Download Image quality vs radiation dose of four cone beam computed

Dentomaxillofacial Radiology (2008) 37, 309–318
’ 2008 The British Institute of Radiology
http://dmfr.birjournals.org
RESEARCH
Image quality vs radiation dose of four cone beam computed
tomography scanners
M Loubele*,1,2, R Jacobs2,3, F Maes1, K Denis4, S White5, W Coudyzer6, I Lambrichts7,
D van Steenberghe2,3 and P Suetens1
1
ESAT-PSI, Katholieke Universiteit Leuven, Belgium; 2Perdiodontology, Katholieke Universiteit Leuven, Belgium; 3Oral Imaging
Center, Katholieke Universiteit Leuven, Belgium; 4EMAP, Xios Hogeschool Limburg, Belgium; 5School of Dentistry, University of
California at Los Angeles, USA; 6Radiology, UZ Leuven, Belgium; 7Morphology Section, Universiteit Hasselt, Belgium
Objectives: To evaluate image quality by examining segmentation accuracy and assess
radiation dose for cone beam CT (CBCT) scanners.
Methods: A skull phantom, scanned by a laser scanner, and a contrast phantom were used
to evaluate segmentation accuracy. The contrast phantom consisted of a polymethyl
methacrylate (PMMA) cylinder with cylindrical inserts of air, bone and PMMA. The
phantoms were scanned on the (1) Accuitomo 3D, (2) MercuRay, (3) NewTom 3G, (4) iCAT and (5) Sensation 16. The structures were segmented with an optimal threshold.
Thicknesses of the bone of the mandible and the diameter of the cylinders in the contrast
phantom were measured across lines at corresponding places in the CT image vs a ground
truth. The accuracy was in the 95th percentile of the difference between corresponding
measurements. The correlation between accuracy in skull and contrast phantom was
calculated. The radiation dose was assessed by DPI100,c (dose profile integral 100,c) at the
central hole of a CT dose index (CTDI) phantom.
Results: The results for the DPI100,c were 107 mGy mm for (1), 1569 mGy mm for (2),
446 mGy mm for (3), 249 mGy mm for (4) and 1090 mGy mm for (5). The segmentations in
the contrast phantom were submillimeter accurate in all scanners. The segmentation accuracy
of the mandible was 2.9 mm for (1), 4.2 mm for (2), 3.4 mm for (3), 1.0 mm for (4) and
1.2 mm for (5). The correlation between measurements in the contrast and skull phantom
was below 0.37 mm.
Conclusions: The best radiation dose vs image quality was found for the i-CAT.
Dentomaxillofacial Radiology (2008) 37, 309–318. doi: 10.1259/dmfr/16770531
Keywords: cone beam computed tomography; image quality; radiation dose
Introduction
During the last decade, there has been a trend towards
using three-dimensional (3D) information to assist
dentomaxillofacial diagnostics and surgical planning.1
This could first be realized by the use of conventional
single and later multislice CT (MSCT).2 Nevertheless,
because conventional CT protocols are generally
associated with relatively high radiation dose levels,3
alternative CT protocols for bone visualization and
modelling that would allow the lowering of the effective
*Correspondence to: Reinhilde Jacobs, Centrum voor Orale Beeldvorming,
Kapucijnvoer 7 blok a bus 7001, 3000 Leuven, Belgium; E-mail:
[email protected]
Received 11 June 2007; revised 7 October 2007; accepted 12 October 2007
radiation dose for the patient without significant loss of
image quality are being investigated. Examples of these
protocols are modified protocols on MSCT scanners4
or the use of cone beam CT (CBCT).5,6
Recently, an impressive number of CBCT scanners
have been introduced in the field of dentomaxillofacial
radiology. Unfortunately, hardly any research-based
evidence is available for either optimal image quality or
radiation dose levels. Besides the development and
research of clinical evaluation protocols of these
scanners,7,8 it is essential to develop a technical test
protocol aiding optimization of the individual scanner
parameters according to the ALARA (as low as
reasonably achievable) principle.9 Such optimization
Image quality and radiation dose of CBCT
M Loubele et al
310
procedures can only be approached by a combined
research strategy of radiation dose assessment for
various image quality levels. A technical method for
the evaluation of radiation dose in MSCT is the CT
dose index (CTDI).10 Because of the shortcomings of
these measurements,11 Mori et al12 suggested the use of
the dose profile integral (DPI). The advantage of such a
DPI is efficient data acquisition. The link to the
effective patient dose can then be established by
correlating effective dose levels with those technical
measures.13 For the evaluation of the image quality,
analysis on physical8,14 as well as anthropomorphic
phantoms8 can be used.
