Effect Of Voxel Size On Accuracy Of Linear Measurement With Cone Beam CT On
Mandibular Implant Sites
Penporn Luangchana1,*, Suchaya Pornprasertsuk-Damrongsri 2,#, Sirichai
Kiattavorncharoen3, and Bundhit Jirajariyavej4
1
Master of science in implant dentistry, Faculty of Dentistry, Mahidol University, Thailand;
2
Department of Oral and Maxillofacial Radiology, Faculty of Dentistry, Mahidol University,
3
Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, Mahidol University,
4
Department of Prosthodontics, Faculty of Dentistry, Mahidol University,
*e-mail; jee.ji@hotmail.com, #e-mail: schdamrongsri@gmail.com
Abstract
Introduction: Linear measurement is necessity for accurate treatment plan in dental implant.
This study was designed to investigate the accuracy of linear measurement made on conebeam computed tomography (CBCT) images and to determine the influence of different
voxel sizes on the linear measurement accuracy of 3D Accuitomo®170.
Methods: Twenty one mandibular implant sites from four dry mandibles were marked with
gutta-percha markers to standardize the transverse cross-sectional plane and path of
measurement. The skulls were imaged with the 3D Accuitomo® 170 (J Morita MFG. Corp.,
Kyoto, Japan) machine using three protocols with different voxel sizes (0.125, 0.160 0.250
mm3). Electronic linear measurement of bone height was recorded from reformatted sections
by two observers using One data viewer program. Physical measurement was directly
recorded from the bone at the same location as the gold standard. All image measurement
was compared with physical measurement to determine its accuracy. The absolute error (AE),
absolute percentage error (APE), intraexaminer reliablility were calculated for each protocol
and compared with each other.
Results: The intraclass correlation coefficient for the overall assessment of the three
protocols and the dry mandible using the digital caliper was more than 0.99. The mean of
absolute error was 0.32±0.10 mm for the 0.125 mm3 voxel group, 0.27±0.13 mm for the
0.160 mm3 voxel group and 0.46±0.17 mm for the 0.250 mm3 voxel group. There was
statistically significant difference between the mean for 0.250 mm3 voxel group and physical
measurement (P = 0.029). For other protocols, there were no statistically significant
differences from physical measurement (P › 0.05).
Conclusions: The linear measurement of 0.125 and 0.160 mm3 voxel CBCT data sets made
with the 3D Accuitomo® 170 machine is significant accurate when compared with physical
measurement. The voxel size associates with the accuracy of linear measurement of CBCT.
Keywords: cone-beam computed tomography, dental implant, accuracy, voxel, linear
measurement
Introduction
Since the dental implants emerged in the 1980s, they have been the preferred
treatment options for edentulism due to their predictable success rates of 94% to 100%.(1-6)
However, successful dental implant treatment is based on complete treatment planning. This
includes the use of radiographs for adequate diagnostic information about the potential
implant site. Accurate evaluation of the quality and quantity of the available bone, angulation
of bone, the precise location of anatomical structures, and verification of the absence of
pathology are necessary for successful treatment.(7-10) As The American Academy of Oral
and Maxillofacial Radiology (AAOMR) holds the position that the success of dental implant
restoration is, in part, dependent on adequate diagnostic information about bony structures of
the oral region. Furthermore radiographic imaging analysis has directly contributed to the
implant’s long-term success.(11)
Cone-beam computed tomography (CBCT) is a new medical imaging technique that
generates 3D images. Richard Robb developed the first CBCT machine in 1982 and was
mainly used for angiography.(12) In 1998, Mozzo et al. presented CBCT machines in
European Radiology for use in craniofacial region.(13) CBCT enhances image quality with
significant reduction of radiation exposure(14, 15) and shorter acquisition scan times, as well
as lower costs compared with conventional computed tomography.(16-18) For implantology,
CBCT dramatically improved the diagnosis and treatment planning because it provides crosssectional reconstruction of the implant site which other imaging or combinations of imaging
techniques cannot provide. These reformatted images allowed three-dimensional evaluation
of vital structures and related oral anatomy.(19-21)CBCT assists the clinician in many
treatment procedures such as pre-implantologic anatomy assessment(22), implant planning,
implant placement(23) , surgical guidance and postimplant and/or post grafting evaluation.
