| X-axis | 5.48 | 5.81 (0.15) | 5.96 (2.92) A | 5.52 (0.12) | 0.61 (2.14) B |
| Titanium | |||||
| Y-axis | 5.50 | 5.61 (0.11) | 2.00 (1.92) A | 5.34 (0.14) | −2.85 (2.56) A |
| Z-axis | 5.50 | 5.77 (0.18) | 4.92 (3.33) B | 5.24 (0.07) | −4.71 (1.21) A |
| X-axis | 5.50 | 5.77 (0.09) | 4.92 (1.70) B | 5.51 (0.11) | 0.21 (2.03) B |
| Zirconium | |||||
| Y-axis | 5.50 | 5.54 (0.12) | 0.65 (2.13) A | 5.50 (0.14) | −0.03 (2.57) A |
| Z-axis | 5.35 | 5.60 (0.13) | 4.67 (2.35) B | 5.44 (0.22) | 1.73 (4.11) A |
| X-axis | 5.35 | 5.69 (0.11) | 6.36 (2.06) B | 5.69 (0.11) | 6.26 (2.06) B |
Different uppercase letters indicate statistically significant differences among the dimensions for each material and each MAR condition, according to ANOVA.
Table 3.
OP300 system, anterior region: tomographic measurements in millimeters, and difference between the measured and the physical values in percentage, according to materials and MAR condition
| Materials and axes | Physical measurements in mm | Without MAR | With MAR | ||
|---|---|---|---|---|---|
| Tomographic measurement in mm | Relative difference in % | Tomographic measurement in mm | Relative difference in % | ||
| Mean (SD) | Mean (SD) | Mean (SD) | Mean (SD) | ||
| Amalgam | |||||
| Y-axis | 5.50 | 6.45 (0.12) | 17.32 (3.78) A | 6.10 (0.16) | 10.91 (2.88) A |
| Z-axis | 5.35 | 6.28 (0.22) | 17.40 (4.13) A | 6.57 (0.14) | 22.75 (2.56) C |
| X-axis | 5.35 | 6.32 (0.18) | 18.11 (2.94) A | 6.50 (0.10) | 19.87 (4.32) B |
| Co-Cr | |||||
| Y-axis | 5.50 | 6.36 (0.25) | 15.64 (4.50) A | 6.20 (0.17) | 12.71 (3.28) A |
| Z-axis | 5.40 | 6.22 (0.21) | 15.19 (3.91) A | 6.52 (0.17) | 20.80 (3.21) B |
| X-axis | 5.40 | 6.28 (0.17) | 16.27 (3.22) A | 6.54 (0.14) | 20.00 (3.45) B |
| Gutta-percha | |||||
| Y-axis | 5.50 | 6.24 (0.15) | 13.37 (2.75) A | 6.14 (0.22) | 11.59 (3.98) A |
| Z-axis | 5.50 | 6.13 (0.15) | 11.42 (2.67) A | 6.40 (0.16) | 16.36 (2.87) B |
| X-axis | 5.50 | 6.26 (0.15) | 13.86 (2.64) A | 6.45 (0.13) | 17.27 (2.30) B |
| Titanium | |||||
| Y-axis | 5.50 | 6.17 (0.08) | 12.18 (1.48) A | 6.01 (0.10) | 9.18 (1.91) A |
| Z-axis | 5.50 | 6.15 (0.09) | 11.77 (1.71) A | 6.43 (0.10) | 16.85 (1.88) B |
| X-axis | 5.50 | 6.23 (0.12) | 13.35 (2.16) A | 6.34 (0.26) | 15.23 (4.68) B |
| Zirconium | |||||
| Y-axis | 5.50 | 6.34 (0.26) | 15.30 (4.69) A | 6.07 (0.21) | 10.30 (3.89) A |
| Z-axis | 5.35 | 6.17 (0.17) | 15.30 (3.23) A | 6.61 (0.19) | 23.47 (3.55) B |
| X-axis | 5.35 | 6.27 (0.18) | 17.10 (3.40) A | 6.61 (0.11) | 23.54 (2.09) B |
Different uppercase letters indicate statistically significant differences among the dimensions for each material and each MAR condition, according to ANOVA.
Table 4.