This paper is based on a combined research strategy
reported in two of our previous studies.15,16 In the first
study, a method was developed which made it possible
to evaluate the bone segmentation accuracy based on a
ground truth acquired with a laser scanner.15 In the
second study, the physical image quality was compared
with the radiation dose for four different CBCT
scanners and one MSCT scanner.16 In the present
study, the evaluation of the image quality on an
anthropomorphic phantom and a physical phantom is
combined with the radiation dose for four CBCT
scanners and one MSCT scanner.
Materials and methods
Radiation dose assessment
The radiation dose was measured by means of the DPI100,c,
Equation (1), measured at the central hole of a dedicated
CTDI-phantom. For the i-CAT (Imaging Sciences
International, Hatfield, PA) and the NewTom 3G
(Quantitative Radiology, Verona, Italy) the DPI100,c was
measured with an electrometer and an ion chamber with an
active length of 10 cm, calibrated for measurements in CT
scanners (Radcal Corporation, Monrovia, CA). For the
measurement of the DPI100,c on the Accuitomo 3D
(Morita, Kyoto, Japan) and the MercuRay CB (Medico
Technology Corporation, Kashiwa, Japan), there was no
ion chamber available on site and therefore the DPI100,c
was measured with strips of thermoluminescent dosemeters (TLD) of type TLD 100 (Li:Mg:Ti) (Bicron, Solon,
OH) placed 1.5 cm from each other. The TLDs for the
Accuitomo 3D were read out on a fully automated
Harshaw 6600 reader (Bicron) in our hospital. The TLDs
for the MercuRay were read out with a fully automated
Harshaw 8800 Card Reader Workstation (Bicron). The
DPI100,c was then calculated by approximation of the
integral in (1) using the trapezium rule.
Figure 1 The volume rendering of the laser model of mandible is
shown
based on the CTDI values as listed by ImPACT, London,
UK (see ImPSCT website http://www.impactscan.
org/ctditables.htm/). After appropriately adapting the
listed CTDI values to the selected tube current settings of
each evaluated MSCT protocol and correcting for
collimation as described on the ImPACT website, the
DPI was calculated by multiplying the adapted CTDI
value with a length of 60 mm, which was sufficient to
acquire the complete mandible. In this way the radiation
dose was truncated corresponding to the approach for
the dose measurements in CBCT.
ð1Þ
Image quality assessment
For the evaluation of the image quality of CBCT and
MSCT, we applied the method previously reported for
observer measurements.17,18 We therefore measured the
thickness of different structures in the CBCT images in
an automated way. By comparing the measured
thickness of the structures in the CT images with the
real thickness of the structures, the accuracy of the
measures can be determined. Because it is important to
have an upper boundary for the accuracy measurements, the 95th percentile of the difference between the
thickness measurements on the CT image and the
ground truth image can be used. This procedure was
applied on both a skull phantom and a physical
phantom (contrast phantom) belonging to the image
quality kit of the Accuitomo 3D. The skull phantom
was used to mimic the diagnostic situation. The test on
the contrast phantom was used to see whether
information about diagnostic quality could be learned
based on a physical phantom. In the sections below, the
complete procedure for the evaluation of the image
quality will be further elaborated.
