The Effects on Absorbed Dose Distribution in Intraoral X-ray Imaging When Using Tube Voltages of 60 and 70 kV for Bitewing Imaging

Abstract

Efforts are made in radiographic examinations to obtain the best image quality with the lowest possible absorbed dose to the patient. In dental radiography, the absorbed dose to patients is very low, but exposures are relatively frequent. It has been suggested that frequent low-dose exposures can pose a risk for development of future cancer. It has previously been reported that there was no significant difference in the diagnostic accuracy of approximal carious lesions in radiographs obtained using tube voltages of 60 and 70 kV. The aim of this study was, therefore, to evaluate the patient dose resulting from exposures at these tube voltages to obtain intraoral bitewing radiographs.
The absorbed dose distributions resulting from two bitewing exposures were measured at tube voltages of 60 and 70 kV using Gafchromic(®) film and an anatomical head phantom. The dose was measured in the occlusal plane, and ± 50 mm cranially and caudally to evaluate the amount of scattered radiation. The same entrance dose to the phantom was used. The absorbed dose was expressed as the ratio of the maximal doses, the mean doses and the integral doses at tube voltages of 70 and 60 kV.
The patient receives approximately 40 - 50% higher (mean and integral) absorbed dose when a tube voltage of 70 kV is used.
The results of this study clearly indicate that 60 kV should be used for dental intraoral radiographic examinations for approximal caries detection.

Full text

JOURNAL OF ORAL & MAXILLOFACIAL RESEARCH Hellén-Halme and Nilsson
The Effects on Absorbed Dose Distribution in Intraoral X-ray
Imaging When Using Tube Voltages of 60 and 70 kV for Bitewing
Imaging
Kristina Hellén-Halme1, Mats Nilsson1,2
1Department of Oral and Maxillofacial Radiology, Faculty of Odontology, Malmö University, Malmö, Sweden.
2Department of Radiation Physics, Skåne University Hospital, Malmö, Sweden.
Corresponding Author:
Kristina Hellén-Halme
Department of Oral and Maxillofacial Radiology, Malmö University
SE-205 06 Malmö
Sweden
Phone: +46 40 665 8414
E-mail: Kristina.Hellen-Halme@mah.se
ABSTRACT
Objectives: Efforts are made in radiographic examinations to obtain the best image quality with the lowest possible
absorbed dose to the patient. In dental radiography, the absorbed dose to patients is very low, but exposures are
relatively frequent. It has been suggested that frequent low-dose exposures can pose a risk for development of
future cancer. It has previously been reported that there was no signicant difference in the diagnostic accuracy of
approximal carious lesions in radiographs obtained using tube voltages of 60 and 70 kV. The aim of this study was,
therefore, to evaluate the patient dose resulting from exposures at these tube voltages to obtain intraoral bitewing
radiographs.
Material and Methods: The absorbed dose distributions resulting from two bitewing exposures were measured at
tube voltages of 60 and 70 kV using Gafchromic® lm and an anatomical head phantom. The dose was measured
in the occlusal plane, and ± 50 mm cranially and caudally to evaluate the amount of scattered radiation. The same
entrance dose to the phantom was used. The absorbed dose was expressed as the ratio of the maximal doses, the
mean doses and the integral doses at tube voltages of 70 and 60 kV.
Results: The patient receives approximately 40 - 50% higher (mean and integral) absorbed dose when a tube
voltage of 70 kV is used.
Conclusions: The results of this study clearly indicate that 60 kV should be used for dental intraoral radiographic
examinations for approximal caries detection.
Keywords: dental radiography; dental digital radiography; bitewing radiography; radiation dosage; radiographic
image enhancement.
Accepted for publication: 7 July 2013
To cite this article:
Hellén-Halme K, Nilsson M. The Effects on Absorbed Dose Distribution in Intraoral X-ray Imaging When Using Tube
Voltages of 60 and 70 kV for Bitewing Imaging.
