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RESEARCH ARTICLE
Evaluation of the effective dose of cone beam CT and multislice
CT for temporomandibular joint examinations at optimized
exposure levels
1,2
N Kadesj¨
o,
1
D Benchimol,
3
B Falahat,
1
KN
¨
asstr¨
om and
1
X-Q Shi
1
Oral Facial Diagnostics and Surgery, Department of Dental Medicine, Karolinska Institutet, Huddinge, Sweden;
2
Medical
Radiation Physics, Karolinska University Hospital, Huddinge, Sweden;
3
Department of Radiology in Huddinge, Karolinska
University Hospital, Huddinge, Sweden
Objectives: To compare the effective dose to patients from temporomandibular joint
examinations using a dental CBCT device and a multislice CT (MSCT) device, both before
and after dose optimization.
Methods: A Promax
®
3D (Planmeca, Helsinki, Finland) dental CBCT and a LightSpeed
VCT
®
(GE Healthcare, Little Chalfont, UK) multislice CT were used. Organ doses and
effective doses were estimated from thermoluminescent dosemeters at 61 positions inside an
anthropomorphic phantom at the exposure settings in clinical use. Optimized exposure
protocols were obtained through an optimization study using a dry skull phantom, where
four observers rated image quality taken at different exposure levels. The optimal exposure
level was obtained when all included criteria were rated as acceptable or better by all
observers.
Results: The effective dose from a bilateral examination was 184 mSv for Promax 3D and
113 mSv for LightSpeed VCT before optimization. Post optimization, the bilateral effective
dose was 92 mSv for Promax 3D and 124 mSv for LightSpeed VCT.
Conclusions: At optimized exposure levels, the effective dose from CBCT was comparable to
MSCT.
Dentomaxillofacial Radiology (2015) 44, 20150041. doi: 10.1259/dmfr.20150041
Cite this article as: Kadesj¨
o N, Benchimol D, Falahat B, N¨
asstr¨
om K, Shi X-Q. Evaluation of
the effective dose of cone beam CT and multislice CT for temporomandibular joint exami-
nations at optimized exposure levels. Dentomaxillofac Radiol 2015; 44: 20150041.
Keywords: temporomandibular joint; cone beam computed tomography; thermoluminescent
dosimetry
Introduction
Diagnosis is crucial in every medical setting and at all
levels of healthcare. Radiographic examination is an
important diagnostic tool to assess morphological and
structural alterations of the osseous components of the
temporomandibular joint (TMJ).
1
The modalities used
to evaluate TMJ bony changes include panoramic ra-
diography, conventional tomography and CT, with
helical or multislice CT (MSCT) or CBCT. MSCT
has been the modality of choice for evaluation of TMJ
osseous changes. However, European guidelines, by
SedentexCT, concluded that CBCT could be considered
as an alternative to MSCT, if radiation dose from
CBCT was shown to be lower.
2
The two main technical differences between MSCT
and CBCT that affect the radiation dose are the de-
tector and the use of volume-of-interest imaging.
Some early CBCT models used image intensifiers but
newer models use flat-panel detectors. Flat-panel
detectors have much smaller detector elements than
Correspondence to: Associate Professor Xie-Qi Shi. E-mail: xie.qi.shi@ki.se
Received 3 February 2015; revised 4 May 2015; accepted 1 June 2015
Dentomaxillofacial Radiology (2015) 44, 20150041
ª2015 The Authors. Published by the British Institute of Radiology
birpublications.org/dmfr
conventional CT detector arrays, which allow for
higher spatial resolution in CBCT images.
3,4
Dental
CBCT units use longer rotation times than conven-
tional CT. This reduces the problems with scintillator
afterglow, thus allowing the use of slower scintillator
materials, such as caesium iodide (CsI), in CBCT.