For the technical evaluation of the radiation dose on
the MSCT scanner, the DPI100,c value was calculated
Skull phantom
For the study, the dry skull from a person who donated
his body to research and which was kindly provided by
l
ð2
DPIl,j ~
d ðzÞdz ½mGy:mm
{12
Dentomaxillofacial Radiology
Image quality and radiation dose of CBCT
M Loubele et al
the Department of Morphology, University of Hasselt
(Belgium) was used for the construction of this
phantom.15 The skull was cut into four parts: the
mandible, the calvarium, the left zygoma and left part
of the maxilla, and the right zygoma and part of the
maxilla. The four different sections were scanned with
an XC50 Cross Scanner (Metris, Leuven, Belgium) with
three laser planes, mounted on a Wenzel LH57 3D
coordinate measurement machine. With this laser
scanner it is possible to obtain surfaces in the form of
a point cloud (Figure 1) with an accuracy of 15 mm.
Because the skull was cut into four sections, it was
possible to acquire the outer and inner bone surface in
one acquisition, allowing the measurement of bone
thickness along the point cloud. After acquiring the
laser scan, the skull was placed in a head mould
enclosure in the form of a face filled with water for softtissue simulation. For further analysis of the image
quality, the segmentation accuracy of the mandible will
be evaluated.
a
b
(mm)
c
Figure 2 Linear measurements are used for image quality assessment. Measurement lines are defined on (a) the computer model of the
phantom and transferred to (b) the CT image. After proper
registration between them, (c) the one-dimensional CT intensity
profile along each line is extracted by interpolation and segmented by
thresholding in order to measure the thickness of the structure of
interest
311
Registration of point cloud to CT data
To assess accuracy, bone thickness measurements on
CT are compared with those of the ground truth. To
achieve a reliable assessment technique, there should be
absolute agreement regarding the position to be
measured on CT and the laser model. An accurate
registration of the point cloud to the CT data was
therefore needed. For this registration, the MIRIT
Software19 was used. This software calculates a sixparameter rigid transformation T (i.e. a combination of
a 3D translation and a 3D rotation), mapping every
location in a floating image, A, to a corresponding
location in the reference image B by maximizing the
statistical dependence between intensity values of
corresponding voxels in both images. A point cloud
exists of a set of 3D coordinates which represent a
surface and not a volume. This means that a point
cloud does not contain voxels. Therefore, a function
was needed, indicating when a specific point was
located inside or on the bone surface and when it
was located outside the bone surface. This function was
calculated based on the FastRBF Interpolation
Toolbox (FarField Technology Limited, Christchurch,
New Zealand), which computes an implicit function
through the point cloud of the laser data in such a way
that this function evaluates to zero on the surface,
negative inside the surface and positive outside. The
function was evaluated on a cubical grid in which the
complete mandible was situated. This grid had a voxel
size of 0.1 mm. As a result, the discretization of the grid
did not influence the registration accuracy.20
Definition of quality measure
For the evaluation of the CT image quality, a measure
needs to be defined. In this paper, the error on the bone
thickness measured on the bone model will be used as the
quality measure, as was done in a previous study.15,16 To
evaluate this error, measurement lines are defined along
the bone surface in the laser model. These measurement
lines are transferred to the CT image based on the
transformation calculated in the previous section. For the
definition of the measurement lines, a cylindrical grid
with the vertical axis through the centre of mass of the
object as z-axis was calculated. This is the central axis of a
cylindrical grid defined by a vertical increment Dz and an
angular increment Dh. In all elements (iDz, jDu) of the
cylindrical grid where bone exists, a measurement line is
defined by two points on the bone surface along a radial
through this element (one point nearest to the z-axis and
one point farthest to the z-axis). With this method, 3589
measurement lines were found. Across these measurement lines, 1D intensity profiles are calculated by a 3D
trilinear interpolation of the image intensity at equidistant points between the beginning and the end point of
each line. The sample distance was 0.1 mm. The
intersections of each measurement line with the bone
surface are extracted by thresholding of its 1D intensity
profile by using a global bone threshold. Linear
interpolation of the profile values is used to locate
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Image quality and radiation dose of CBCT
M Loubele et al
312
candidate intersection points at sub-voxel precision. If
more than two candidates are found, which is typically
the case when the bone consists of two cortical plates
surrounding an inner spongiosa, the locations that are
retained are those closest to the reference bone surface
derived from the laser data.
