Download Dose reduction in maxillofacial imaging using low dose

European Journal of Radiology 56 (2005) 413–417
Dose reduction in maxillofacial imaging using low dose Cone Beam CT
Kostas Tsiklakis a,∗ , Catherine Donta a , Sophia Gavala a , Kety Karayianni a ,
Vasiliki Kamenopoulou b , Costas J. Hourdakis b
a
Department of Oral Diagnosis and Oral Radiology, School of Dentistry, University of Athens, Greece
b Licensing and Inspection Division, Greek Atomic Energy Commission, Greece
Received 14 January 2005; received in revised form 13 May 2005; accepted 18 May 2005
Abstract
Objectives: (a) To measure the absorbed dose at certain anatomical sites of a RANDO phantom and to estimate the effective dose in radiographic
imaging of the jaws using low dose Cone Beam computed tomography (CBCT) and (b) to compare the absorbed and the effective doses
between thyroid and cervical spine shielding and non-shielding techniques.
Study design: Thermoluminescent dosimeters (TLD-100) were placed at 14 sites in a RANDO phantom, using a Cone Beam CT device
(Newtom, Model QR-DVT 9000, Verona, Italy). Dosimetry was carried out applying two techniques: in the first, there was no shielding device
used while in the second one, a shielding device (EUREKA!, TRIX) was applied for protection of the thyroid gland and the cervical spine.
Effective dose was estimated according to ICRP60 report (EICRP ). An additional estimation of the effective dose was accomplished including
the doses of the salivary glands (ESAL ). A Wilcoxon Signed Ranks Test was used for statistical analysis.
Results: In the non-shielding technique the absorbed doses ranged from 0.16 to 1.67 mGy, while 0.32 and 1.28 mGy were the doses to the
thyroid and the cervical spine, respectively. The effective dose, EICRP , was 0.035 mSv and the ESAL was 0.064 mSv. In the shielding technique,
the absorbed doses ranged from 0.09 to 1.64 mGy, while 0.18 and 0.95 mGy were the respective values for the thyroid and the cervical spine.
The effective dose, EICRP , was 0.023 mSv and ESAL was 0.052 mSv.
Conclusions: The use of CBCT for maxillofacial imaging results in a reduced absorbed and effective dose. The use of lead shielding leads
to a further reduction of the absorbed doses of thyroid and cervical spine, as well as the effective dose.
© 2005 Elsevier Ireland Ltd. All rights reserved.
Keywords: Absorbed dose; Effective dose; Lead shielding; Low dose Cone Beam CT; Thyroid gland; Cervical spine; Maxillofacial imaging
1. Introduction
In the last few years, computed tomography (CT) has
become one of the most useful and significant examinations
for the maxilla and mandible. Since implant treatment has
become the preferred method in cases of partial or total edentulism, the use of CT has increased mainly due to its high
diagnostic accuracy [1,2].
It has been well documented that CT produces reliable data
that facilitate the assessment of bone dimensions (both height
and width) and/or the localization of important anatomical
landmarks such as the mandibular canal, the mental foramen,
∗
Corresponding author. Tel.: +30 210 7461179.
E-mail address: [email protected] (K. Tsiklakis).
0720-048X/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.ejrad.2005.05.011
the nasopalatal duct and the maxillary sinus [3]. Presently, CT
is being widely used for the examination of temporomandibular joint and sinus pathology, trauma in the maxillofacial
region and, routinely, in oncology [1,4–7].
However, the increasing use of CT technique in dentistry
carries the risk of patient overexposure to radiation, which
must be one of the dentist’s greatest concerns [8]. It is critical
that radiation exposure be reduced to a minimum without any
loss of diagnostic information [9,10]. Eventually, exposure
minimization is more important for children and young adults
[11,12].
The development of Cone Beam CT (CBCT) reduces
exposure by using lower radiation dose, compared to conventional CT [13,14]. Cone Beam CT utilizes a cone shaped
X-ray beam instead of the collimated fan beam in the spiral
and conventional CT [15].
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K. Tsiklakis et al. / European Journal of Radiology 56 (2005) 413–417
The purpose of this study was: (a) to measure the absorbed
radiation dose at 14 anatomical sites in a RANDO phantom and to estimate the effective dose in the radiographic
examination of the jaws using CBCT and (b) to compare the
absorbed and the effective doses between thyroid and cervical
spine shielding and non-shielding techniques.