5
The columnar structure of CsI acts as a light guide,
maintaining high spatial resolution even for thicker
scintillator layers. This allows for detectors with both
high sensitivity and high spatial resolution. Dental
CBCT reconstructs three-dimensional (3D) volumes
with isotropic voxels, usually between 0.1 and 0.3 mm
in size.
6
However, there are some drawbacks with flat-
panel CT, such as reduced low-contrast resolution and
lower detective quantum efficiency than conventional
CT detectors.
3,4
The other major difference between dental CBCT and
conventional CT is how the field of view (FOV) is de-
fined. With medical CT, the diameter of a volume is
fixed, covering a complete cross section of the head, and
the scan length is adjustable, whereas CBCT devices
have several pre-defined FOVs. The X-ray field may be
collimated up to 3–4 cm in diameter in CBCT images,
thus reducing the radiation dose to tissue outside of
the FOV.
For all types of radiographic examinations, the ra-
diation risk, in terms of effective dose to the patient,
and the potential diagnostic benefit are two major
aspects when considering the choice of the image mo-
dality. Most previous dosimetry studies on CBCT have
been performed with FOVs in dentoalveolar or cra-
niofacial region, and there is a lack of scientific reports
regarding radiation dose from TMJ examinations. To
our knowledge, no published study compares TMJ
examinationwithbothCBCTandMSCT,takingboth
effective dose and image quality into consideration.
Since both medical MSCT and dental CBCT are
commonly used for TMJ imaging, the present study
aimed to compare the effective dose of one CBCT unit
and one MSCT unit using their current clinical pro-
tocols for TMJ examination. Furthermore, the image
quality at sequential exposure levels was assessed for
CBCT and MSCT, in order to optimize exposure
levels.
Methods and materials
A Promax
®
3D (Planmeca, Helsinki, Finland) CBCT
was used at 90 kV tube voltage and 8.0-mm aluminium
half-value layer. This CBCT unit uses a 210° scan angle.
For the MSCT unit, a GE LightSpeed VCT (GE
Healthcare, Little Chalfont, UK) 64-slice CT at 120 kV
tube voltage and medium bowtie filter (6.4-mm alu-
minium half-value layer) was used.
Dosimetry
Measurements were performed with TLD-100 thermo-
luminescent dosemeters (TLDs), read with a Harshaw
5500 (Thermo Scientific, Waltham, MA) TLD reader.
The TLDs were calibrated for dose to water using the
in-air method from the American Association of Phys-
icists in Medicine protocol for 40- to 300-kV X-ray beam
dosimetry.
7
AVictoreen
®
Model 550-4-T (Victoreen,
Cleveland, OH) ion chamber, calibrated at the Swedish
Secondary Standard Dosimetry Laboratory, was used for
the cross-calibration.
The effective dose was calculated by multiplying or-
gan doses with the weighting factors from the In-
ternational Commission on Radiological Protection
(ICRP) publication 103, shown in Table 1.
8
An Alder-
son Rando
®
(Alderson Research Laboratories, New
York, NY) adult male anthropomorphic phantom was
used to determine the organ dose. The TLD detectors
were placed at 61 sites within the head and neck region,
two detectors at each site. The mean reading of each
detector pair was used when determining the organ
dose. Consistency between detector readings was eval-
uated by interclass correlation (1,1), using SPSS
®
v. 22
(IBM Corporation, Armonk, NY). The number of
measurement points for each organ is presented in
Table 1. These sites were chosen to provide a good es-
timate of the mean dose to each organ of interest. The
dose contribution to these organs from outside the head
and neck region was assumed to be negligible. For
organs only partially positioned inside the head and
neck region, the measured organ doses were multiplied
with the fraction of that organ which was irradiated, to
obtain the mean organ dose. The fractions of active
bone marrow positioned inside the cranium (7.6%),
Table 1 Mean organ doses, organ-weighting factors and effective dose for the CT and CBCT examinations pre optimization
Organ Weighting factor
8
Dosemeter sites
Organ dose (mGy)
ProMax
®
3D LightSpeed VCT
®
Active bone marrow 0.12 23 215 240
Endosteum 0.01 23 566 621
Brain 0.01 6 1018 1302
Oesophagus 0.04 2 15 20
Extrathoracic airways 0.12/13 7 1355 2349
Lymphatic nodes 0.12/13 17 119 98
Oral mucosa 0.12/13 6 710 1675
Salivary glands 0.01 12 2195 1681
Thyroid 0.04 5 183 234
LightSpeed VCT obtained from GE Healthcare, Little Chalfont, UK; ProMax 3D obtained from Planmeca, Helsinki, Finland.
