Accuracy of linear measurements from cone-beam computed tomography-derived surface models of different voxel sizes



Accuracy of linear measurements from cone-beam computed tomography-derived surface models of different voxel sizes




American Journal of Orthodontics and Dentofacial Orthopedics, 2010-01-01, Volume 137, Issue 1, Pages 16.e1-16.e6, Copyright © 2010 American Association of Orthodontists


Introduction

The aims of this study were to determine the linear accuracy of 3-dimensional surface models derived from a commercially available cone-beam computed tomography (CBCT) dental imaging system and volumetric rendering software and to investigate the influence of voxel resolution on the linear accuracy of CBCT surface models.

Methods

Glass sphere markers were fixed on 10 dry mandibles. The mandibles were scanned with 0.40 and 0.25 voxel size resolutions in 3 sessions. Anatomic truth was established with 6 direct digital caliper measurements. The surface models were rendered by a volumetric rendering program, and the CBCT measurements were established as the mean of the 3 measurements.

Results

The intraclass correlation coefficients between the physical measurements and the measurements of the CBCT images of 0.40 and 0.25 voxels were all more than 0.99. All CBCT measurements were accurate. There was no difference between the accuracy of the measurements between the 0.40 and 0.25 voxel size groups. The smallest detectable differences of the CBCT measurements were minimal, confirming the accuracy of the CBCT measurement procedure.

Conclusions

The measurements on 3-dimensional surface models of 0.25 and 0.40 voxel size data sets made with the 3D eXam CBCT scanner (KaVo Dental GmbH, Bismarckring, Germany) and SimPlant Ortho Pro software (version 2.00, Materialise Dental, Leuven, Belgium) are accurate compared with direct caliper measurements. An increased voxel resolution did not result in greater accuracy of the surface model measurments.

Because of the high cost and relatively high radiation exposure of helical computed tomography (CT) imaging methods, cone-beam CT (CBCT) is used more frequently for craniofacial assessment in orthodontics and oral maxillofacial surgery. CBCT captures the craniofacial structures with a single 360° rotation of a tube-detector unit. This is contrary to classic CT, in which imaging is performed in sections or layers. During the rotational scanning, many single projections are produced and these 2-dimensional images are churned by the reconstruction algorithm directly into a 3-dimensional (3D) or volumetric data set.

Drawing an object with a computer is called rendering. The object is given some characteristics to make it appear to be a real-world object with shadows and transparency. To draw a 3D image, the raw CT data are transformed to vector data by constructing a surface of many triangles covering the object of interest. Volumetric rendering programs are used to construct the 3D surface models from imported CBCT data sets by implementing an algorithm that is usually unique for each program. The 3D surface model allows for actions such as indicating landmarks, making measurements, moving bone fragments, and performing virtual osteotomies. The accuracy of the derived surface model is therefore of utmost importance, not only for diagnostic purposes but also for treatment planning and outcome.

The accuracy of CBCT images has been confirmed with various CBCT scanners. However, the accuracy of surface models derived from CBCT seems to vary. Some authors illustrated differences that, even though statistically significant, were not considered clinically relevant. These studies used anatomic landmarks on the surface models, which are subject to identification errors in the segmentation process. These factors might influence the accuracy of the measurement procedure. Therefore, the accuracy of the measurement procedures should be calculated to fully determine whether there is a significant difference between surface models and anatomic truth. To overcome the problem of landmark identification, Mischkowski et al used gutta-percha markers and concluded that the CBCT device provides satisfactory information about linear distances. Lagravere et al used titanium markers with a hollow cone on a synthetic mandible and concluded that volumetric renderings from the CBCT device produce a 1-to-1 image-to-reality ratio.