cylinders with a diameter of 10 mm consisting of
aluminum, PMMA, bone equivalent plastic and air
(Figure 2a,b)). Based on the phantom, image-based
measurements of the diameters of the cylinders at
various positions were also performed in a similar way
to those for the skull phantom. Because the different
parts of this phantom exist as homogeneous materials,
the threshold value was calculated as the average of the
mean intensity of the bone/aluminium and PMMA.
The evaluation procedure is briefly depicted in
Figure 2. For a more elaborate explanation, refer to
the previous study.16
Calculation of threshold value
If a human observer measures bone thickness on a CT
image, they delineate the bone in their mind by seeking the
transition between soft-tissue and bone. Such delineation
is performed by the computer using a segmentation
algorithm. The easiest segmentation algorithm to use is
the use of a global threshold value. This means that a
single threshold value is used to segment the whole object
everywhere in the image. Some pilot tests were performed
at the start of the study, and the results of this test showed
that the rule applied by Wiemker and Zwartkruis21
performed best, therefore this rule was used. They showed
that the transition from a region to segment and their
background corresponds to a local optimum in the
cumulative Laplace-weighted histogram. In some cases
an optimum of the surface, the mean gradient, the volume
or the sphericity histogram can give more information to
find the ideal threshold value. For the calculation of the
histograms, a cuboid region was indicated on the 3D
images of the CBCT or MSCT images. The materials
present in this cuboid region were air, water and bone. To
find the maximum which corresponds to the transition
between bone and soft tissue on the cumulative histogram
of the Laplacian, first the intensity values which
correspond to soft tissue and bone needed to be selected.
This intensity region was found by inspection of the
histogram of the image intensities.
Experiments
Dose measurements and image quality assessment were
performed in one MSCT and four different CBCT
scanners. An overview of the protocols is given in
Table 1. The MSCT scanner was the Somatom
Sensation 16 (Siemens, Erlangen, Germany). The
CBCT scanners were the NewTom 3G, the i-CAT,
the MercuRay and the Accuitomo 3D. When we
initiated this study, the different CBCT scanners were
not available at many sites and hence measurements
needed to be performed at different institutions. The
measurements with the Somatom Sensation 16 and the
Accuitomo 3D were performed at the University
Hospital Leuven (Leuven, Belgium), the measurements
with the NewTom 3G were performed at the UCLA
School of Dentistry (Los Angeles, CA), the measurements with the MercuRay CB at were performed at
SmartScan Imaging (Orange, CA) and the measurements with the i-CAT were performed at Golden State
X-ray Lab (North Hollywood, CA) and Imaging
Sciences International (Hatfield, PA). For all CBCT
scanners except for the MercuRay, the protocols suited
for the planning of oral implants were evaluated. Due
to practical reasons, we could not evaluate the implant
protocol on the MercuRay and therefore the protocol
with the largest diameter was evaluated. A comparison
between the radiation dose of the MercuRay and the
other CBCT scanners would therefore not be fair. It is,
however, possible to estimate the radiation dose of an
MSCT scanner with a similar height as the MercuRay.