OP300 system, posterior region: tomographic measurements in millimeters, and difference between the measured and the physical values in percentage, according to materials and MAR condition
| Materials and axes | Physical measurements in mm | Without MAR | With MAR | ||
|---|---|---|---|---|---|
| Tomographic measurement in mm | Relative difference in % | Tomographic measurement in mm | Relative difference in % | ||
| Mean (SD) | Mean (SD) | Mean (SD) | Mean (SD) | ||
| Amalgam | |||||
| Y-axis | 5.50 | 6.41 (0.32) | 16.52 (5.81) A | 6.36 (0.25) | 15.60 (4.63) A |
| Z-axis | 5.35 | 6.10 (0.22) | 14.02 (4.09) A | 6.47 (0.28) | 20.89 (5.20) B |
| X-axis | 5.35 | 6.15 (0.18) | 14.86 (3.45) A | 6.38 (0.17) | 17.49 (3.32) AB |
| Co-Cr | |||||
| Y-axis | 5.50 | 6.38 (0.25) | 16.08 (4.53) B | 6.29 (0.20) | 14.32 (3.66) A |
| Z-axis | 5.40 | 6.13 (0.16) | 13.46 (2.88) AB | 6.56 (0.17) | 21.47 (3.19) C |
| X-axis | 5.40 | 6.08 (0.14) | 12.50 (2.60) A | 6.43 (0.15) | 18.01 (3.56) B |
| Gutta-percha | |||||
| Y-axis | 5.51 | 6.24 (0.10) | 13.23 (2.16) B | 5.98 (0.22) | 8.56 (3.14) A |
| Z-axis | 5.48 | 6.18 (0.12) | 12.70 (2.22) B | 6.36 (0.19) | 15.94 (3.57) B |
| X-axis | 5.48 | 6.01 (0.08) | 9.65 (1.52) A | 6.25 (0.11) | 13.74 (2.40) B |
| Titanium | |||||
| Y-axis | 5.50 | 6.20 (0.09) | 12.74 (1.68) B | 6.05 (0.15) | 10.08 (2.81) A |
| Z-axis | 5.50 | 6.13 (0.13) | 11.52 (2.39) AB | 6.51 (0.21) | 18.39 (3.87) C |
| X-axis | 5.50 | 6.10 (0.10) | 10.89 (1.83) A | 6.26 (0.07) | 13.73 (1.33) B |
| Zirconium | |||||
| Y-axis | 5.50 | 6.37 (0.14) | 15.85 (2.46) B | 6.21 (0.22) | 12.90 (3.94) A |
| Z-axis | 5.35 | 6.19 (0.14) | 15.78 (2.59) B | 6.55 (0.22) | 22.48 (4.06) B |
| X-axis | 5.35 | 6.07 (0.08) | 13.49 (1.51) A | 6.39 (0.12) | 19.42 (2.24) B |
Different uppercase letters indicate statistically significant differences among the dimensions for each material and each MAR condition, according to ANOVA.
Image acquisition
The four cylinders of the same material were inserted in two anterior and two posterior perforations of the phantom, while the perforations that were not used were filled with PMMA cylinders (Figure 1c). Two CBCT systems were studied, Picasso Trio (E-Woo Technology Co., Ltd./Vatech, Giheung-gu, Korea; manufacturing year: 2010) and OP300 Maxio (Instrumentarium Dental, Tuusula, Finland; manufacturing year: 2017).
The acquisition parameters for Picasso Trio were fixed at 8.5 × 12 cm FOV, 0.2 mm voxel, 90 kVp, 5 mA, 24 s of scanning time, 450 basis images, and a contrast resolution of 16 bits. Considering OP300 Maxio, the acquisition parameters were set at 8 × 15 cm FOV, 0.25 mm voxel, 90 kVp, 5 mA, 24.3 s of scanning time, 312 basis images, and a contrast resolution of 13 bits. For both CBCT systems and the five materials, images were acquired with MAR enabled and disabled. For standardization of the phantom positioning for image acquisition, a Styrofoam platform with orientation lines was used as a guide to align with the guiding light of the CBCT system, thus ensuring that the phantom was always centered in the FOV in both horizontal and vertical directions (Figure 1d, e). After the change of the cylinders, the first scan was always analyzed to ensure the positioning was kept the same. Each experimental condition was scanned three times in order to consider potential fluctuations of the X-ray tube operation parameters (kV, mA), thus 60 CBCT scans were acquired (five materials × two CBCT scans × 2 MAR conditions × three repetitions).