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mandible (0.8%) and cervical vertebrae (3.9%) were
taken from Cristy.
9
The fractions of endosteum (bone
surface) inside the cranium (16.3%), mandible (0.4%),
cervical vertebrae (2.1%) and the fraction of lymphatic
nodes (6.3%) inside the head and neck region were ap-
plied according to ICRP 110.
10
The fraction of the oe-
sophagus inside the head and neck region was estimated
at 10%. The contribution to the effective dose from the
skin and muscle was considered negligible and was not
included.
Doses to International Committee on Radiological
Units four-component soft tissue were calculated and
used for all organs examined, with conversion factors
taken from the American Association of Physicists in
Medicine protocol for 40–300 kV X-ray beam dosime-
try.
7
For the osteoprogenitor cells, the new definition of
the surrogate tissue was used according to ICRP 110.
10
In addition, the terminology of “endosteum”was ap-
plied instead of the obsolete “bone surface”, according
to ICRP 116.
11
Dosimetry of active bone marrow and
endosteum is complicated owing to the complex ana-
tomical structure inside the spongiosa. In the tissue
close to the trabecular bone, there will be a contribution
of additional electrons from the bone into the endos-
teum and active marrow, resulting in a higher dose. To
account for this increase, we multiplied dose to the soft
tissue with dose enhancement factors calculated by
Johnson et al,
12
as an approximation of the interface
effects. The dose enhancement factors were calculated
by interpolating from the values tabulated for different
energies by Johnson et al and combining these values
with simulated X-ray spectrums for both devices
(Table 2).
For both the CBCT and the MSCT units, the ef-
fective dose was determined based on our clinically
used exposure protocols. For CBCT, the manufac-
turer-recommended settings were used, whereas for
MSCT exposure parameters optimized by the Kar-
olinska University Hospital were used. A lateral
scout image was included for MSCT, and two scout
images, frontal and lateral, were included for CBCT.
The following exposure parameters were used for
Promax 3D: 90 kV tube voltage, 12 mA tube current,
12 s exposure time with a 4 35 cm cylindrical FOV,
resulting in a dose–area product of 606 mGy cm
22
.
For LightSpeed VCT, the following parameters were
used: a helical scan with 120 kV tube voltage, 73 mA
tube current, 0.5 s rotation time, 0.969 pitch with
a scan length of 3 cm, resulting in a volume CT dose
index of 7.42 mGy and a dose–length product of
38.26 mGy cm
21
.
Dose optimization
In order to establish optimized exposure levels, a simple
image quality assessment study was performed at dif-
ferent exposure levels. For this part of the study, a dif-
ferent anthropomorphic phantom was used, comprising
a human dry skull inside simulated soft tissue, shown in
Figure 1. Images of the phantom’s right TMJ were ac-
quired at five levels of tube current, with all other
parameters identical to the dosimetric study. For the
MSCT, tube currents between 90 and 50 mA, with an
interval of 10 mA, were used. Iterative reconstruction
was not used. For CBCT, tube currents between 4 and
12 mA, with an interval of 2 mA, were used. For both
modalities, the sagittal and coronal slices were recon-
structed through the long axis of the condyle. The voxel
sizes were 0.16 mm for CBCT images and 0.293 3
0.293 30.625 mm for MSCT images. All the slices were
eventually viewed with 1-mm thickness in order to be
comparable between the two types of images.