Physical phantom
For the evaluation of the image quality based on the
physical phantom, the results acquired for the segmentation of the contrast phantom in a previous study were
used.16 The contrast phantom of the Accuitomo 3D
was used for the evaluation of the physical performance. This phantom is made of polymethyl
methacrylate (PMMA) with an insert of four smaller
Table 1
Overview of the parameters of the evaluated protocols
Scanner
Protocol
Tube potential
(kVp)
Tube
current
(mAs)
Diameter
(mm)
Height
(mm)
Rotation
time (s)
Exposure
time (s)
Voxel size (mm)
Kernel
NewTom 3G
NewTom 3G
NewTom 3G
i-CAT
i-CAT
i-CAT
i-CAT
MercuRay
Accuitomo 3D
Sensation 16
Sensation 16
N1
N2
N3
I1
I2
I3
I4
M1
A1
S1
S2
110
110
110
120
120
120
120
120
80
120
120
22
22
22
11
11
20
40
150
72
90
90
100
100
100
160
160
160
160
193
40
90
90
100
100
100
60
60
60
60
193
30
60
193
36
36
36
10
10
20
40
10
18
–
–
7.2
7.2
7.2
1.92
1.92
3.67
7.19
10
18
–
–
0.1860.1861
0.1860.1860.4
0.1860.1860.4
0.360.360.3
0.460.460.4
0.460.460.4
0.260.2 0.2
0.3860.3860.38
0.1360.1360.5
0.2560.2560.4
0.2560.2560.4
Standard
High
Very high
N/A
N/A
N/A
N/A
N/A
N/A
H60s
H60s
N/A, not applicable
Dentomaxillofacial Radiology
Image quality and radiation dose of CBCT
M Loubele et al
The results of the analysis of the segmentation
accuracy can be found in Table 2. The highest accuracy
for the segmentation of the mandible was found for the
i-CAT (protocols I2 and I3), this accuracy was also
higher than for the Somatom Sensation 16. The lowest
accuracy was found for the MercuRay. The high
accuracy for the i-CAT could be achieved because this
image suffered less from the intensity inhomogeneity
and it was therefore possible to calculate a threshold
value based on the complete mandible. The MercuRay
suffered most from the intensity inhomogeneity and
there was therefore a lower accuracy for the bone
segmentation.
For the segmentation of the cylinder of bone and
aluminium in the contrast phantom, for all protocols an
accuracy better than 1 mm was achieved. The Pearson
correlation coefficient between the accuracy achieved
on the mandible and the bone equivalent plastic was
0.37 and the correlation coefficient between the
accuracy achieved on the mandible and the aluminium
cylinder was 0.03. This means that based on the
analysis of the physical phantom, no conclusion can
be drawn about the segmentation of the mandible.
For the protocols used for oral implant placement,
the DPI100,c was the highest on the Somatom Sensation
16. The highest radiation dose for the CBCT scanners
was for the NewTom 3G and the lowest for the
Accuitomo 3D. The i-CAT had the highest accuracy
combined with a low radiation dose and had as a
surplus a submillimeter accuracy for the segmentation
of the mandible. We can therefore say that this scanner
performed the best on this test.
When the radiation doses of the MercuRay and the
Somatom Sensation 16 are compared, one can see that
the radiation dose levels are similar. Because the
radiation dose of the Somatom Sensation 16 is higher
than the CBCT scanners for placement of oral
implants, one may therefore expect that if a protocol
with a lower height is used on the MercuRay, we will
also have a radiation dose for the MercuRay that is
Therefore a comparison will be made between the
MercuRay and the Somatom Sensation 16.
Data and statistical analysis
For the evaluated protocols on the different scanners,
descriptive statistics were used to express the mean and
standard deviation between the ground truth values
measured on the laser model and the bone thickness
measured on the CT images for all measurement lines.
Positive values indicated an underestimation of the true
thickness; negative values indicate an overestimation.
Also the 95th percentile of the absolute difference
between the ground truth and the measured thickness
was calculated. A similar analysis was performed in the
previous study for the images of the contrast phantom.16 Finally, the Pearson correlation coefficient
between the accuracy achieved on the mandible and
the segmentation of the cylinder in bone and aluminum
were calculated.