A factor that could possibly influence the accuracy of the surface models is the voxel resolution. Volume is composed of voxels, which can be considered tiny cubes arranged next to each other. Each voxel is a value (brightness or gray-scale color) that represents the x-ray density of the corresponding structure. Reducing the voxel resolution can result in a lower-quality image, more noise and artifacts, and less detailed anatomic information. Spatial resolution is lower at faster scanning times and larger voxel sizes. Greater spatial and voxel resolution results in generally “smoother” images by increasing the signal-to-noise ratio, with fewer artifacts from metallic restorations. However, greater voxel resolution is accomplished with an increased scanning time, exposing the patient to a higher radiation dosage, but there is also an increased risk of patient movement. Therefore, the influence of voxel resolution on the linear accuracy of CBCT-rendered surface models needs further investigation, since the result might be clinically relevant.

Our aims in this study were to determine the linear accuracy of CBCT-derived surface models, to investigate the influence of voxel resolution on the linear accuracy of CBCT-derived surface models, and to determine the accuracy of the measuring procedures.


Material and methods

The sample included 10 dry anonymous partially dentate adult mandibles, selected from the collection of dry skulls at the Department of Orthodontics, University Medical Center Groningen. Mandibles with teeth containing metallic restorations were not used because of possible scattering and artifact formation. Twelve areas were prepared in the cortical bone of the mandibles with a round surgical bur. Spherical glass markers with a diameter of 2.4 mm (KGM Kugelfabrik Gebauer GmbH, Fulda, Germany) were fixed in the prepared areas with cyanoacrylate glue (Pattex, Uni-rapide Gold, Henkel, Nieuwegein, The Netherlands). The spherical glass markers were used to minimize inherent differences in landmark identification and to establish fiducial anatomic locations. Twenty-five linear distances, representing all 3 planes of space, were measured between the landmarks ( Fig , A ). The midpoint of the outer-most part of the sphere from the direct frontal view, opposite where it was glued to the mandible, was the reference mark. The distances between the reference marks were determined with an electronic digital caliper (GAC, Bohemia, NY) on 6 occasions, at least 3 days apart, by 2 observers (J.D. and Z.F.). The mean of the measurements was designated as the reference value, or anatomic truth.

A and B , 25 linear distances measured between the 12 markers; C and D , 3D rendered surface models of a mandible with glass markers used in the study. Surface models were made with 0.4 voxel ( C ) and 0.25 voxel ( D ) sizes.
Fig
A and
B , 25 linear distances measured between the 12 markers;
C and
D , 3D rendered surface models of a mandible with glass markers used in the study. Surface models were made with 0.4 voxel (
C ) and 0.25 voxel (
D ) sizes.

To provide soft-tissue equivalent attenuation, a latex balloon filled with water was placed in the lingual area of the mandible. Before imaging in the CBCT scanner, the mandible was adjusted with the mandibular plane parallel to the floor and the sagittal laser reference coinciding with pogonion. The CBCT images were acquired with a 3D eXam scanner (KaVo Dental GmbH, Bismarckring, Germany). There were 3 scanning sessions of the mandibles at least a week apart. Each mandible was scanned twice during each session: once with a 0.40 voxel resolution and once with a 0.25 voxel resolution. Ultimately, each voxel size group consisted of 30 3D images of scanned mandibles. The preset parameters of the scanner are summarized in Table I . The CBCT data were exported from the eXamVisionQ software (Imaging Sciences International LCC, Hatfield, Pa) in DICOM multi-file format and imported into SimPlant Ortho Pro software (version 2.00, Materialise Dental, Leuven, Belgium) on an Aspire 7730G laptop computer (Acer, Hertogenbosch, The Netherlands) with a dedicated 512-mb video card (Geforce 9600M-GT, NVIDIA, Santa Clara, Calif). All measurements were performed on the surface models on a 17-in CrystalBrite LCD flat-panel color screen (Acer) with a maximum resolution of 1440 × 900 pixels.

Table I
Preset scanning parameters for a field of view of 10 cm of the 3D eXam CBCT scanner
Voxel size Projections (n) kV mAs Scanning time (s)
0.40 236 120 18.54 8.9
0.25 400 120 37.07 26.9
kV, Kilovolts;
mAs , milliampere-seconds.