Image evaluation
CLINIVIEW TM v. 10.2.6 (KaVo Dental GmbH, Biberach an der Riß, Germany) and Ez3D Plus Professional (Vatech, Giheung-gu, Korea) software were used to reconstruct and export the DICOM data sets of OP300 and Picasso Trio, respectively. The CBCT images were then randomized for evaluation. Measurements were performed by two oral radiologists, who were blinded to the acquisition parameters, by using OnDemand 3D software (Cybermed Inc., Seoul, Republic of Korea) in a dimmed-lit environment. Before the evaluation, the responsible researchers carried out a training session to demonstrate and establish the methods for the evaluation according to the proposed aims and methodology, and then explained to the observers these standards for the measurements. The observers measured in millimeters (mm) the linear dimensions of the cylinders in three axes: Y-axis (height), Z-axis (antero posterior diameter), and X-axis (latero-lateral diameter) (Figure 2). Y-axis was measured in the sagittal reconstruction considering the center of the cylinders, and both Z-axis and X-axis were measured in the axial reconstruction also in the center of the cylinder (Figure 2). Observers subjectively defined the center of the cylinders with the aid of the orientation lines of the OnDemand software. The measurements were performed according to the observers’ discretion of the boundaries of the cylinders considering the interface between the hyperdense image and the phantom, using the measurement tool available on the OnDemand software. Brightness and contrast adjustments and the use of filters were not allowed. A total of 1440 linear measurements were performed (60 volumes × 3 axes × 4 cylinders × two observers). After 30 days, 30% of the sample was reevaluated in order to measure the intraobserver reproducibility.
Figure 2.
On the left, a 3D-segmented cylinder demonstrating the X, Y, and Z-axes. On the right, an example of how dimensions were measured in each reconstruction.
Statistical analysis
Data were analyzed using SPSS v.25 (IBM Corp. Armonk, USA) and GraphPad Prism v.7.0 (GraphPad, La Jolla, USA). Intraclass correlation coefficient (ICC) was used to verify the intra- and interobservers reproducibility.11 Data were expressed as the mean of the tomographic measurements in mm. The differences in percentage between the tomographic measurements and the physical measurements (namely, TMPM) were regarded as the numeric expression of dimension change, considering each cylinder. TMPM values were compared among the axes (X, Y, and Z) using repeated measures ANOVA (one-way) for each factor under study (material, anatomical region and MAR condition) considering each CBCT system. The Tukey post-hoc test was applied with a significance level of 5% (α = 0.05). The null hypothesis was that the TMPM were not different in the three axes, regardless of materials, anatomical region, MAR condition and CBCT system.
Results
Measurements reproducibility was “almost perfect” to “perfect” for both interobserver (ICC = 1.0) and intraobserver (ICC = 0.99 for observer 1, and ICC = 1.0 for observer 2). Data regarding the physical measurements, tomographic measurements and TMPM according to the different dental materials, anatomical regions, CBCT system and MAR condition are displayed in Tables 1–4. The tomographic and TMPM values correspond to the mean of the measurements obtained by the two observers in the three CBCT repetitions for each protocol. To interpret the results, statistically significant differences (p < 0.05) between two or more axes were considered as shape distortion, while magnification was considered when there was no difference among the axes (p ≥ 0.05). Few cases of negative TMPM were considered as material contraction.
Table 1 shows data regarding anterior region in the Picasso Trio system. When images were acquired without MAR, statistically significant differences among axes were observed for all materials under study (p < 0.05). Amalgam, gutta-percha, titanium, and zirconium showed shape distortion in all axes but greater in the X-axis, whilst Co-Cr presented shape distortion on the horizontal plane (Z and X-axes) (p < 0.05) (Figure 3). When images were acquired with MAR, gutta-percha and titanium presented an evenly contraction in all axes (p ≥ 0.05); the other materials kept similar behaviour to those without MAR.
Figure 3.
Representative scheme of the dimensional changes of the cylinders according to dental material, CBCT system, anatomical region and MAR condition. Dark grey cylinders represent physical cylinder; light grey cylinder represent tomographic cylinder. Am, Amalgam alloy; Co-Cr, Cobalt-chromium alloy; Gu, Gutta-percha; Ti, Titanium; Zi, Zirconium.
In the posterior region using the Picasso Trio system (Table 2), without MAR, gutta-percha presented magnification (p ≥ 0.05). All other materials showed shape distortion (p < 0.05), depicted in the X-axis for amalgam, and at the horizontal plane (both Z and X-axes) for Co-Cr, titanium and zirconium. When images were acquired with MAR, gutta-percha presented an uneven contraction mostly in the Z-axis, while titanium contracted in both Y and X-axes (p < 0.05). Amalgam, Co-Cr and zirconium presented shape distortion in the X-axis (p < 0.05).
Table 3 shows data regarding the anterior region in the OP300 system. There was no statistically significant difference (p ≥ 0.05) among the axes for any material without MAR application (Figure 3), which represents a magnification of all materials in all three axes. Nevertheless, images in the anterior region acquired with MAR activated showed shape distortion in both X and Z-axes (i.e., horizontal plane) for all materials (p < 0.05).