URL: http://www.ejomr.org/JOMR/archives/2013/3/e2/v4n3e2ht.pdf
doi: 10.5037/jomr.2013.4302
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JOURNAL OF ORAL & MAXILLOFACIAL RESEARCH Hellén-Halme and Nilsson
INTRODUCTION
The purpose of all radiographic examinations is to
provide reliable diagnostic information allowing rapid
and suitable treatment of the patient. These examinations
must be performed with great care to ensure sufcient
image quality while exposing the patient to the lowest
dose possible. In order to increase the sensitivity and
specicity of a particular diagnostic method, every
link in the diagnostic chain must be optimized and
evaluated for the specic task at hand. Many studies
have been performed in dental digital radiography to
evaluate digital detectors [1-5], monitors [6-8], viewing
conditions [7,9,10] and tube voltage [11-14].
The effect of tube voltage on radiographic image quality
and diagnostic accuracy for dental carious lesions has
been investigated by several authors. Svenson et al.
[12] concluded that an optimal balance was obtained
between the absorbed dose to the patient and diagnostic
accuracy with an analogue lm technique using a
tube voltage of 60 kV. In a later study using a digital
sensor technique [13] no signicant difference was
found in the diagnostic accuracy of approximal carious
lesions when using tube voltages of 60 kV and 70 kV.
In another previous study by Vandenberge and Jacobs
[15] it was concluded that 63 kV and 70 kV provided
a similar diagnostic accuracy and image quality for
periodontal disease. The main opinion among vendors
and many users is that digital sensors often perform
with a higher subjective image quality at a higher tube
voltage, although no studies could be found supporting
this belief.
Optimization of any radiological procedure is a matter of
obtaining adequate image quality at the lowest possible
absorbed dose to the patient. In general dental practice
radiographs are often taken every time the patient
attends the clinic. Carious lesions are small, faint objects
in the X-ray image, superimposed on a background of
anatomical structures, which may impede detection.
The dose administered by standard dental X-ray units
can be adjusted by changing the exposure time or the
tube voltage.
Self-developing Gafchromic® lm (XR-QA2,
International Specialty Products, Wayne, NJ, USA)
has been used previously to measure absorbed dose
and its distribution in phantoms simulating the clinical
situation [16-18]. This offers a simple and accurate way
of mapping the dose distributions from radiographic
examinations. In a recent study [19] some support was
found of the hypothesis that exposure to dental X-rays,
particularly multiple exposures, may be associated with
an increased risk of thyroid cancer. Since it has been
shown that reducing the voltage from 70 kV to 60 kV
does not reduce image quality, we have investigated the
effect of voltage reduction on the absorbed dose to the
patient at these two voltages.
MATERIAL AND METHODS
Self-developing Gafchromic® lm (XR-QA2,
International Specialty Products, Wayne, NJ, USA) was
used to measure the absorbed dose and its distribution
in phantoms simulating the clinical situation. This lm
has a sensitive layer containing a crystalline diacetylene
monomer which polymerises and, as a result, darkens
when irradiated. This provides a simple and accurate
way of mapping the dose distributions from radiographic
examinations. The response of Gafchromic® lm is not
linear to the absorbed dose [20]. The response curve of
the lm was obtained by irradiating the lm with X-rays
when it was placed adjacent to a calibrated ionisation
chamber (Radcal 10X6-6, Radcal Corporation,
Monrovia, CA, USA) which measured the absorbed
dose to the lm. The lm, in which different sections
were irradiated with different absorbed doses, was
digitalized using a high-quality at-bed scanner (Epson
Perfection 4990, Seiko Epson Corporation, Nagano,
Japan). The results were used to obtain a polynomial
calibration curve which was then used to calculate the
actual absorbed dose distributions in the lms irradiated
in the phantom. Response curves were also obtained
for 60 and 120 kV, respectively, and were found to be
identical to that for 70 kV. Therefore, the same response
curve could be used for the experiments with 60 and 70
kV, respectively.