The 3D surface models of all mandibular images were generated by the preset threshold value for bone (250-3071 Hounsfield units) as specified by the rendering software. The SimPlant Ortho Pro software provides various views by rotating and translating the rendered image. The reference points were identified on the spherical glass markers by using a cursor-driven pointer. After landmark identification, a preprogrammed analysis provided the distances to the nearest 0.01 mm of the 25 linear measurements described in the Figure , A . The values were then exported and saved in Excel file format (Microsoft, Redmond, Wash). Each CBCT image was rendered and measured on 3 occasions by 1 observer (J.D.). The mean of the 3 measurements was called the CBCT measurement value.


Statistical analysis

The accuracy of these measurements was expressed by means of the absolute error (AE) and absolute percentage error (APE). Absolute error was defined as the CBCT measurement value subtracted from the reference value. Absolute percentage error was calculated with the following equation: APE = 100 ∗(AE/reference value). Means and standard deviations were calculated.

As a measure of reliability, the intraclass correlation coefficient (ICC) for absolute agreement based on a 2-way random-effects analysis of variance (ANOVA) was calculated between the 3 measurement techniques (digital caliper, 0.40 voxel size, and 0.25 voxel size) used in the study.

To determine the linear accuracy of the measurement procedures (direct caliper and CBCT measurements), the standard error of measurement (SEM) of the 3 consecutive CBCT sessions was calculated as the variance of the random error (interaction between locations and measurement session) from the 2-way random-effects ANOVA. SEM values were calculated for each voxel size and the physical measurements. The smallest detectable difference (SDD) was then calculated as 1.96 ∗ √2 ∗ SEM 2 . All statistical analyses were performed with a standard statistical software package (version 14, SPSS, Chicago, Ill).