2. Materials and methods
Seventy-five thermoluminescent dosimeter chips (TLD100) fabricated by Harshaw Chemical Co., Solon, USA,
in the form of lithium fluoride, were placed in an adult
male tissue-equivalent RANDO human phantom (Alderson
Research Laboratories, Stanford, CN, USA). Although
RANDO phantom is designed for radiotherapy purposes, the
differences in (µab /ρ) coefficient for tissue equivalent materials allow the use of this phantom in diagnostic radiology
energies with an uncertainty (error estimation) less than 15%
depending on the anatomical sites [16]. This is due to the
differences between a cadaver head and the phantom in relation to soft/hard tissues between the tube and the measured
organs [17]. Before radiation exposure, TLD dosimeters
were calibrated in X-ray beams with qualities similar to those
used in dental radiography. Thus, the relation between TLD
signal and X-ray doses for such beam qualities was established and used for the estimation of tissue/organ doses in the
phantom.
In order to avoid contamination from dust, moisture or
grease, the TLDs were placed in sealed polyethylene bags
and fixed at the anatomical locations of the tissues/organs
of interest. These anatomical locations were defined using
the manufactured canals within the slices of the phantom.
The tissues/organs in which the absorbed radiation dose
was measured were: the brain, the eyes, the parotid and
the submandibular salivary glands, the thyroid gland, the
bone marrow of the mandible, the bone marrow of the
cervical spine (second, fourth and sixth cervical vertebra),
the stomach, the lungs, the breasts, the oesophagus and
the skin. In each exposure, 25 dosimeters were used. The
anatomical regions and the phantom level, where the TLDs
were put, are shown in Table 1. Two TLDs were used to
measure background radiation, which was estimated 0.4 nC.
The lowest TLD signal used to provide dosimetric data was
three times higher than background. Background values
were subtracted from TLD readings.
CBCT (Newtom Model QR-DVT 9000, Verona, Italy) was
used with an automatic exposure depending on bone volume
and density. The milliamperage was 3.4 mA, the tube rotation
time was 72 s, the mean exposure time was 17 s, the tube
voltage was kept constant at 110 kV and the total filtration was
8 mm Al. The parameters used for the phantom correspond
to an average weight 45-year-old male.
Two techniques were applied during the study, using the
same parameters. They differed in that during the second
technique a lead shield device (EUREKA!) fabricated by
TRIX especially for Newtom, was applied for the protection
of the thyroid gland and the cervical spine (Fig. 1). Additionally, the use of this device facilitates the stabilization of the
head during the examination. This shield device is composed
of a base structure, a chin support combined with a front lead
protection, a pillow, two support stirrups and a back lead
protection. The two support stirrups were fixed on the base
structure. This assemblage was put on CT patient’s couch
nearby the scanning area. The chin support and the lead protection were fixed after the placement of the phantom, since
their level depends on the size of the head and neck. Each
technique was repeated three times to ensure reliability.
All TLDs were annealed before irradiation in a PTWTLDO oven, for 1 h at 400 ◦ C followed by 2 h at 100 ◦ C.
After irradiation TLDs were annealed for 10 min at 100 ◦ C.
The thermoluminescent reader was a Harshaw, Model 4500.
The type of X-rays used for the calibration was RQR Narrow
(according to ISO 4037-1, 1996), of different energies. The
findings were assessed statistically using a Wilcoxon Signed
Ranks Test.
Table 1
Mean absorbed doses in mGy at pre-selected anatomical regions of the phantom for non-shielding and shielding techniques
Anatomical region
Phantom level
Mean absorbed dose
(mGy) non-shielding
S.D.
Mean absorbed dose
(mGy) shielding
S.D.
p-Value
Thyroid
Bone marrow (mandible)
Bone marrow (cervical spine second,
fourth, sixth vertebra)
Skin (zygomatic area)
Lens of eyes
Submandibular salivary glands
Parotid salivary glands
Brain
Lung
Breast
Stomach
Oesophagus
10
7
6, 7, 9
0.32
1.67
1.28
0.07
0.09
0.04
0.18
1.64
0.95
0.04
0.12
0.07
0.04*
0.88
0.04*
4
4
8
5
3
17
15
17
10
0.77
0.61
1.28
1.12
0.32
0
0
0
0.16
0.12
0.09
0.09
0.07
0.12
–
–
–
0.10
0.76
0.62
1.25
1.10
0.30
0
0
0
0.09
0.07
0.05
0.09
0.12
0.08
–
–
–
0.06
0.89
0.98
0.79
0.82
0.98
–
–
–
0.07
p-Value is demonstrated according to Wilcoxon Signed Ranks test.
* p < 0.05.
K. Tsiklakis et al. / European Journal of Radiology 56 (2005) 413–417
415
calculated as: Average salivary gland dose × 0.025 +
remainder (brain) / 10 × 0.025 [22].
3. Results
Fig. 1. RANDO phantom with thyroid and cervical spine shielding placed
in Cone Beam CT device.
