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Physica Medica (2008) 24, 149e158
available at www.sciencedirect.com
journal homepage: http://intl.elsevierhealth.com/journals/ejmp
ORIGINAL PAPER
Polymer gel dosimetry on a multislice computed
tomography scanner: Effect of changing parameters
on CTDI
B. Hill a,b,*, A.J. Venning c, C. Baldock b
a
Medical Physics, The Canberra Hospital, ACT Health, P.O. Box 11, Woden, Canberra, ACT 2606, Australia
Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia
c
Wellington Blood and Cancer Centre, Wellington Hospital, Private Bag 7902, Wellington South, New Zealand
b
Received 23 July 2007; received in revised form 20 November 2007; accepted 22 November 2007
Available online 4 March 2008
KEYWORDS
Computed tomography;
Normoxic polymer gel
dosimetry;
Radiation;
Dosimeter;
CT;
MRI;
Dose profile
Abstract Polymer gel dosimetry undertaken on a multislice CT scanner provides an alternative method to conventional dosimetry measurements. Polymer gel dosimeters were used to
measure CT radiation doses and compared to TLD and ionization chamber measurements in different diameter phantoms. CTDI was investigated for each of these phantoms for a range of
mAs (100e400 mAs), tube voltage (100e135 kV) and nominal slice width (2e32 mm). Linear fits
of the CTDI values for mAs show for the smallest phantom diameter an increase in CTDI of 60%
for both TLD and polymer gel dosimeters. A similar increase in CTDI of 50% at 100 kVp and 100%
for 135 kVp was also noted. It was also shown that slice width variation measured with either
polymer gel or TLD was greatest with the smallest slice widths. In summary, it was found that
polymer gels can be used as an alternative dosimeter to TLD for the determination of SWDP and
subsequent CTDI calculations.
ª 2007 Published by Elsevier Ltd on behalf of Associazione Italiana di Fisica Medica.
Introduction
Polymer systems for the use of radiation dosimetry were first
proposed as early as 1954, where Alexander et al. discussed
the effects of ionising radiation on polymethylmethacrylate
* Corresponding author. Medical Physics, The Canberra Hospital,
ACT Health, P.O. Box 11, Woden, Canberra, ACT 2606, Australia.
Tel.: þ61 2 62442256.
E-mail address: [email protected] (B. Hill).
[1]. Following this, Hoecker and Watkins [2] in 1958 investigated the dosimetry of radiation-induced polymerization
in liquids, and in 1961, Boni [3] used polyacrylamide as
a gamma dosimeter. In 1992, a polymer formulation was suggested that consisted of acrylamide (AA) and N,N0 -methylene-bis-acrylamide (bis) monomers infused in an aqueous
agarose matrix. This polymer gel formulation did not have
the diffusion limitations of Fricke gels [4]. In 1994, Maryanski
et al. modified the formulation by replacing agarose with
gelatine and named the, now commercial, product BANG
1120-1797/$ - see front matter ª 2007 Published by Elsevier Ltd on behalf of Associazione Italiana di Fisica Medica.
doi:10.1016/j.ejmp.2007.11.005
150
gel [5]. A procedure for manufacturing what is now commonly referred to in the literature as PAG, an acronym for
polyacrylamide gel, was later described [6]. Upon exposure
of the radiological tissue equivalent [7] PAG dosimeter to
radiation, polymerization of the co-monomers is induced
by the free radical products of water radiolysis resulting in
a 3D insoluble polymer network infused within the gel
matrix. Since the extent of the resulting polymer structure
is a function of dose, MRI can be used to evaluate the relaxation rates which can be related to dose by means of an R2
(1/T2) versus dose calibration curve. The polymerization in
the irradiated PAG is known to be inhibited by oxygen and
therefore, requires that the polymer gel be produced in an
oxygen free or hypoxic environment [6]. Over the years
many formulations of polymer gels have been published
[8e10] demonstrating its use mainly in radiotherapy dosimetry [11e14]. In 2001, a method was described for developing polymer gels in an oxygen or normoxic environment. The
formulation became known as MAGIC polymer gel which is an
acronym for Methacrylic acid (MAA), Ascorbic acid (AscA),
Gelatine, Initiated by Copper [15]. The addition of the
AscA oxygen scavenger into the formulation resulted in
one of the major limitations of polymer gel dosimeters being
overcome, allowing the polymer gel to be manufactured
under normal atmospheric conditions upon the bench top.