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Accuracy of linear measurements from cone-beam computed tomography-derived surface models of different voxel sizes Janalt Damstra , Zacharias Fourie , James J.R. Huddleston Slater and Yijin Ren American Journal of Orthodontics and Dentofacial Orthopedics, 2010-01-01, Volume 137, Issue 1, Pages 16.e1-16.e6, Copyright © 2010 American Association of Orthodontists Introduction The aims of this study were to determine the linear accuracy of 3-dimensional surface models derived from a commercially available cone-beam computed tomography (CBCT) dental imaging system and volumetric rendering software and to investigate the influence of voxel resolution on the linear accuracy of CBCT surface models. Methods Glass sphere markers were fixed on 10 dry mandibles. The mandibles were scanned with 0.40 and 0.25 voxel size resolutions in 3 sessions. Anatomic truth was established with 6 direct digital caliper measurements. The surface models were rendered by a volumetric rendering program, and the CBCT measurements were established as the mean of the 3 measurements. Results The intraclass correlation coefficients between the physical measurements and the measurements of the CBCT images of 0.40 and 0.25 voxels were all more than 0.99. All CBCT measurements were accurate. There was no difference between the accuracy of the measurements between the 0.40 and 0.25 voxel size groups. The smallest detectable differences of the CBCT measurements were minimal, confirming the accuracy of the CBCT measurement procedure. Conclusions The measurements on 3-dimensional surface models of 0.25 and 0.40 voxel size data sets made with the 3D eXam CBCT scanner (KaVo Dental GmbH, Bismarckring, Germany) and SimPlant Ortho Pro software (version 2.00, Materialise Dental, Leuven, Belgium) are accurate compared with direct caliper measurements. An increased voxel resolution did not result in greater accuracy of the surface model measurments. Because of the high cost and relatively high radiation exposure of helical computed tomography (CT) imaging methods, cone-beam CT (CBCT) is used more frequently for craniofacial assessment in orthodontics and oral maxillofacial surgery. CBCT captures the craniofacial structures with a single 360° rotation of a tube-detector unit. This is contrary to classic CT, in which imaging is performed in sections or layers. During the rotational scanning, many single projections are produced and these 2-dimensional images are churned by the reconstruction algorithm directly into a 3-dimensional (3D) or volumetric data set. Drawing an object with a computer is called rendering. The object is given some characteristics to make it appear to be a real-world object with shadows and transparency. To draw a 3D image, the raw CT data are transformed to vector data by constructing a surface of many triangles covering the object of interest. Volumetric rendering programs are used to construct the 3D surface models from imported CBCT data sets by implementing an algorithm that is usually unique for each program. The 3D surface model allows for actions such as indicating landmarks, making measurements, moving bone fragments, and performing virtual osteotomies. The accuracy of the derived surface model is therefore of utmost importance, not only for diagnostic purposes but also for treatment planning and outcome. The accuracy of CBCT images has been confirmed with various CBCT scanners. However, the accuracy of surface models derived from CBCT seems to vary. Some authors illustrated differences that, even though statistically significant, were not considered clinically relevant. These studies used anatomic landmarks on the surface models, which are subject to identification errors in the segmentation process. These factors might influence the accuracy of the measurement procedure. Therefore, the accuracy of the measurement procedures should be calculated to fully determine whether there is a significant difference between surface models and anatomic truth. To overcome the problem of landmark identification, Mischkowski et al used gutta-percha markers and concluded that the CBCT device provides satisfactory information about linear distances. Lagravere et al used titanium markers with a hollow cone on a synthetic mandible and concluded that volumetric renderings from the CBCT device produce a 1-to-1 image-to-reality ratio. A factor that could possibly influence the accuracy of the surface models is the voxel resolution. Volume is composed