As the output from a dental intraoral X-ray unit is
very low and the Gafchromic® lm has low sensitivity,
a standard X-ray tube for medical radiology (A-196,
Varian Medical Systems, Inc., Salt Lake City, UT, USA)
with a standard collimating device (Svendx SX100-
MF, Santax Medico A/S, Aarhus, Denmark) was - for
practical reasons - used to irradiate the lm in the head
phantom. The output from a medical radiology X-ray
tube is 50 - 100 times higher than that from an intraoral
X-ray tube. This means that the experiments could be
carried out using a few exposures with the medical unit
instead of having to make more than 500 exposures
with the intraoral unit. The radiation eld produced
by the medical X-ray unit was compared (uniformity,
penumbra regions axial and transverse) with that of a
standard intraoral dental unit (Planmeca Intra, Planmeca
Oy, Helsinki, Finland). For that purpose, two pieces of
Gafchromic® lm were irradiated with identical eld
size and focal distance. The ltration of the beam from
the standard X-ray tube was adjusted so that the half-
value layer was the same as for the intraoral unit.

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JOURNAL OF ORAL & MAXILLOFACIAL RESEARCH Hellén-Halme and Nilsson
The basis for comparison of the two X-ray tube voltages
was that the signal-to noise ratio in the images was the
same. The ratio was measured in images produced in
a geometry used in a previous study [10] simulating
the clinical case with extracted teeth mounted in
PRESIDENT putty (Coltène Whaledent AG, Cuyahoga
Falls, Ohio, USA). In order to obtain the same signal-
to-noise ratio, the exposure time for 70 kV had to be
reduced with 20%. This reduction also resulted in an
approximately equal entrance skin dose for 60 and
70 kV, respectively.
To simulate a dental patient, the head of an anatomical
phantom (Rando/RAN100, The Phantom Laboratory,
Salem, NY, USA) was used. The Rando head phantom
consists of natural bone, full dentition and a soft plastic
simulating tissue, and is well suited and frequently
used in dosimetry studies. The Gafchromic® lm was
cut with a pair of scissors to t between the slices of
the anatomical phantom (Figure 1). The anatomical
phantom was irradiated corresponding to two bitewing
exposures. This was done by using the same entrance
angle for the X-ray eld as for normal intraoral units.
The dose distributions were measured in the occlusal
plane, and ± 50 mm cranially and caudally to evaluate the
primary and scattered dose distributions, respectively.
Following irradiation the lms were digitized in the
scanner and read into an image processing program
(ImageJ, NIH, Bethesda, MD, USA). The measured
pixel values were converted to absorbed dose using the
polynomial calibration curve. The dose distributions
were recalculated in order to correspond to exposure of
the patient from two standard bitewing images.
The sensor used when obtaining a radiographic image
in a patient is in itself an efcient beam stopper.
When placed intraorally, the absorbed dose behind the
sensor is drastically reduced. However, an intraoral
digital sensor could not be placed inside the phantom.
Therefore, the attenuation of two types of sensors:
Planmeca DIXI2 (Planmeca Oy, Helsinki, Finland)
and a CDR wireless sensor (Schick Technologies, Inc.,
Long Island City, NY, USA) was measured at 60 and
70 kV with an ionization chamber (Radcal 10X6-6,
Radcal Corporation, Monrovia, CA, USA) with 4 cm
of plexiglass in front of the sensor in order to produce
a similar amount of scatter as in the clinical case. Both
sensors are scintillation detectors using CsI (Tl).The
dose distributions behind the position where the sensor
would have been placed in the mouth were corrected
for sensor attenuation by scaling the dose values in the
region affected by attenuation of the sensor.
RESULTS
The radiation eld of the standard X-ray tube used was
found to have properties very similar to those of the
dental X-ray unit, as can be seen in Figure 2, where dose
proles along the main axes of the radiation eld are
shown. The use of the standard X-ray unit was therefore
considered representative of the clinical situation.
The transmission of the DIXI2 intraoral sensor is
4.4% for a tube current of 60 kV and 4.6% for 70 kV.
The corresponding values for the Schick CDR sensor
were 2.4% and 2.7%. The lower values for the Schick
sensor are explained by the fact that this sensor is
wireless, and is powered by a small battery which
increases its attenuation. The dose distributions in the
Gafchromic® lm for the two different tube voltages
are shown in Figure 3, while Figure 4 shows the dose
distributions as isodose curves for the same entrance
dose at tube voltages of 60 and 70 kV. It should be
noted that the absorbed dose outside of the primary
radiation eld, i.e. in the cranial and caudal sections,
is only a few percent of that inside the primary eld.