De Deene et al. investigated the various components of
the original MAGIC polymer gel formulation and proposed
some alternative oxygen scavengers, in particular tetrakis
(hydroxymethyl) phosphonium chloride (THPC), which was
originally proposed in 1996 by Billingham [16] and from
which MAGAT polymer gel was developed [17,18]. Further
formulations were also proposed such as MAGAS and PAGAS
[17,18] for which radiological tissue equivalence was investigated [19]. In 2005, Venning et al. investigated a normoxic
formulation based on the original hypoxic PAG polymer gel
formulation of AA and bis dissolved in a gel matrix with the
novel addition of THPC and hydroquinone (HQ). This formulation was given the acronym PAGAT polymer gel [7]. PAGAT
polymer gel was subsequently shown to be useful for both
radiotherapy and diagnostic dosimetry with high spatial resolution being achieved when imaged with MRI in the relevant
clinical dose ranges [14].
In addition to the MRI evaluation of polymer gels other
evaluation modalities such as optical tomography [20e23],
vibrational spectroscopy [24,25], ultrasound [26e30] and Xray CT [31e33] have been studied. Further investigations of
normoxic polymer gel dosimeters using X-ray CT were subsequently pursued due to the potential advantage, due to
the availability and ease of use of X-ray CT compared to
MRI [34,35]. It was whilst undertaking these studies that
the discovery was made that normoxic polymer gels polymerize due to diagnostic X-ray CT doses. A further investigation demonstrated that polymer gel dosimeters could be
used for the measurement of the diagnostic X-ray slice
width dose profile (SWDP) on helical CT scanners [36]. It
was shown that normoxic polymer gel dosimeters potentially provide an alternative to conventional dosimeters
for diagnostic CT scanner SWDP measurements and subsequent CT dose index (CTDI) calculation. Furthermore, polymer gel dosimeters also provide an advantage over
conventional dosimeters since the dose profile can potentially be acquired in any direction allowing evaluation of
B. Hill et al.
the complete volume when the delivered CT scanner dose
profile is recorded in 3D in the irradiated phantom. Given
the potential application of this dosimeter in acceptance
testing and for research purposes a review of various CT dosimetry studies was assessed in order to further compare
polymer gels to more conventional methods of dosimetry
on a multislice CT scanner.
It is an accepted requirement to measure CTDI from CT
scanners as a part of acceptance and quality assurance
procedures [37e39]. Current protocols recommend the use
of simulated adult head and body phantoms (of perspex diameters 16 and 32 cm, respectively). This may be a limitation in the current protocols as it does not consider the
relationship between diameter and dose (or CTDI) and
does not take into account dose determination at alternative diameters. Several authors have identified the importance of the CT scanning parameters, such as tube
current time (mAs), tube voltage (kV), nominal slice width,
pitch and length of scan on radiation dose when measured
with a 100 mm ionization chamber as a function of phantom
diameter. Siegel et al. evaluated the effects of varying the
CT parameters, mAs and kV on radiation dose and image
quality on a multislice CT scanner with a 100 mm ionization
chamber placed in phantoms of diameters 8, 16, 24, 32 cm
and a 28 18 cm oval phantom [40]. Boone et al. characterised CT contrast-to-noise (CNR) factors and thus image
quality for similar CT parameters using an ionization chamber for various containers of diameters 10, 13, 16, 20, 25,
28 and 32 cm also on a multislice CT scanner [41]. Nickoloff
et al. identified the exponential relationship of CTDI with
an ionization chamber placed in phantoms of diameters 6,
10, 16, 24 and 32 cm for consecutive axial scans and helical
scans of variable pitch [42]. Huda et al. identified the relationships of CT CNR ratios for CT parameters, mAs and kV in
phantoms of varying weight and diameters [43]. Generally
these studies found that through the optimisation of CT
protocols for varying phantom diameters significant dose
reductions could be achieved.
Traditionally both thermoluminescent detectors (TLD)
and radiographic film have been used for determination of
SWDP [37,44]. In the case of TLD, SWDP can be determined
by stacking the 1D dosimeters in the z-direction of the CT
gantry to enable 2D measurement of the X-ray beam profile
and therefore SWDP. SWDP measurements at depth in
a phantom provide a means of determining the dose profile
for a given CT X-ray collimation setting. The SWDP can be directly compared to the X-ray detection systems effective
acquisition width in order to determine co-incidence. This
is undertaken to determine z-efficiency of the CT scanner
and is useful in ensuring overshoot or misalignment of the
X-ray beam to X-ray detector system is minimized [44].
TLDs can also be used to measure the relative or absolute
distribution of dose as a result of phantom diameter and
other CT parameters. Cheung et al. assessed the effects of
integrating lengths on the calculation of CTDI in standard
head and body phantoms of 16 and 32 cm diameters [45].