of voxels, which can be considered tiny cubes arranged next to each other. Each voxel is a value (brightness or gray-scale color) that represents the x-ray density of the corresponding structure. Reducing the voxel resolution can result in a lower-quality image, more noise and artifacts, and less detailed anatomic information. Spatial resolution is lower at faster scanning times and larger voxel sizes. Greater spatial and voxel resolution results in generally “smoother” images by increasing the signal-to-noise ratio, with fewer artifacts from metallic restorations. However, greater voxel resolution is accomplished with an increased scanning time, exposing the patient to a higher radiation dosage, but there is also an increased risk of patient movement. Therefore, the influence of voxel resolution on the linear accuracy of CBCT-rendered surface models needs further investigation, since the result might be clinically relevant. Our aims in this study were to determine the linear accuracy of CBCT-derived surface models, to investigate the influence of voxel resolution on the linear accuracy of CBCT-derived surface models, and to determine the accuracy of the measuring procedures. Material and methods The sample included 10 dry anonymous partially dentate adult mandibles, selected from the collection of dry skulls at the Department of Orthodontics, University Medical Center Groningen. Mandibles with teeth containing metallic restorations were not used because of possible scattering and artifact formation. Twelve areas were prepared in the cortical bone of the mandibles with a round surgical bur. Spherical glass markers with a diameter of 2.4 mm (KGM Kugelfabrik Gebauer GmbH, Fulda, Germany) were fixed in the prepared areas with cyanoacrylate glue (Pattex, Uni-rapide Gold, Henkel, Nieuwegein, The Netherlands). The spherical glass markers were used to minimize inherent differences in landmark identification and to establish fiducial anatomic locations. Twenty-five linear distances, representing all 3 planes of space, were measured between the landmarks ( Fig , A ). The midpoint of the outer-most part of the sphere from the direct frontal view, opposite where it was glued to the mandible, was the reference mark. The distances between the reference marks were determined with an electronic digital caliper (GAC, Bohemia, NY) on 6 occasions, at least 3 days apart, by 2 observers (J.D. and Z.F.). The mean of the measurements was designated as the reference value, or anatomic truth. Fig A and B , 25 linear distances measured between the 12 markers; C and D , 3D rendered surface models of a mandible with glass markers used in the study. Surface models were made with 0.4 voxel ( C ) and 0.25 voxel ( D ) sizes. To provide soft-tissue equivalent attenuation, a latex balloon filled with water was placed in the lingual area of the mandible. Before imaging in the CBCT scanner, the mandible was adjusted with the mandibular plane parallel to the floor and the sagittal laser reference coinciding with pogonion. The CBCT images were acquired with a 3D eXam scanner (KaVo Dental GmbH, Bismarckring, Germany). There were 3 scanning sessions of the mandibles at least a week apart. Each mandible was scanned twice during each session: once with a 0.40 voxel resolution and once with a 0.25 voxel resolution. Ultimately, each voxel size group consisted of 30 3D images of scanned mandibles. The preset parameters of the scanner are summarized in Table I . The CBCT data were exported from the eXamVisionQ software (Imaging Sciences International LCC, Hatfield, Pa) in DICOM multi-file format and imported into SimPlant Ortho Pro software (version 2.00, Materialise Dental, Leuven, Belgium) on an Aspire 7730G laptop computer (Acer, Hertogenbosch, The Netherlands) with a dedicated 512-mb video card (Geforce 9600M-GT, NVIDIA, Santa Clara, Calif). All measurements were performed on the surface models on a 17-in CrystalBrite LCD flat-panel color screen (Acer) with a maximum resolution of 1440 × 900 pixels. Table I Preset scanning parameters for a field of view of 10 cm of the 3D eXam CBCT scanner Voxel size Projections (n) kV mAs Scanning time (s) 0.40 236 120 18.54 8.9 0.25 400 120 37.07 26.9 kV, Kilovolts; mAs , milliampere-seconds. The 3D surface models of all mandibular images were generated by the preset threshold value for bone (250-3071 Hounsfield units) as specified by the rendering software. The SimPlant Ortho Pro software provides various views by rotating and translating the rendered image. The