Effective dose, EICRP , has been estimated, according to
the ICRP60 [18]. The tissues/organs considered to contribute
to the effective dose were the thyroid, the bone marrow, the
oesophagus, the bone surface and the skin. The contribution
of the whole body active bone marrow was estimated as
0.8% for the mandible and 3.9% for the cervical spine. These
proportions depend on the age and gender and correspond to
40-year-old male [19]. Likewise, concerning the fractional
contribution of the skin it was calculated that only 5% of the
skin was present in the primary beam [22]. The bone surface
was estimated, according to Frederiksen et al., by multiplying
the mean absorbed dose of the bone marrow with a factor of
4.64, which is the ratio of f-factors (the factors of conversion
of exposure to absorbed dose) for bone and soft tissue [20].
The respective proportions of the mandible and cervical spine
in relation to the whole body bone surface are 1.33% [21]
and 1.4% [20].
In our study, absorbed doses were measured also in
the submandibular and the parotid salivary glands. These
tissues are not referred to as “remainder” in the ICRP60
report, because they are not radiosensitive enough in order
to contribute to the effective dose [18]. Nevertheless, in our
study the salivary glands were considered as “remainder”,
since the doses, which were received by these organs were
high in order of magnitude as the other radiosensitive
tissues. In this way there is an overestimation of the effective
dose.
Since the salivary glands are not included in the ICRP60
list of individually weighted tissues or remainder organs, an
additional calculation of effective dose has been performed
[8,22]. Particularly, ESAL was calculated as EICRP with an
addition of the salivary glands as part of the remainder
organs. Since the absorbed doses of the salivary glands were
found among the highest of the measured weighted organs,
a weighting factor of 0.025 should be applied to the average
dose of the salivary glands and a weighting factor of 0.025
to the average dose of the 10 remainder organs and was
Table 1 shows the mean organ dose (left and right) (D), the
standard deviation (S.D.) and the p-value for the statistical
analysis in the non-shielding and the shielding techniques. In
the non-shielding technique, the doses ranged from 0.16 mGy
in oesophagus to 1.67 mGy in bone marrow of the mandible.
It is noteworthy that the absorbed doses of bone marrow of
the mandible, bone marrow of the cervical spine and salivary
glands are the highest among the tissues of interest.
In the shielding technique the absorbed radiation doses
ranged from 0.09 mGy in oesophagus to 1.64 mGy in the
bone marrow of the mandible. It is obvious that radiation
doses of bone marrow of the mandible and salivary glands
are the highest among the tissues of interest.
The other organs (stomach, lungs and breasts), which
are not included in Table 1, received no dose or negligible
dose, which was accepted as zero, in both shielding and nonshielding technique.
In the non-shielding technique, the thyroid gland received
0.32 mGy, while in the shielding technique the respective
value was 0.18 mGy. Additionally, the cervical spine
received 1.28 mGy as an average of the three vertebrae in
the non-shielding technique and 0.95 mGy in the shielding
technique. The p-value for both thyroid and cervical spine
absorbed doses was 0.04, statistically significant (p < 0.05).
In contrast, there was no statistically significant difference
(p > 0.05) between the two techniques in the doses received
by the other organs (Table 1).
Table 2 shows the effective dose calculations according
to ICRP60 [18]. In the non-shielding technique, the EICRP
value was 0.035 mSv. Adding the average dose of the salivary glands in the remainder organs, the ESAL value was
0.064 mSv. Furthermore the EICRP value in the shielding technique was 0.023 mSv and the ESAL was 0.052 mSv.
4. Discussion
In this study, using CBCT, the average absorbed radiation
doses have been estimated at certain anatomical areas of the
head, neck and upper body of a RANDO Alderson phantom.
The findings have shown that the absorbed doses were low
compared to the doses received during a conventional CT
[23,24]. High-resolution CT can provide diagnostic information about bone structure and soft tissues as well. Additionally, there are CT dose reduction protocols by using lower mA
and increasing pitch. The dose reduction leads to an image
noise increase, which results to a lower image quality [6,10].
In this study, the bone marrow in the body of the mandible
at molar area and in the cervical spine, as well as the salivary
glands, received the highest doses bilaterally, probably
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Table 2
Effective dose according to ICRP Publication 60 guidelines (EICRP ) and including salivary glands as remainder organs (ESAL )
Anatomical region
Weighting factor (WT )
Percentage irradiated (%)
Effective dose (mSv)
non-shielding
Effective dose (mSv)
shielding
Thyroid
Bone marrow (mandible)
Bone marrow (cervical spine)
Skin
Oesophagus
Bone surface (mandible)
Bone surface (cervical spine)
Remainder (brain)
Total EICRP
0.05
0.12
0.12
0.01
0.05
0.01
0.01
0.050
100
0.8
3.9
5
100
1.33
1.4
100
0.016
0.001
0.006
0
0.008
0.001
0.001
0.002
0.035
0.009
0.001
0.005
0
0.005
0.001
0.001
0.001
0.023
Remainder (brain)
Remainder (salivary glands)
Total ESAL
0.025
0.025
100
100
0.001
0.030
0.064
0.001
0.029
0.052
because these areas are directly irradiated by the X-ray
beam. The findings resulting from the study are in agreement
with other investigations, which have identified the highest
absorbed radiation doses in these areas [10,15,22,24].