McNitt-Gray et al. investigated the relative effects of collimation and pitch in a cylindrical body phantom of 32 cm
diameter and found that the use of smaller collimations increases dose and that higher pitch lowers dose [46]. Tsai
et al. investigated the relationship of scatter-to-primary
dose from TLD SWDP measurements in standard head and
Polymer gel dosimetry on a multislice CT scanner
body phantoms from which a method of modeling organ
dose using Monte Carlo was reported [47]. TLDs have also
been used to determine relative dose in anthropomorphic
phantoms of different diameters identifying the relationship between phantom diameter or patient size and dose
[48,49]. Dixon used TLDs to measure SWDP for multislice
CT scanners in standard head and body phantoms providing
experimental evidence to support observed limitations in
the use of 100 mm length ionization chambers for making
measurements and subsequent calculation of CTDI [50].
In the case of radiographic film, a 2D dosimeter, SWDP is
acceptable for use either in air or sandwiched in plastic
phantom material [37,44]. More recently computer radiography equipment has been used in measuring SWDP on a CT
scanner [51]. Other types of detectors have also been used
experimentally for CT SWDP measurements. Nakonechny
et al. demonstrated SWDP measurement using a diamond
detector [52].
The purpose of the current research was to compare the
use of polymer gel dosimeters to measure SWDP and
subsequently calculate CTDI and compare with other
conventional dosimeters. Changes in CTDI were investigated as a function of mAs, tube voltage and nominal slice
width for different diameter phantoms.
Materials and methods
Each of the polymer gel dosimeters were prepared, poured
into cylindrical phantom containers and allowed to set.
Each polymer gel dosimeter was then irradiated using the
CT scanner in axial mode, and left to polymerize before
evaluation using MRI. The polymerized area of the polymer
gels were visualized and evaluated using a single MRI slice
and from that slice an associated R2 map calculated. Relative dose profiles were determined in any direction within
the MRI R2 map from which the integrated dose under the
profile was used to calculate CTDI. In order to calibrate
the R2 map as a function of dose in the SWDP plane of
the CT scanner an ionization chamber was used. By placing
the ionization chamber at the centre of a ‘non-polymer’
gelatine gel phantom identical in geometry to the irradiated polymer gel phantom and exposing it to the same CT
protocol the dose at the centre of polymer gel could be determined. At this point the method of calibration of the
polymer gel varies from that previously described [36].
This method does not require the use of a water filled phantom, instead has the ionization chamber at the centre of
the gelatine phantom for which dose measured with the
ionization chamber is determined and directly related to
the number of single axial slices required to deliver that
particular dose to the phantom (Table 1). For all subsequent polymer gels the calibration result obtained using
the ionization chamber for cumulative single axial slices
was used to relate the R2 map of the exposed polymer gel
to dose and therefore enable the calculation of CTDI.
Polymer gel dosimeter preparation
Polymer gel dosimeters were manufactured according to
a formulation and method described previously [7]. The
PAGAT polymer gel formulation by % mass consisted of
151
Table 1 CTDI(100C) obtained using an ionization chamber
in a 6 cm diameter phantom for cumulative single axial slices with the following CT parameters: 135 kV, 100 mAs and
16 mm slice width
No of slices
Ionization chamber
dose (mGy)
Ionization chamber
CTDI (mGy)
0
1
2
5
10
20
40
0.0
20.8
41.6
103.8
208.0
416.5
832.9
0.0
34.3
68.7
172.9
345.6
691.0
1381.2
4.5% bis (Sigma Aldrich, Sydney), 4.5% AA (Sigma Aldrich,
Sydney), 5% gelatine (300 bloom) (Sigma Aldrich, Sydney)
and 86% de-ionized water, 10 mM THPC (Sigma Aldrich,
Sydney) and 5 mM HQ (Sigma Aldrich, Sydney). Costs of
amounts to make several batches of polymer gels are in
the hundreds of dollars. All components were mixed on
the bench top under a fume hood. The gelatine was left to
soak in de-ionized water for 10 min before heating commenced. The gel was heated to 48 C and then bis was added
and stirred until dissolved. The AA was added followed by
THPC and HQ. The length of time for preparation was 3 h.
The polymer gel was left to cool to 30 C and then poured
into the cylindrical Barex containers (CIRS, Virginia, USA)
and sealed with an airtight cap and placed in a refrigerator
for 12 h before irradiation. The cylindrical Barex containers varied in length with the largest 14 cm, medium
12 cm and smallest 10 cm. They also varied in diameter
with largest 16 cm, medium 9 cm and smallest 6 cm.