CBCT volumetric data or field of view (FOV) comprise of voxels. In CBCT, the
voxel is isotropic (that is, height, width, and depth are equal) and varies in the same CBCT
device based on the reason for the examination, suspected disease presentation and region of
interest. The smaller the FOV, the smaller the voxel size and the better the spatial
resolution.(24) The small FOV has specific indication in evaluation that requires high
resolution such as accessory canals, radicular fractures or resorption, bone fenestration and
bone height. There are some disadvantages in this protocol which use for implant planning in
extended partial or complete edentulous patient cause it requires many times of exposure
result in high radiation doses. The medium FOV is recommended in oral surgery, TMJ
assessment, study of bone lesions and implantology. The large FOV is the best choice when a
full facial analysis is required.
For implant placement, it is essential to accurately measure the height of bone
available to avoid compromising vital structures such as the inferior alveolar nerve or
maxillary sinus. Use of linear measurements is a necessity for accurate treatment plan in
dental implant. In the preoperative evaluation for implants , measurements are considered to
be acceptable within an measurement error (ME) of 1 mm(25, 26) to 2 mm.(9) A better
understanding of the accuracy of the linear measurements for each of protocol will provide
the best protocol to be used in planning for multiple dental implants.
Thus, the aims of this study were to investigate the accuracy of linear measurement
made on cone-beam computed tomography (CBCT) images and to determine the influence of
different voxel sizes on the linear measurement accuracy of 3D Accuitomo® 170.
Methodology
Preparation of samples
Four human dry mandibles with partially or complete edentulous regions, no damage, no
anomalies or severe resorption were provided by the Anatomy department of Faculty of
Medicine Chiang Mai University, after approval by the institution’s Ethics Committee. Only
edentulous areas were used for linear measurement. Various regions from 4 specimens are
shown in Table 1.
Table 1. Distribution of twenty-one evaluated region from four specimens
No. of Specimen
1
2
3
4
Incisor
( number )
2
2
-
Canine
(number )
2
2
-
Premolar
(number )
2
2
1
1
Molar
(number )
2
2
1
2
The alveolar ridges were flattened to expose areas of bone which would facilitate the physical
sectioning of the bone while maintaining the height of the ridge at the site of sections. The
entire body of the mandible was enclosed by acrylic resin. The acrylic resin was drilled with
round surgical bur at crestal, buccal, lingual and inferior of mandible of incisor, canine and
molar region and was inserted with gutta-percha. For premolar region, the acrylic resin was
drilled with round surgical bur at crestal, buccal and lingual and was inserted with guttapercha.
Evaluated regions
The evaluated regions were analyzed as follow: incisor (1 cm distal from the median sagittal
plane), canine (1 cm distal from the incisive region), premolar (at the mental foramen), molar
(1 cm from the mental foramen distal), denominated I, C, PM and M respectively. (Table 2)
Table 2. Evaluated region for CBCT and physical measurement
Evaluated region
Description
I (incisor)
1 cm distal from the median sagittal plane
C (canine)
1 cm distal from the incisive region
P (premolar)
at the mental foramen
M ( molar)
1 cm distal from premolar region
Imaging of the jaws
The mandible was immobilized with the mandibular plane parallel to the horizontal plane, as
recommended by the CBCT patient positioning protocol. The CBCT study was performed
with a 3DAccuitomo® 170 machine (3D Accuitomo, J Morita MFG. Corp., Kyoto, Japan) as
recommended by the manufacturer. Each mandible was scanned using three different voxel
sizes: 0.125, 0.160 and 0.250 mm3 (protocol 1, 2, 3 respectively). The images were processed
with i-Dixel imaging software, respectively.