Results
The setup for the experiment did not fit in the
Accuitomo 3D and therefore a smaller box was used
for the experiment on the Accuitomo 3D. Typical
histograms, i.e. the histogram of the image intensities
and the cumulative histogram of the Laplacian of the
image, are shown in Figures 3–6 for the different CBCT
scanners. For each image, the region of interest (ROI) is
indicated with highlighted intensities. Due to the
intensity of the grey values in the NewTom 3G and
the MercuRay, several ROIs needed to be tested before
a cumulative histogram of the Laplacian was found
which was suitable for analysis. Such a histogram could
be achieved when only a small region was taken into
account. In the image histograms the intensities which
were taken into account to find the bone threshold are
indicated in a thicker line.
a
313
b
c
Figure 3 (a) A slice of the NewTom (protocol N2) together with a highlighted rectangle representing the region of interest. (b) Represents the
histogram of the image intensities and (c) represents the cumulative Laplacian histogram. The image intensities which are taken into account are
shown in a thicker line
Dentomaxillofacial Radiology
Image quality and radiation dose of CBCT
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314
a
b
c
Figure 4 (a) A slice of the Accuitomo 3D. (b) Represents the histogram of the image intensities and (c) represents the cumulative Laplacian
histogram. The image intensities that are taken into account are shown in a thicker line
similar to Somatom Sensation 16 and thus higher than
the other CBCT scanners
the X-ray beam at two opposite positions. Because of
the larger distance between the source of the X-ray
beam and the object, and considering the limited
collimation of the X-ray beam, the irradiated area will
only be small; therefore, one can expect much of the
scattered radiation to be covered when an ion chamber
with a length of over 10 cm is used. In Figure 7b, a
similar composition is made, but this time for the iCAT scanner. The figure has been made for a protocol
with a height of 13 cm. Because the scanned height is
even longer than the 10 cm of the ion chamber, it is
obvious that not all scattered radiation is covered by
the ion chamber. If we divided the DPI by the slice
thickness, for achieving the CTDI, we could obtain a
completely wrong dose descriptor. In Figure 7c, we give
the configuration for the Accuitomo 3D. For this
scanner, a similar reflection can be made as for the
i-CAT, except that the scanned height here is smaller
than 10 cm. The use of longer phantoms as proposed
by Mori et al12 is not feasible in CBCT scanners for
dentomaxillofacial applications.11
Another problem for the CTDI measurement is the
way that different CTDI measurements are combined.11
For the CTDI measurements in MSCT scanners, the
Discussion
In this paper, the radiation dose and image quality of
different CBCT scanners was compared with the
radiation dose and image quality of an MSCT scanner.
The radiation dose was evaluated by a technical dose
measurement, the DPI100,c measured in the central hole
of a CTDI phantom. Recently, a lot of discussion for
and against the use of the CTDI or another technical
dose measurement has been performed.11,12,22,23 We will
explain two of the shortcomings of the use of the CTDI.
The main difference between the CTDI and the DPI is
that the CTDI is the DPI divided by the slice thickness.
One of the most important shortcomings of the
CTDI is that it is measured over a length of 10 cm,
which is too short to include all the scattered radiation
dose in the protocol of a CBCT scanner. We show this
in Figure 7. Figure 7a represents the configuration of
the Somatom Sensation 16. The contour of the CTDI
phantom is presented on an axial slice, together with
a
b
c
Figure 5 (a) A slice of the MercuRay together with a highlighted rectangle representing the region of interest. (b) Represents the histogram of
the image intensities and (c) represents the cumulative Laplacian histogram. The image intensities which are taken into account are shown in a
thicker line
Dentomaxillofacial Radiology
Image quality and radiation dose of CBCT
M Loubele et al
a
b
315
c
Figure 6 (a) A slice of the i-CAT (protocol N2) together with a highlighted rectangle representing the region of interest. (b) Represents the
histogram of the image intensities and (c) represents the cumulative Laplacian histogram. The image intensities which are taken into account are
shown in a thicker line
weighted sum of the CTDI at the central position and
the average of four peripheral measurements with
weights of one-third and two-thirds, respectively, is
used as global dose estimation. For the dose measurement in this study, only a measurement at the central
hole was performed and not at the peripheral positions
because there currently exists no similar rule to combine
these into one dose estimate. This is intuitively
explained in Figure 7. For the Somatom Sensation 16
(Figure 7a), the cross-section of the X-ray beam with
the phantom is almost rectangular. For the i-CAT
(Figure 7b) and the Accuitomo 3D (Figure 7c), this
cross-section has a conical shape, which depends on the
geometry of the scanner and the protocol which was
used. It is obvious that the contribution of the
dosimetry at the central and peripheral holes in the
phantom does not make the same contribution to the
radiation dose as for the traditional formula of the
CTDI.