Four dentomaxillofacial radiologists assessed the
image quality in terms of how well they could identify
the intra-articular joint space, the cortical bone and the
trabecular bone of the TMJ, as well as the subjective
experience of noise level in the images. All the ques-
tions were assessed on a one to three scale, with three
being excellent, two being acceptable and one being
unacceptable. The optimized exposure level was de-
fined as the lowest possible level where all observers
rated all four criteria as at least acceptable. Apart from
these optimization criteria, the observers subjectively
rated the overall image quality for each image on the
same one to three scale. Two sets of images, CBCT and
MSCT separated, were randomly displayed; both sets
using the same model of monitor (RadiForce MX191;
EIZO, Hakusan, Japan), with a built-in digital imag-
ing and communications in medicine setting. The
radiographs were evaluated under dimmed room light
and a viewing distance of about 50 cm. The observers
were allowed to adjust the window setting for light
intensity and contrast according to their own prefer-
ences. The CBCT stacks were assessed using the
Romexis
®
software (Planmeca), while the MSCT
images were assessed using the Sectra PACS (Sectra
AB, Link ¨
oping, Sweden).
Table 2 Dose enhancement factors for active bone marrow and endosteum in different bones for the X-ray spectra of the two devices. Calculations
based on the method and simulations of Johnson et al
12
Organ
ProMax
®
3D LightSpeed VCT
®
Active marrow Endosteum Active marrow Endosteum
Cranium 1.216 1.727 1.192 1.671
Mandible 1.017 1.796 1.014 1.747
Cervical vertebrae 1.113 1.736 1.102 1.679
LightSpeed VCT obtained from GE Healthcare, Little Chalfont, UK; ProMax
®
3D obtained from Planmeca, Helsinki, Finland.
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o
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Results
Table 1 shows the mean organ doses, organ-weighting
factors and their corresponding effective dose before
dose optimization for the unilateral CBCT and bi-
lateral MSCT TMJ examination. Interclass correla-
tion was 0.999 and 0.998 for CBCT and MSCT,
respectively. The LightSpeed VCT examination
resulted in higher organ doses for all organs except
the salivary glands and lymphatic nodes, with a 20%
higher effective dose than one Promax 3D examina-
tion. However, if both TMJs should be examined, the
resulting effective dose from Promax 3D would be
184 mSv, which is 60% higher than that from Light-
Speed VCT examination.
By applying our image quality assessment criteria, the
optimized exposure levels were 6 mA for Promax 3D and
80 mA for LightSpeed VCT. Therefore, the estimated
effective dose using the optimized exposure parameters
were 92 mSv for a bilateral Promax 3D examination and
124 mSv for a LightSpeed VCT. Figure 2 demonstrates
the overall assessment of image quality based on four
observers at different exposure levels for CBCT and
MSCT, respectively. At optimized exposure levels, the
rating of the overall image quality by Observers 1–4was
3, 3, 2 and 3 for CBCT and 3, 2, 2 and 2 for MSCT.
Figure 1 Anthropomorphic phantom used for image quality assessment: (a) photograph of the phantom, (b) multislice CT slices at 80 mA and (c)
CBCT slices at 6 mA.
Figure 2 Overall assessment of image quality for multislice (MSCT) and CBCT, based on four observers.
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Discussion
Most of the published studies on effective doses from
CBCT considered the FOVs in the dental alveolar or
large craniofacial region. The present study focuses on
TMJ examinations, irradiating a different area, as well
as using both higher kilovoltage and smaller FOV
compared with most dental studies. Librizzi et al
13
reported the effective dose from bilateral TMJ exami-
nation using a different CBCT device, CB MercuRay
(Hitachi Medical, Twinsburgh, OH), resulting in 550 m
Sv for either one 9-inch FOV or two 6-inch FOVs.