Measurement
The four gutta-percha markers with equal in same width for incisor, canine and molar region
were chosen. For premolar region, the three gutta-percha markers with equal in same width
were chosen. The linear measurement was made directly on the image using the linear
measuring tools of the One data viewer software. The path of measurement of each evaluated
region is shown in Figure 1. The measurement was recorded directly from the computer
monitors. The images were viewed on identical liquid crystal display (LCD) monitors with
resolution 2560 × 1600 (EIZO® RX430, EIZO Nanao Corp., Ishikawa, Japan). Measurement
was recorded twice by two observers. Both observers were blinded each other and to any
protocols.
Figure 1. The path of height measurement on transverse cross-sections. a) Incisor region (I) and canine
region (C), b) Premolar region (P) , c) Molar region (M)
After imaging, the mandibles were sectioned using a diamond disc to obtain
transverse cross-sections of the jawbones at the sites of the gutta-percha markers. Only the
bone sections that consist of the innermost portion of the four gutta-percha markers were
included. The path of measurement were marked on the bone with a pencil, and the direct
bone measurement was recorded using a digital caliper (Mitutoyo Absolute Digimatic
Caliper, Mitutoyo Corporation, Kawasaki, Japan) with 0.01 mm resolution and ± 0.02 mm
accuracy. All the measurement was recorded twice by the two observers. The observers were
blinded each other and blinded to which bone sections corresponded to the previously
examined image.
Statistical analysis
The accuracy of measurement was expressed as by means and standard deviation of
the absolute error (AE) and absolute percentage error (APE). (27) Absolute error and absolute
percentage value were calculated with the following equation:
Absolute Error (AE) = |Physical measurement – CBCT measurement|
Absolute Percentage Error (APE) = |Physical measurement – CBCT measurement|
Physical measurement
Intraexaminer reliability was evaluated with the intraclass correlation coefficient
(ICC) and confirmed by the calculation of Cronbach’s alpha.
To determine the linear accuracy of the measurement procedures (direct caliper and
CBCT measurement), the Friedman, Kruskal-Wallis, and Wilcoxon testes were used to
compare the mean between each protocol and physical measurement by a standard statistical
software package (version 17, Spss, Chicago, I11). Results with a P-value < 0.05 were
considered to be statistically significant.
Results
Two hundred and fifty two measurement was performed, 84 for each protocol and 84
measurement on the dry mandible (gold standard). Height measurement was recorded at 21
sites.
Accuracy of the measurement was determined by the AE and APE. The mean of
absolute error was 0.02 to 0.99 mm (0.32±0.10 mm) for the 0.125 mm3 voxel group, 0.04 to
0.82 mm (0.27±0.13 mm) for the 0.160 mm3 voxel group, 0.03 to 1.08 mm (0.46 ±0.17 mm)
for the 0.250 mm3 voxel group. The APE values were 2.39% for the 0.125 mm3 voxel group,
1.78% for the 0.160 mm3 voxel group and 3.25% for the 0.250 mm3 voxel group. (Table 3)
Table 3. Absolute errors (AE) and absolute percentage errors (APE) from 0.125, 0.160 and 0.250 mm 3 voxel
groups
Physical measurement
(mm)
Measurement
areas
Mean
SD
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
26.88
27.62
12.16
9.85
26.89
29.85