reference points were identified on the spherical glass markers by using a cursor-driven pointer. After landmark identification, a preprogrammed analysis provided the distances to the nearest 0.01 mm of the 25 linear measurements described in the Figure , A . The values were then exported and saved in Excel file format (Microsoft, Redmond, Wash). Each CBCT image was rendered and measured on 3 occasions by 1 observer (J.D.). The mean of the 3 measurements was called the CBCT measurement value. Statistical analysis The accuracy of these measurements was expressed by means of the absolute error (AE) and absolute percentage error (APE). Absolute error was defined as the CBCT measurement value subtracted from the reference value. Absolute percentage error was calculated with the following equation: APE = 100 ∗(AE/reference value). Means and standard deviations were calculated. As a measure of reliability, the intraclass correlation coefficient (ICC) for absolute agreement based on a 2-way random-effects analysis of variance (ANOVA) was calculated between the 3 measurement techniques (digital caliper, 0.40 voxel size, and 0.25 voxel size) used in the study. To determine the linear accuracy of the measurement procedures (direct caliper and CBCT measurements), the standard error of measurement (SEM) of the 3 consecutive CBCT sessions was calculated as the variance of the random error (interaction between locations and measurement session) from the 2-way random-effects ANOVA. SEM values were calculated for each voxel size and the physical measurements. The smallest detectable difference (SDD) was then calculated as 1.96 ∗ √2 ∗ SEM 2 . All statistical analyses were performed with a standard statistical software package (version 14, SPSS, Chicago, Ill). Results Accuracy of the measurements was determined by the AE and APE ( Table II ). The AE values were small: 0.01 to 0.15 mm (0.05 ± 0.04 mm) for the 0.4-voxel group and 0.00 to 0.16 mm (0.07 ± 0.05 mm) for the 0.25-voxel group. The APE values were 0.25% ± 0.37% and 0.33% ± 0.47% for the 0.40- and 0.25-voxel groups, respectively. The ICC values between the physical measurements and measurements of the CBCT images of the 2 groups were all more than 0.99. Table II Means, standard deviations, absolute errors (AE), and the mean percentage measurement errors CBCT measurements 0.4 voxel CBCT measurements 0.25 voxel Physical measurement (mm) T1 T2 T3 AE T1 T2 T3 AE Measurement Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD 1 26.14 5.43 26.09 5.32 26.09 5.33 26.08 5.35 0.09 0.02 26.17 5.43 26.13 5.42 26.13 5.41 0.02 0.01 2 24.52 2.96 24.45 3.02 24.43 3.00 24.42 2.97 0.05 0.05 24.44 2.92 24.39 2.85 24.44 2.88 0.10 0.03 3 23.73 2.84 23.73 2.84 23.63 2.70 23.68 2.82 0.14 0.03 23.72 2.81 23.73 2.77 23.73 2.81 0.00 0.01 4 10.27 0.94 10.15 0.93 10.10 0.96 10.13 0.93 0.06 0.04 10.07 0.96 10.07 0.96 10.07 0.99 0.20 0.00 5 27.41 2.64 27.36 2.59 27.51 2.57 27.43 2.52 0.03 0.04 27.50 2.53 27.50 2.52 27.49 2.49 0.09 0.01 6 10.65 0.64 10.63 0.66 10.58 0.71 10.65 0.71 0.05 0.01 10.62 0.68 10.63 0.71 10.57 0.67 0.04 0.03 7 28.69 2.19 28.64 2.11 28.63 2.09 28.64 2.09 0.13 0.04 28.74 2.05 28.72 2.12 28.71 2.06 0.03 0.02 8 30.61 2.31 30.45 2.13 30.46 2.10 30.52 2.15 0.01 0.01 30.56 2.10 30.51 2.11 30.52 2.09 0.08 0.03 9 29.16 2.51 29.17 2.43 29.15 2.33 29.18 2.38 0.01 0.02 29.25 2.32 29.29 2.35 29.24 2.34 0.10 0.03 10 36.51 3.94 36.55 3.86 36.51 3.84 36.51 3.82 0.01 0.00 36.50 3.85 36.50 3.87 36.47 3.89 0.02 0.02 11 11.10 0.29 11.09 0.30 11.11 0.28 11.09 0.28 0.02 0.01 11.02 0.27 11.09 0.30 11.07 0.31 0.04 0.04 12 37.85 4.06 37.88 4.14 37.86 4.10 37.84 4.16 0.08 0.01 37.73 4.18 37.72 4.10 37.72 4.15 0.13 0.01 13 39.47 3.59 39.39 3.69 39.38 3.66 39.39 3.67 0.05 0.02 39.33 3.66 39.33 3.64 39.32 3.67 0.14 0.01 14 38.04 3.73 38.08 3.69 37.99 3.71 37.97 3.73 0.07 0.03 37.93 3.75 37.94 3.69 37.90 3.74 0.12 0.02 15 28.49 2.38 28.44 2.32 28.42 2.39 28.39 2.42 0.15 0.02 28.45 2.42 28.47 2.40 28.50 2.37 0.02 0.02 16 10.12 1.38 9.98 1.34 9.99 1.36 9.95 1.31 0.06 0.02 9.94 1.36 9.96 1.36 9.97 1.36 0.16 0.02 17 29.45 2.71 29.53 2.62 29.51 2.61 29.50 2.66 0.01 0.01 29.49 2.59 29.53 2.70 29.57 2.64 0.08 0.04 18 31.88 2.43 31.88 2.33 31.86 2.35 31.86 2.37 0.03 0.02 31.88 2.37 31.95 2.40 31.94 2.36 0.04 0.04 19 29.67 2.33 29.71 2.26 29.70 2.28 29.66 2.32 0.04 0.02 29.64 2.23 29.66 2.28 29.68 2.27 0.02 0.01 20 20.67 3.88 20.64 3.90 20.61 3.89 20.65 3.90 0.01 0.01 20.64 3.87 20.56 3.86 20.56 3.87 0.08 0.05 21 22.38 4.56 22.39 4.52 22.40 4.48 22.39 4.51 0.01 0.02 