Lukat et al
14
reported an effective dose of 220 mSv from
a CB MercuRay using a 9-inch FOV, and 20 mSv from
a Kodak 9000 3D (Carestream, Rochester, NY) CBCT
using two 5.0 33.7 cm FOVs. The difference between
the effective doses determined for CBCT and MSCT in
the present study, 92 and 124 mSv, respectively, were
minor compared with the very large range of effective
doses from CBCT examinations of the TMJ in the lit-
erature, 20–550 mSv. The large difference in effective
dose from different CBCT models is partly owing to the
FOV used. A wide range of FOVs for different di-
agnostic purposes is important for dose optimization.
For bilateral TMJ examination, we recommend either
a FOV of at least 12 cm width but no more than 5 cm
height, or the use of two small FOVs about 4 34cmin
size. Owing to the lack of suitable FOVs, some CBCT
models might be, from a dose perspective, unsuitable for
TMJ examinations. In the case of CB MercuRay with
9-inch FOV, there is also a large difference in effective
dose between studies, owing to different exposure pro-
tocols being used. Owing to the large range of reported
effective doses and large technical differences between
the CBCT models, dose comparison between CBCT
and MSCT for TMJ diagnostics is complex. Currently,
there is not enough evidence in the literature to declare
that one modality gives lower doses than the other. The
data seem to indicate that dose optimization, in forms
of suitable FOVs and optimized exposure parameters
for various diagnostic tasks, is equally or more impor-
tant than the choice of CT modality.
The authors want to stress the importance of taking
into consideration diagnostic tasks and image quality
when comparing effective doses between different mo-
dalities. Optimization studies have shown a large po-
tential for dose reduction in both dentomaxillofacial
MSCT and CBCT examinations.
15–17
Dawood et al
15
studied the potential for dose reduction from 68 patients
undergoing pre-implant evaluation with CBCT. In their
study, low-dose protocols, down to 12.5% of the man-
ufacturers’standard value, were used with no significant
difference in the surgeons’confidence in judging bone
height and bone width. TMJ diagnosis is more sensitive
to noise than implant planning and would thus be
expected to require a higher dose. The present study
indicates a potential dose reduction of up to 50% for
TMJ imaging using Promax 3D, compared with the
manufacturer’s recommended exposure parameters.
The de facto standard detector placement for effective
dose measurements in the dental field, using 24 mea-
surement points, was introduced by Ludlow et al
18
in
2006. However, in 2010, Pauwels et al
19
used 150
measurement points and showed that measurements at
24 points provided insufficient accuracy, especially for
small FOVs, such as the ones suitable for TMJ imaging,
with organ doses deviating up to 80%. In the present
study, we reduced the measuring points to 61, mostly by
eliminating points at organs with no or negligible con-
tribution to the effective dose, such as the eyes, skin and
muscle.
Our dosimetric method included dose enhancement
factors for active bone marrow and endosteum, cor-
recting for the influence of the nearby trabecular bone.
These corrections are usually not included when mea-
suring organ doses. However, since the cranium has the
highest active bone marrow dose enhancement factors
of all bones and a considerable portion of the effective
dose, about 25% in the present study, came from the
active marrow, this correction was initially deemed
relevant to perform. Still, the correction affected only
the effective dose with 7.5% for Promax 3D and 6% for
LightSpeed VCT, a difference that was minor when
considering the uncertainties in determining the effec-
tive dose from TLD measurements. Thus, owing to the
comparatively small influence on effective dose, it is not
essential to perform the relatively difficult correction for
the influence of the trabecular bone.
In conclusion, the effective doses determined for the
Promax 3D CBCT and LightSpeed VCT MSCT, 92 and
124 mSv, respectively, were comparable. There seemed
to be a large potential for dose reduction compared with
the manufacturers’standard values; in our case, 50% for
CBCT. The use of appropriate FOV and optimized
exposure parameters are essential for obtaining a low
effective dose.
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