14.31
11.44
23.33
24.9
12.71
13.38
23.14
23.68
10.41
10.39
10.35
7.32
18.05
15.98
16.41
0.035
0.02
0.145
0.015
0.07
0.01
0.01
0.065
0.02
0
0
0.095
0.01
0.02
0.01
0.03
0.02
0.04
0.15
0.025
0.07
Mean of AE and APE
0.125-voxel group
AE
Mean SD
APE
(%)
0.160-voxel group
AE
Mean SD
APE
(%)
0.250-voxel group
AE
Mean SD
APE
(%)
0.16
0.15
0.36
0.17
0.22
0.12
0.13
0.21
0.12
0.02
0.44
0.06
0.05
0.20
0.40
0.99
0.71
0.24
0.46
0.59
0.87
0.03
0.18
0.01
0.04
0.20
0.13
0.06
0.04
0.22
0.07
0.09
0.04
0.17
0.17
0.07
0.14
0.01
0.06
0.25
0.13
0.04
0.60
0.53
2.98
1.70
0.82
0.40
0.89
1.86
0.53
0.08
3.46
0.43
0.22
0.84
3.87
9.51
6.81
3.21
2.52
3.71
5.30
0.34
0.13
0.18
0.04
0.82
0.29
0.40
0.20
0.04
0.52
0.49
0.47
0.06
0.19
0.08
0.14
0.58
0.27
0.11
0.37
0.05
0.07
0.06
0.04
0.06
0.59
0.15
0.40
0.01
0.07
0.08
0.06
0.02
0.15
0.08
0.07
0.19
0.10
0.08
0.07
0.22
0.18
1.27
0.45
1.48
0.38
3.05
0.97
2.78
1.73
0.19
2.07
3.84
3.51
0.25
0.78
0.72
1.37
5.6
3.66
0.6
2.33
0.34
0.15
0.17
0.03
0.42
1.08
0.44
0.20
0.26
0.10
0.87
0.87
0.64
0.09
0.30
0.08
1.04
0.65
0.68
0.66
0.85
0.11
0.10
0.07
0.01
0.21
0.03
0.13
0.16
0.17
0.49
0.07
0.12
0.06
0.12
0.07
0.04
0.24
0.11
0.95
0.16
0.09
0.14
0.56
0.62
0.25
4.27
4.03
1.48
1.4
2.27
0.43
3.5
6.87
4.76
0.39
1.26
0.79
10.04
6.3
9.33
3.63
5.34
0.69
0.32
0.10
2.39
0.27
0.13
1.78
0.46
0.17
3.25
The intraexaminerreliability was very high for the overall assessment of the three
protocols (varying from 0.999 to 1). The ICC for the measurement on the dry mandible using
the digital caliper was 1.
There was statistically significant difference between the mean for 0.250 mm3 voxel
group and physical measurement (P= 0.029). For other protocols, there were no statistically
significant differences from physical measurement (P › 0.05).
a
b
c
Figure 2. Example of reformatted cross-sectional images obtained from cone beam CT at canine region
with different voxel sizes: a) a voxel of 0.125 mm3, b) a voxel of 0.160 mm3, c) a voxel of 0.250 mm3
a
b
c
Figure 3. Example of reformatted cross-sectional images obtained from cone beam CT at molar region
with different voxel sizes: a) a voxel of 0.125 mm3, b) a voxel of 0.160 mm3, c) a voxel of 0.250 mm3
The CBCT values had a tendency to underestimate the direct bone measurement
(physical measurement). This occurred in 90.48% of the measurement for the 0.125 mm3
voxel group, 71.43% of the measurement for the 0.160 mm3 voxel group and 76.19% of the
measurement for the 0.250 mm3 voxel group. On the other hand, the measurements were
overestimated for 9.52% for the 0.125 mm3 voxel group, 28.57% for the 0.160 mm3 voxel
group and 23.80% for the 0.250 mm3 voxel group. (Table 4)
Table 4. Frequency and percent of CBCT measurement errors according to < 0.5 mm or 0.5 - 1 mm
underestimation or overestimation
Voxel group
0.125
0.160
0.250
Number of sites(%)with
underestimation
< 0.5 mm
0.5 – 1 mm
14 (66.67%)
5 (23.81%)
13 (61.90%)
2 (9.52%)
9 (42.86%)
7 (33.33%)
Number of sites (%)with
Overestimation
< 0.5 mm
0.5 – 1 mm
2 (9.52%)
0 (0%)
5 (23.81%)
1 (4.76%)
3 (14.29%)
2 (9.52%)
Discussion
Radiological evaluation is necessary for implant placement especially an adequate
information on quantity, quality of bone available and the anatomical structures. The
measurement error is generally required to be less than 1mm on images made for implant
treatment(28). All the measurement error from this study was found to be less than 1 mm.