22.48 4.54 22.42 4.54 22.37 4.56 0.05 0.05 22 25.57 4.72 25.60 4.69 25.57 4.73 25.58 4.71 0.04 0.02 25.59 4.77 25.56 4.78 25.60 4.75 0.02 0.01 23 94.53 2.75 94.50 2.76 94.51 2.79 94.47 2.79 0.02 0.01 94.58 2.72 94.56 2.73 94.53 2.73 0.03 0.03 24 86.49 4.88 86.52 4.82 86.50 4.85 86.50 4.88 0.04 0.04 86.56 4.82 86.52 4.80 86.49 4.82 0.03 0.04 25 72.83 3.76 72.79 3.75 72.75 3.69 72.83 3.77 0.09 0.02 72.78 3.74 72.76 3.75 72.74 3.74 0.07 0.02 Mean absolute measurement error 0.05 0.07 0.05 Mean percentage measurement error 0.25 0.33 0.46 T, Scanning session. Measurements of the CBCT images and the digital caliper showed excellent intraoperator reliability, with ICC values of 1.00. The SDD values calculated to determine the accuracy of the CBCT measurement procedure were 0.03 mm for the 0.40-voxel group and 0.02 mm for the 0.25-voxel group. The SSD value for the direct caliper measurements was 0.03 mm. The mean values and standard deviations of the reference values (anatomic truth) and the CBCT measurements for the 0.40 and 0.25 voxel sizes are summarized in Table II . The CBCT values had a tendency to underestimate the reference values. This occurred in 61.3% of the measurements for the 0.40-voxel group (0.06 ± 0.05 mm) and in 60% of the measurements for the 0.25-voxel group (0.08 ± 0.06 mm). However, the measurements were overestimated for 29.3% of the measurements for the 0.40-voxel group (0.03 ± 0.02 mm) and in 33.3% of the measurements for the 0.25-voxel group (0.06 ± 0.03 mm). Discussion This study was performed to establish the accuracy of the CBCT-derived surface models and to investigate the possible influence of voxel resolution on the accuracy thereof. We used the 3D eXam CBCT scanner and the SimPlant Ortho Pro software to produce the surface models. Our results showed that linear measurements made on CBCT surface renderings of 0.40- and 0.25-voxel resolutions are accurate and confirmed the accuracy of CBCT surface models reported in previous studies. Our results justify the use of CBCT-derived surface models for orthodontic and craniofacial treatment planning. There was no difference between the CBCT measurements of the 0.4-mm and 0.25-mm voxel resolution groups compared with anatomic truth. These results confirm the results of Ballrick et al and suggest that 0.4-mm voxel resolution is adequate for measurement of craniofacial structures. The increased voxel resolution did not cause a difference in accuracy of the surface models. Therefore, the benefits of a shorter scanning time (ie, lower radiation exposure and less patient movement) might outweigh the poorer resolution. However, care must be taken when interpreting this result. The diagnostic ability of CBCT images appears to be influenced by voxel size. Liedkte et al investigated simulated external root resorption of tooth roots imaged with voxel sizes of 0.40, 0.30, and 0.20 mm. They concluded that, even though the results from the different voxel sizes were the same, diagnosis was easier at a smaller voxel size of 0.30 or 0.20 mm. Although the benefits of a shorter scanning time satisfy the “as low as reasonably achievable” principle, the risks of misdiagnosis and treatment complications must also be weighed. Therefore, a scanning protocol with a 0.40-mm voxel size might not be suitable for every patient; voxel size should depend on the patient's problems and treatment plan. The mean differences between the CBCT and the caliper measurements were small: 0.05 ± 0.04 mm for the 0.4-voxel group and 0.07 ± 0.05 mm for the 0.25-voxel group. These values were similar to those previously reported in the literature for differences between 3D CBCT renderings and direct caliper measurements. Stratemann et al used chromium balls of 2.4-mm diameter as markers and reported small mean differences (0.00 and 0.07 mm). However, the standard deviations of the mean differences were significantly larger (0.41 and 0.22 mm) compared with our results. Mischkowski et al used prepared holes filled with gutta-percha markers and reported a mean absolute difference of 0.26 mm. In a pilot study, we found that metal and chromium markers caused significant artifacts when rendering the surface models because of scattering, whereas the prepared gutta-perchs markers was not clearly visible on our surface models. These markers were not considered for this study. Hassan et al used anatomic landmarks that resulted slightly larger differences of 0.10 to 0.39 mm between the 3D renderings and the caliper measurements. The mean difference between