Unfortunately, when we performed a study on the
technical dose that was reported in the manuals of the
evaluated CBCT scanners, we noticed that they were
aware of the problem with the CTDI and had therefore
adapted the definition of the CTDI based on their own
opinion. However, we propose that besides the CTDI
the measurement of the DPI100 should be given because
this measurement can be obtained with the same
measurement procedure. The aim of future research in
technical dosimetry will consist of striving for a
correlation between the DPI or a new technical
recording method and the effective radiation dose that
a patient receives. Also, correction factors will be
calculated and tabulated to compensate for scattered
radiation, which was not measured by the ion chamber
of 10 cm, and similar correction factors will be
tabulated to compensate for the geometry of the
scanner.
We assessed the segmentation accuracy of bone
models segmented from CBCT and MSCT images by
comparing them with a model acquired from a laser
scanner. Our method relies on a correct geometric
alignment or registration of the CT images with a
volumetric model with a voxel size of 0.1 mm acquired
from the laser scanner. The voxel size of this model was
defined as smaller than the voxel sizes of the CT images
and the registration accuracy was therefore not
determined by the laser model but by the CT images.20
The availability of an accurate ground truth made it
possible to evaluate some rules for the finding of a
threshold value for bone. It was found that, certainly
Table 2 Results of the analysis of the image quality and the radiation dose
Skull phantom
Contrast phantom
Mandible
Bone equivalent plastic
Aluminium
Protocol
Prct 95
Mean ¡
Prct 95
Mean ¡ SD
Prct 95
Mean ¡ SD
CTDI phantom
DPI100,c
N1
N2
N3
I1
I2
I3
I4
M1
A1
S1
S2
3.5
3.5
3.4
1.4
1.0
1.0
1.1
4.2
2.9
1.2
1.2
20.7¡1.5
20.72¡1.5
20.7¡1.5
0.03¡0.7
20.03¡0.6
20.08¡0.5
20.01¡0.6
20.1¡2.0
0.5¡1.1
0.14¡0.6
0.14¡0.6
0.73
0.59
0.42
0.34
0.6
0.45
0.25
0.42
0.3
0.24
0.24
20.06¡0.17
20.02¡0.13
20.07¡0.16
0.03¡0.2
20.1¡0.19
20.04¡0.24
20.1¡0.08
20.04¡0.38
20.17¡0.09
20.07¡0.06
20.07¡0.06
0.37
0.37
0.4
0.26
1.03
1.07
0.25
1.01
0.3
0.22
0.22
20.06¡0.17
20.02¡0.13
20.07¡0.16
0.05¡0.10
0.08¡0.32
0.12¡0.29
20.19¡0.06
20.04¡0.38
20.48¡0.57
20.07¡0.06
20.07¡0.06
446
446
446
71
71
124
249
1569
107
1090
1677
CTDI, CT dose index; Prct, percentile, SD, standard deviation
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for the i-CAT scanner and the MSCT scanner, the use
of the rule indicated by Wiemker et al21 gave satisfying
results. Unfortunately, this rule gave no accurate results
when the images suffered more from intensity inhomogeneity.