The effect of using sensors in the clinical situation on
Figure 1. The anatomical head phantom and the Gafchromic® lm cut to t between the different layers of the phantom.

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Figure 4. Dose distributions represented as isodose lines. The values given on the
right are absorbed doses in mGy for exposures of 0.12 s at 60 kV and 0.1 s at 70 kV:
A = cranial level, B = occlusal level, C = caudal level.
Figure 3. Gafchromic® lm after exposure in the
phantom at 60 kV and 70 kV: A = cranial level,
B = occlusal level, C = caudal level.
Figure 2. Signal proles along the minor and major axes for the standard X-ray unit used in this study and for a conventional dental X-ray
unit, showing the similarity between them.
Dental unit
Standard unit
Minor axis
Major
axis
Signal proles - major axis
Signal proles - minor axis
0 200 400 600
0 200 400 600
210
200
190
180
170
160
150
210
200
190
180
170
160
150
Relative lm blackening Relative lm blackening
Position
Position
60 kV 70 kV
A
B
C
60 kV 70 kV
5
4
3
2
1
Absorbed dose (mGy)Absorbed dose (mGy)Absorbed dose (mGy)
175
150
100
50
25
10
5
5
4
3
2
1
A
B
C

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the dose distributions is illustrated in Figure
5. The lack of scatter caused by the presence
of the sensor can be estimated by integrating
the dose distribution that would have been
shadowed by the sensor in relation to the
total integrated absorbed dose within the
primary beam. Table 1 gives the absorbed
dose expressed as the ratio of the maximal,
mean and integral doses resulting from
exposure using tube voltages of 70 and
60 kV for the same entrance dose.
DISCUSSION
The purpose of this study was to evaluate
how tube voltage affected the absorbed
dose within the primary radiation eld,
and outside the primary radiation eld
Figure 5. The values of absorbed dose at the occlusal level without a sensor and
the simulated levels with a sensor in place.
Without sensor - 60 kV With sensor - 60 kV
Absorbed dose (mGy)
175
150
100
50
25
10
5
Table 1. Ratios of the absorbed doses (mGy) resulting from exposure using tube
voltages of 70 kV and 60 kV
Layer Maximal dose Mean dose Integral dose
50 mm cranially 2.46 1.4 1.4
Occlusal 0.99 1.5 1.49
50 mm caudally 2.49 1.43 1.4
due to scattered radiation. The major principles when
undertaking any radiological procedure are justication
and optimisation. Optimisation means that the absorbed
dose to the patient is kept as low as reasonably
achievable while the diagnostic value of the procedure
is maintained. It can be argued that the absorbed dose,
and hence the effective dose, are very low for a dental
intraoral exposure. On the other hand, the number of
intraoral X-ray examinations performed is relatively
high, and is the most common X-ray procedure in
the Western world. Despite the low individual dose,
the effects on the population as a whole cannot be
neglected. In a recent publication, the risk of thyroid
cancer as a result of dental X-ray examinations was
extensively discussed, and it was concluded that dose
optimization in dental radiography should be urgently
addressed [19].
In intraoral imaging, only a few parameters that affect
the absorbed dose to the patient can be altered. Given
proper ltration and collimation, only the tube voltage
and the exposure time can be adjusted to change the
absorbed dose. When using digital detectors, it is the
responsibility of the dentist to use a dose at which the
quantum noise will not impair the diagnostic accuracy.
Therefore, the parameter affecting the dose which
should be studied in detail is the tube voltage. Today,
the lowest tube voltage (kV) permitted and used in the
Western world is 50 kV [21,22]. In Europe, there is an
on-going discussion on increasing the lower limit to
60 kV [22]. In Sweden, the permitted tube voltage
interval is 60 - 75 kV for general dental practitioners
[23]. This study was based on a comparison of 60 and
70 kV, which are the two most common tube voltages
used in Sweden.