the rendered 3D surface models and the caliper measurements was less than the relevant error of 0.5 mm postulated by Marmulla et al and less than the voxel size of the image, and can therefore not be regarded as clinically relevant for craniofacial measurements. The results show that the CBCT values had a tendency to underestimate the reference values; however, it was not as severe as previously reported. The CBCT values were underestimated for 60.7% of the total measurements in this study, but this is significantly less than the 94.4% reported by Ballrick et al. Lascala et al also reported smaller computer-based linear measurements than direct digital caliper measurements of dry skulls. However, the CBCT measurements in these studies were made on axial, coronal, and sagittal cuts of the 3D image rather than on the 3D surface renderings; this probably accounted for the differences of underestimation. In this study, the SDD was used to determine the accuracy of the measurement procedures. The SDD was proposed as an adequate measure for quantitative and statistically significant difference between measurements. The SDD is expressed in the same unit as the measurement device used and is generalizable to all included facets (observers, techniques, measurement times, repeated measures). The SDD values for the CBCT measurements were small: 0.03 mm for the 0.40-voxel group and 0.02 mm for the 0.25-voxel group. The small SDD values confirmed the accuracy of the measurement procedure we used. The SDD values indicate that the measurement procedure for the surface models was just as accurate as direct caliper measurements. For a statistically significant difference between 2 observations, the difference must be at least the SDD of the measurement procedure. If this is not the case, and the SSD is larger than the reported difference, the difference could be result of inaccuracies in measurement rather than a true difference between the observations. In this study, the CBCT measurement procedure had the power to detect differences of 0.03 mm. If the measurement procedure is less accurate (ie, influenced by the segmentation process and landmark identification error), the SDD will be larger. A large SDD could have a significant effect on the interpretation of the differences between 2 observations, especially when the reported differences are small. The CBCT measurements are accurate because the landmark identification error was reduced by using opaque glass spheres as fiducial markers. Additionally, the spherical glass markers are likely to be less affected by the segmentation process because of their uniform density. The glass spheres we used were produced from soda-lime-silica-glass, the most prevalent type of glass that is commonly used for windows and glass containers (bottles and jars). The main advantage of glass vs metallic markers is that glass markers produce no scattering and artifacts when rendered to surface models ( Fig , C and D ). This is because bone and glass spheres have similar values on the Hounsfield scale. This study showed the technical limits of the CBCT scanner and rendering software, but these might not directly apply to patient care. The mandibles we used did not move and had fiducial markers for measurement; this is not the case with patients. We placed a latex balloon filled with water in the lingual area of the dry mandible to simulate soft-tissue attenuation, a method also used by Brown et al and Periago et al. An alternative method used to simulate soft-tissue attenuation is a water bath, which might be problematic during positioning in the CBCT scanner and might damage the dry skulls. In addition, absorption of water by the dry mandibles could influence measurement accuracy because of expansion of the bone. Neither the balloon filled with water nor the water-bath method equates in either quantity or distribution to patients' soft tissues. Although the water-filled balloon in the lingual area provided some soft-tissue attenuation, the lack of peripheral attenuation material might have allowed for increased contrast of the landmarks. Conclusions Linear measurements on 3D surface models of 0.25- and 0.40-voxel CBCT data sets made with the 3D eXam CBCT scanner and SimPlant Ortho software are accurate when compared with direct caliper measurements. Increasing the voxel resolution from 0.40 to 0.25 mm to construct a 3D surface model did not result in increased accuracy of the CBCT measurements. 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