Therefore, the measurement from 3DAccuitomo® 170 machine is sufficiently accurate for
dental implant clinical use. As for the CBCT sections where an absolute error of more than
0.5 mm were found, almost all were associated with an unsharp or unclear anatomical
boundary. It should be noted for our study that the inaccurate of CBCT measurement may
relevant with the landmark identification error such as inferior alveolar nerve or unclear
crestal ridge. Grinding of the crestal ridge during preparation of the samples may lead to
difficulty in localization of the bone margin because the edentulous ridge is not covered by
compact bone.
The selected mandibular sites for this study and the path of height measurement were
the same situation in clinic for implant treatment planning. For mandibular premolar and
molar regions, the superior borders of inferior alveolar canal were used as reference points.
These points were not as clearly seen as those of incisor and canine sites which used the
inferior borders of mandible as reference points. Further studies are needed to evaluate the
accuracy between various implant sites.
In our study, the accuracy of vertical measurement using CBCT in 0.125, 0.160 and
0.250 mm3 voxel group was compared with the measurement performed on the dry mandible.
There was statistically significant difference only the 0.250 mm3 voxel group, which is the
largest voxel size of this machine. The mean of the CBCT absolute error value was shown
maximum in the same group (0.250 mm3 voxel group). This finding is in agreement with the
results of Maret et al.(29) They compared two isotropic voxel size (0.2 and 0.3 mm3) and
found slightly underestimated which statistically significant only 0.3 mm3 voxel group. They
concluded that the accuracy of CBCT connected to the size of voxels. Baba et al.(30) also
concluded that the refined isotropic resolution is the factor that lead to satisfactory results
regarding accuracy. Although, the study of Marianna et al.(31) which compared 0.2, 0.25, 0.3
and 0.4 mm3 voxel to direct measurement on human mandibles found no statistical significant
difference between them ( P = 0.606 ).
The mean of the CBCT absolute error value was shown minimum in 0.160 mm3 voxel
group. A considerable dose reduction without a loss of diagnostic information is the most
suitable protocol for multiple dental implant planning in clinical practice. Thus, we
recommend that the protocol which used 0.160 mm3 voxel was preferable in the evaluation of
the linear measurement as in multiple implant planning.
From our study, the CBCT values had a tendency to underestimate the direct bone
measurement; however, it was not as much as previously reported. The CBCT values were
underestimated for 79.36% of the total measurement in our study, but this is significantly
less than that (94.4%) reported by Ballrick et al.(32) However, underestimation has been
shown to be safer clinically compared with overestimation because it may preserve vital
structures when placing the dental implants. The high intraexaminer reliability obtained in
this study for all protocols indicates the reliability of the result.
There are many alternative methods to simulate soft-tissue attenuation. For example,
the water bath, the latex balloon filled with water placed in the lingual area of the dry
mandible and the acrylic. A water bath might be problematic during positioning in the CBCT
scanner and damage the dry skull from absorption of water by the dry mandibles. This
absorption may influence measurement accuracy because of expansion of the bone. A latex
balloon filled with water placed in the lingual area of the dry mandible is used in some
studies. Nevertheless, it still lacks of peripheral attenuation material. Thus, we used the
acrylic which are able to simulate soft-tissue attenuation surrounding the samples.
Conclusions
The linear measurement of 0.125 and 0.160 mm3voxel CBCT data sets made with the
3DAccuitomo® 170 machine is significantly accurate when compared with physical
measurement. The voxel size associated with the accuracy of linear measurement of CBCT.
However, the linear measurement from 3DAccuitomo® 170 machine in our three protocols is
sufficiently accurate for mandibular implant sites based on the requirement that the
measurement error less than 1 mm is acceptable. Thus, we recommend that the protocol
which uses 0.160 mm3 voxel is preferable in the evaluation of the linear measurement as in
multiple implant planning because of its best accuracy for linear measurement.
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