Contrary, the thyroid gland and the eyes received the lowest radiation dose during tomography, probably because these
areas are located out of the primary beam.
The irradiation of organs outside the primary beam is
mainly due to X-rays scattered within the patient. Additionally, the extension of the scout view caudally, beyond the
mandibular symphysis, contributes to the total thyroid dose
by doubling it [11]. If the region under examination lies in the
trunk, the shape of the patient makes it difficult to shield adjacent organs from this scattered radiation [11]. Therefore, we
considered that the shielding of radiosensitive organs, wherever possible, would improve patient protection, since it leads
to an overall dose reduction. At this point, it is important to
mention that, using the shielding, there was no difference in
the image quality and no artifacts were observed, since the
shielding device was placed out of the irradiation beam. We
evaluated the two techniques and a statistically significant difference was found between the values of the absorbed doses
received by the thyroid gland and the cervical spine (p < 0.05).
Specifically, the absorbed doses measured at these organs in
the shielding technique have been proved to be lower than
the respective values in the non-shielding technique (Table 1).
Contrary, comparing the absorbed doses received by the other
organs and tissues examined, no statistically significant difference was found between the two techniques (p > 0.05)
(Table 1). Although the doses were low, it is our responsibility
to assure that the patients do not receive any unnecessary dose
of radiation, especially in the thyroid gland of the young individuals. It is noteworthy that in many cases the use of thyroid
shielding may lead to a dose reduction by one half [25,26].
Concerning the salivary glands absorbed radiation doses,
the authors’ argument is about including these glands in
the remainder organ calculations [22]. This is considered as
being consistent with the ICRP60 position that other tissues
or organs either selectively irradiated or later identified as
having a significant risk of induced cancer will be included
either with a specific weighting factor or in the list constituting remainder organs [18]. It has been suggested that the
lymphoid component may be more susceptible to low dose
radiation damage than the parenchyma of the salivary glands
[27].
As it concerns the calculation of the effective dose (E), the
authors estimated E according to the ICRP60 report, but they
also used the salivary glands as remainder organs, because
the reported probability of fatal cancer induction in the salivary glands is of the order of 5 × 10−4 Sv−1 [22]; thus, this
probability is equal to the likelihood of cancer induction
from irradiation of the bone surface, which is included as
a weighted organ in ICRP60 effective dose calculations.
Other investigators have reached the same conclusions
concerning our point in achieving low radiation dose during
radiographic examination using CBCT. In one study, the
authors estimated the effective dose using CBCT and the
E value, which was calculated for the central dose profile,
ranged from 0.05 to 0.06 mSv [15]. In a recent study, while
accomplishing a maxilla/mandible scan using CBCT, the
EICRP value was 0.04 mSv and the ESAL was 0.08 mSv
[22]. Using new software in the same study, the EICRP was
0.04 mSv and the ESAL was 0.09 mSv [22]. The purpose
of using a new version of software was to increase the
signal-to-noise ratio. The thyroid and the cervical spine
absorbed doses were found 0.37 and 1.45 mGy, respectively.
Using new software, the thyroid and the cervical spine
absorbed doses were 0.44 and 1.73 mGy, respectively
[22]. The variability of the results of these studies can
be explained as the interaction of many factors. Inexact
vertical positioning of the TLDs inside the phantom may
influence the results [22]. Variation in the accuracy of TLDs
and the TLD reader also contribute to interexamination
variation [22].
Generally, from the stand point of radiation risk, CBCT
in maxillofacial imaging produces 8- to 10-fold lower effective dose than a conventional CT examination using standard
protocol [6,8,20]. If a low dose protocol is used for conventional CT, the effective dose may be reduced with an increase
of image noise [15]. According to published effective doses
K. Tsiklakis et al. / European Journal of Radiology 56 (2005) 413–417
from panoramic radiography, CBCT appears to have a three
to seven times higher risk compared to a panoramic examination. These results depend on the absorbed radiation doses
at certain anatomical areas, the degree of collimation and the
acquisition software version [17,28,29].
In conclusion, the absorbed radiation doses and the effective doses with and without the salivary glands using CBCT
could be considered low. Additionally, the use of lead shielding leads to a further reduction of the absorbed doses of
thyroid and cervical spine, as well as the effective dose.
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