Also, for the OP300 system, in the posterior region without MAR application (Table 4), only amalgam presented magnification (p ≥ 0.05). Gutta-percha and zirconium presented shape distortion in the Y-axis and the Z-axis (p < 0.05), while Co-Cr and titanium distorted mostly in the Y-axis (p < 0.05). With MAR application, gutta-percha and zirconium presented shape distortion on the horizontal plane (Z and X-axis) (p < 0.05), while amalgam distorted more in the Z-axis compared to the Y-axis (p < 0.05). Co-Cr and titanium distorted in all three axes, with bigger dimensions at the Z-axis, followed by the X-axis, and the Y-axis, respectively (p < 0.05).
Discussion
Our study aimed to assess whether dimensional changes of high-density materials in CBCT reconstruction occurred evenly or unevenly when considering three axes: Y-axis (height), Z-axis (antero posterior diameter) and X-axis (latero-lateral diameter). We identified three types of behaviour: magnification, when the object was evenly overestimated; contraction, when the object was evenly underestimated; and shape distortion, when the object was over or underestimated unevenly. Volume and shape distortions can be considered as a limitation of CBCT because the reconstructed images misrepresent the object’s dimensions and physical presentation.6
When we consider the dimensional changes itself, in general both systems overestimated the cylinders’ physical dimensions, which agrees with previous studies.5,6,9 For OP300 system, a higher frequency of magnification was noticed in the anterior region, and a shape distortion towards the Y-axis was observed in the posterior region, except for amalgam that was magnified. However, when MAR was enabled in OP300, the height was less overestimated than the diameters for both regions. For Picasso Trio system, measurements were more overestimated in the latero-lateral diameter than in the other planes, which indicates a general trend towards shape distortion. On the contrary, MAR algorithm of Picasso Trio presented lower distortion values, and in fact it did underestimated gutta-percha and titanium dimensions in both anterior and posterior regions.
In a previous study,6 an underestimation in the volumes of gutta-percha and titanium was found, which corroborates to our findings, although the measurement methods performed in both studies are different. Both gutta-percha and titanium present lower physical density compared to the other materials under study, and therefore a less pronounced artefacts expression is expected.6,9,12 Since the dimensional changes of these two materials were already slight, and higher intensity of artefacts occurs in the surrounding of these materials,1 Picasso Trio’s MAR may underestimate their dimensions in attempting to correct both artefacts and shape distortion of gutta-percha and titanium.6
Coelho-Silva et al. (2020)6 subjectively observed a shape distortion of the cylinders in the images of Picasso Trio system. Our findings objectively confirm their remark, and add that OP300 also produces a shape distortion, except for the anterior region when MAR was disabled. The effectiveness of MAR algorithm in reducing other artefacts has been supported previously in the literature.12–14 However, some studies have demonstrated that MAR algorithms were not effective in subjectively15 and objectively6 correcting volumetric distortion. MAR condition was not statistically compared, however, considering the axes in which the shape distortion occurred, our results agree with those previous studies. For both CBCT systems studied herein, MAR algorithm did not correct the shape distortion nor the magnification, and sometimes it even changed the distortion plane or altered magnification into shape distortion or into contraction. Another study5 indicated that the performance of MAR algorithm in objectively amending volumetric distortion depends on the CBCT system and on the material. The behaviour of MAR algorithm in the dimensional changes differed according to material, CBCT system and anatomical region in the present study. Considering the different regions, a previous study16 demonstrated that anterior region showed higher expression of artefacts than posterior regions; this partially agrees with our results that showed different behaviour of the three dimensions between these positions.
The reason for having acquired images with four cylinders positioned on the phantom was to reproduce a common clinical situation in which the patient has more than one high-density material in the dental arch, such as the presence of dental implants or teeth restored with gutta-percha or metal posts. Our study assessed two CBCT systems with different characteristics and protocols. Considering this, our acquisition parameters were as similar as possible and kept fixed in all acquisitions, since such parameters may affect the expression of artefacts.3,4,7,9,15 Additionally, some differences between the two systems concerning the type of dimensional alteration can be noticed. In Picasso Trio, shape distortion was predominant, and some specific magnification and contraction were observed. OP300 only demonstrated magnification and shape distortion. These differences might be related to the different manufacturers, the reconstruction algorithms, the MAR algorithms, and to the inherent characteristics of each system related to image acquisition, such as beam geometry.