Previous studies have been carried out to evaluate
different tube voltages in intraoral imaging. Svenson
et al. [12] concluded that 60 kV was preferable when
using analogue lm. Kaeppler et al. [24] showed that
increasing the tube voltage from 60 to 90 kV did not
have any effect on either the local absorbed dose or the
effective dose. They did not investigate how the image
quality or the diagnostic accuracy was affected when
the tube voltage was increased. In a study on a charge-
coupled device (CCD) Kitagawa et al. [25] found that
the estimated signal-to-noise ratio improved at a lower
tube voltage. Results reported by Hayakawa et al. [26]
showed that the low-contrast resolution of a CCD
sensor decreased when the tube voltage was increased
from 60 to 70 kV. In a previous study [10], we found
no signicant difference in the diagnostic accuracy for
any approximal carious lesions when evaluating digital
radiographs using tube voltages of 60 and 70 kV.
Due to the higher photon energies using tube voltage of
70 kV, a larger fraction of the photons is scattered than
at 60 kV. Additionally, the mean energy of the scattered
photons generated at 70 kV is higher than those at
60 kV, and their range is thus longer. This should result
in a higher absorbed dose outside the primary radiation
eld at 70 kV than at 60 kV. This is conrmed by the
results of this study, and is illustrated in Figure 4. The
low dose levels outside of the primary eld should not be
neglected, since the scattered radiation causing the dose
will inevitably hit sensitive tissues as the brain, thyroid
and salivary glands. Here, it is obvious that 70 kV will
cause a signicantly higher dose outside of the primary
radiation eld. It should also be noted that the dose
distributions cranially and caudally of the occlusal plane
were measured without a sensor blocking the primary
photons (as it was not possible to insert a sensor inside
the phantom). Since most of the scattered radiation

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JOURNAL OF ORAL & MAXILLOFACIAL RESEARCH Hellén-Halme and Nilsson
is produced in front of the sensor, due to the high intensity
of photons in this area, the lack of a blocking sensor will
have a small effect on the amount of scattered radiation.
Since the sensor will block the beam to an extent that
approximately 10% less scatter will be generated in the
phantom, the measured dose outside of the primary eld
may be slightly overestimated. Furthermore, as most
scattered radiation is produced in front of the sensor
and has an angular distribution that is generally in the
forward direction with respect to the primary beam, this
overestimation is clearly below 10%.
Several studies [13-15] have shown that the accuracy and
reliability of Gafchromic® lm for dose measurements
are adequate. The results obtained in this study clearly
conrm these previous ndings. Furthermore, the lm
is extremely user friendly and makes it possible to
measure absorbed dose distributions with an almost
unsurpassed spatial resolution. Its only drawback is its
low sensitivity which requires repeated exposures from
X-ray units with low output.
The ndings of this study show that the patient receives
an approximately 40 - 50% higher absorbed dose (mean
value) when using a tube voltage of 70 kV.
Our results indicate that lowering the tube voltage from
70 to 60 kV will result in a lower dose to the patient
without compromising image quality for evaluation of
carious lesions. Further studies are needed to investigate
if this also applies to other diagnostic tasks in bitewing
imaging, i.e. periodontal bone levels.
CONCLUSIONS
The results of this study clearly indicate that 60 kV
should be used for digital bitewing examinations for
approximal caries detection.
ACKNOWLEDGEMENTS AND DISCLOSURE
STATEMENTS
The authors declare that they have no conict of
interests.
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http://www.ejomr.org/JOMR/archives/2013/3/e2/v4n3e2ht.htm J Oral Maxillofac Res 2013 (Jul-Sep) | vol. 4 | No 3 | e2 | p.7
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JOURNAL OF ORAL & MAXILLOFACIAL RESEARCH Hellén-Halme and Nilsson
To cite this article:
Hellén-Halme K, Nilsson M. The Effects on Absorbed Dose Distribution in Intraoral X-ray Imaging When Using Tube
Voltages of 60 and 70 kV for Bitewing Imaging.
J Oral Maxillofac Res 2013;4(3):e2
URL: http://www.ejomr.org/JOMR/archives/2013/3/e2/v4n3e2ht.pdf
doi: 10.5037/jomr.2013.4302
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