In a second experiment, the image quality was
evaluated based on a physical phantom. Only a small
correlation between the results of the segmentation
accuracy on the mandible and the segmentation of the
cylinders in the contrast phantom was found. Several
reasons can be found for this small correlation. The
contrast phantom is a small phantom, only 5 cm in
diameter and 5 cm in height. This means that except for
the Accuitomo 3D the phantom is positioned completely in the field of view and therefore does not suffer
from the truncated view artefact. Secondly, because the
object is so small, there is less scattered radiation which
can influence the image quality. The influence of the
scanner depends on the geometry of the scanner and the
spectrum of the X-ray tube.24 How the scatter finally
will influence the image quality can be determined by
the reconstruction algorithm which is developed by the
a
b
c
Figure 7 The configuration of the X-ray beam is presented for three different scanners: (a) Somatom Sensation 16, (b) i-CAT and (c) Accuitomo
3D. Each figure shows two positions of the X-ray beam in the opposite position, together with the contours of a CT dose index (CTDI) phantom.
The dashed-dotted line together with the dotted lines indicates the central and the two peripheral holes of the CTDI phantom
Dentomaxillofacial Radiology
Image quality and radiation dose of CBCT
M Loubele et al
CBCT manufacturer.24 Because these different factors
are dependent on each scanner, one cannot predict the
image quality of bone segmentation based on the
segmentation of bone equivalent plastic in the contrast
phantom. This does not mean that physical phantoms
are useless for the evaluation of image quality of CBCT
scanners. These phantoms are very useful for continuous quality control of CBCT scanners. A difference,
for example, in contrast and therefore a deterioration of
segmentation accuracy over time can indicate that there
might be a problem with the CBCT scanner. Also,
different parameters such as, for example, the
Hounsfield units without influence of scattered radiation dose can be calculated.
The benefits of this work over the work of previous
studies17,18 in which the accuracy of linear measurements of CBCT was derived is that the measurements
are performed automatically without much user interaction. Therefore, this method does not suffer from
observer and intraobserver variability. In the current
study, water was used to simulate soft tissue. Water for
the use of soft tissues is not very practical and a solid
water simulating or soft tissue simulating material
should therefore be used for the generation of a skull
phantom. However, the use of solid material instead of
water belongs to a future study. Also in future studies,
different parts of the skull phantom should be
incorporated for the evaluation of the image quality.
Besides this, a better definition of the measurement
lines will also be researched.
The Somatom Sensation 16, the MSCT scanner, had
the highest radiation dose together with the MercuRay.
Similar results for the MercuRay were found by
Ludlow et al.25 The NewTom 3G had a higher
radiation dose than the other CBCT scanners, but a
317
larger region was scanned, so this also should be taken
into account. The lowest radiation dose was achieved
for the Accuitomo 3D, but here the smallest region was
scanned. If, for example, a complete mandible needs to
be imaged, this would require at least three scans. As a
result, the Accuitomo 3D would require the highest
radiation dose. If with the i-CAT a region with a similar
height would be scanned with the parameters of
protocol I4, a similar dose to the NewTom 3G would
be achieved. One can therefore state that the radiation
dose of the i-CAT and the NewTom 3G are similar.
However, the i-CAT has better segmentation accuracy
for a similar radiation dose and therefore the i-CAT
performed best in this study.
Conclusion
A framework for the evaluation of image quality and
radiation dose was presented. The image quality was
evaluated by measuring the segmentation accuracy of
the mandible in a skull phantom and the segmentation
of cylinders in a contrast phantom. The results of the
image quality assessment were accumulated in the 95th
percentile of the absolute difference between the
measured thickness on the CT scanners and the ground
truth thickness. The radiation dose was evaluated by a
technical measure, the dose profile integral. The highest
radiation dose was found for the Somatom Sensation
16 and the MercuRay. The lowest was found for the
Accuitomo 3D, which also covered the smallest image
area. The best segmentation accuracy was found for the
i-CAT. No correspondence was found between accuracy in contrast phantom and skull phantom.
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