Clinical considerations
Titanium and zirconium are materials used in the composition of dental implants, and the volumetric changes of these materials can impact the clinical evaluation of patients. Some studies have shown that artefacts did not influence measurement around implants,17,18 but another recent study found influence from artifacts, specifically blooming, in this measurement.8 In addition to that Vanderstuyft et al. (2019)8 found that the thickness of the vestibular bone is underestimated due to the presence of blooming, a fact emphasized by the findings of the present study where the Z and X-axes in the titanium and zirconium materials in general presented distortion or magnification in both devices, mainly in OP300 when MAR was activated. This overestimation may impair the diagnosis of peri-implant bone defects by annulling the respective voxels.
In images acquired with the Picasso Trio, the X-axis was significantly more affected for titanium and zirconium in the anterior region. A similar behavior was observed for these two materials in the posterior region, but with magnification in the Z and X-axis. For the OP300 device, both titanium and zirconium had magnification or shape distortion, regardless of the region studied. Therefore, evaluations of the peri-implant bone should be performed with caution, since the distortion can cover up some part of the cortical bone or simulate close contact with relevant anatomical structures. For the Picasso Trio, such shape distortion or magnification would possibly be minimized with the use of the MAR algorithm for titanium; nevertheless, for zirconium the MAR algorithm does not seem to have much effect. The use of the MAR algorithm for OP300 system seems controversial, as it apparently minimizes distortion on the Y-axis, but increases on the X and Z-axes; therefore, its use depends on the purpose of the exam and this potential shape distortion may be considered.
Recent studies have evaluated the quality of endodontic treatment in CBCT images.19–21 However, according to our findings, researchers must be aware of possible dimensional changes of intracanal materials (gutta-percha and Co-Cr alloys), not only regarding the anatomical region but also the CBCT system. Some degree of distortion (Y-axis, Z-axis and X-axis) was always present for both gutta-percha and Co-Cr alloy. Picasso Trio system showed an underestimation of gutta-percha when MAR was activated, while OP300 overestimated it, regardless of MAR condition. Parameters related to the quality of endodontic treatment, such as underfilling, overfilling, non-homogeneous filling, must always be carefully evaluated in order to avoid false positive and false negative diagnoses in these conditions.
Another important aspect is that the identification of root canal anatomical variations, mainly in the apical region of the teeth, is naturally challenging in CBCT images.22,23 In root-filled teeth, depending on the amount of volumetric overestimation, the detection of these variations can become even more complex. For example, an isthmus can be missed due to overestimation of the intracanal material, highlighted by the increase of the measures in all axes of the cylinders, especially in OP300. However, this hypothesis should be further explored in future studies.
Limitations
The present research has limitations typical of an in vitro study, such as using a phantom and manufactured cylinder, which does not fully mimic a clinical situation. Nevertheless, we must emphasize that using a PMMA phantom, specifically made to accommodate the cylinders,6,10 allowed their standardized positioning for acquisitions, which was a feasible way to test our hypotheses. In addition, this set formed by phantom and cylinders was carefully positioned on the devices, as described in the methodology, in order to avoid biases related to positioning and thus ensuring that measurements were taken exactly on the long axis of the cylinders. This was the reason why homogeneous cylinders were used, although spheres of these same materials are also a good choice for this type of study.
The use of homogeneous cylinders composed by high-density materials and with known physical dimensions was also needed for better reliability of our results. Although the dimensions of the cylinders are slightly different among some materials, due to inherent characteristics of each manufacturing process, this was overcome by the use of the TMPM values of each cylinder for data analysis. Next steps should include ex vivo study design, for example, with the use of skulls and mandibles, and even assessing the impact of these shape distortions, magnifications and contractions on different diagnostic tasks.
Conclusions
The dimensional changes of high-density materials in the two CBCT systems studied can be either a magnification, a contraction or a shape distortion, being the most prevalent the shape distortion. In addition, those dimensional changes differ considering anatomical regions, material, CBCT system and MAR use.
Footnotes
Acknowledgements: This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. The authors are thankful to S.I.N Implantes for providing the titanium cylinders.
Competing interests: The authors declare openly that there are no conflicts of interest in relation with this article.
Contributor Information
Fernanda Coelho-Silva, Email: silva.fernanda.coelho@gmail.com.
Hugo Gaêta-Araujo, Email: hugogaeta@hotmail.com.
Lucas P. Lopes Rosado, Email: lucaslopesrosado@gmail.com.
Deborah Queiroz Freitas, Email: deborahq@unicamp.br.
Francisco Haiter-Neto, Email: haiter@unicamp.br.
Sergio Lins de-Azevedo-Vaz, Email: sergiolinsv@gmail.com.
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