TLD calibration and measurement
LiF:MgTi TLD-100 was used to measure SWDP for the
subsequent calculation of CTDI. Each TLD had dimensions
of 0.35 0.35 0.9 mm3 and energy dependence to within
10.0% for the energy range of 60e140 keV [53]. Calibration
of the TLD was achieved by exposing each TLD using an Xray machine (BuckyDiagnost, Philips, Netherlands) with additional filtration to make the beam quality similar to that
of the CT scanner (6.8 mmAl). Each TLD was exposed under
the same conditions (100 cm sourceeaxisedistance,
10 10 cm2 field size, 135 kV, 50 mAs). The output of the
X-ray machine was determined with a 0.3 cc model 20 3
RadCal ionization chamber used in conjunction with a RadCal 2025 radiation monitor (RadCal Corp., Monrovia, CA,
USA). The resulting exposure was converted to dose within
the dosimeter making it possible to apply a sensitivity correction factor of 0.97 5.0% mGy to each TLD. The annealing of TLDs was performed with a Fimel automatic TLD oven
(Vélizy, France) and readout with a Bicron 5500 (Saint-Gobain Crystals and Detectors, France) TLD reader which
had a cycle time of 4 h for annealing and 2 h to read 50 TLD.
For SWDP measurements using TLDs, the TLDs were
stacked into a holder (Fig. 1) and placed centrally into
each of the gelatine filled phantoms. Up to 25 TLDs at
152
B. Hill et al.
200
Ion chamber in gel phantom 100 mAs
PAGAT phantom 100 mAs
TLD in gel phantom 100 mAs
Ion chamber in gel phantom 200 mAs
PAGAT phantom 200 mAS
TLD in gel phantom 200 mAs
Ion chamber in gel phantom 400 mAs
PAGAT phantom 400 mAs
TLD in gel phantom 400 mAs
180
CTDI (mGy)
160
140
120
100
80
60
40
20
6
7
8
9
10
11
12
13
14
15
16
Diameter (cm)
Figure 1
chamber.
Image of containers, TLD holder and ionization
a time were stacked about the central part of the TLD
holder and a maximum of 3 at a time were stacked at
equal intervals throughout the rest of the holder. The
length of time preparation of TLD in holder was approximately 1 h. Each phantom was subsequently placed centrally in the CT scanner with the TLDs stacked in the
direction of the z-axis and exposed. Three single exposure
rotations from the CT scanner were made each time and
the TLD results averaged. The individually determined
TLD doses were used to calculate the integrated area under the SWDP using Origin (Microcal) and subsequently
CTDI was calculated.
Ionization chamber calibration and measurement
The RadCal 2025 radiation monitor and model
20 5100 mm CT ionization chamber was placed centrally
into each of the gel filled phantoms and each phantom
was centrally placed in the CT scanner and exposed. The
ionization chamber was calibrated to within 5% of its expected reading for the energy range of 60e140 kV by RadCal Corp. (Monrovia, CA, USA). Three measurements were
made with the ionization chamber for each of the CT scanner protocols used. An average dose value and standard deviation was calculated from these measurements.
CT scanner
All CT doses were measured on a Toshiba Aquilion Multislice
CT scanner (Toshiba Corporation, Japan). The total slice
widths that a user can define in a single rotation are 2.0,
4.0, 8.0, 12.0, 16.0, 24.0, or 32.0 mm. Total slice widths
will be used to describe the nominal slice widths from
this point on in this paper. Further information about this
specific CT scanner acquisition details can be obtained
from a previous paper on optimisation of protocols for
this CT scanner [35].
The HVL was measured with the tube in a fixed position
under the table using a 0.3 cc model 20 3 RadCal ionization chamber used in conjunction with a RadCal 2025
Figure 2 CTDI(100C) as a function of diameter at currents 100,
200 and 400 mAs for an ionization chamber, TLD and PAGAT.
Fitted lines are for PAGAT only.
radiation monitor (RadCal Corp., Monrovia, CA, USA) and
found to be 6.8 mmAl at 135 kV.
When tube voltages were compared 100, 120 and 135 kV
were used while 100 mAs and 16 mm nominal slice width
were kept constant. When mAs were compared 100, 200
and 400 mAs were used while tube voltage 135 kV and
16 mm nominal slice width were kept constant. When nominal slice widths were compared, tube voltage of 135 kV and
mAs of 100 mAs were kept constant. For the calibration of
the polymer gel using the ionization chamber at the centre
of the polymer gel a 16 mm nominal slice width was used
with 135 kV and 100 mAs.
Table 2 Mean CTDI(100C) and standard deviation (in
brackets) of ionization chamber, TLD and PAGAT for different CT parameters in 6, 9 and 16 cm diameter phantoms
Set CT parameters
Diameter (cm)
Slice Tube
Current 6
9
16
width voltage time
Mean CTDI(100C) values (SD)
(mm) (kVp)
(mAs)
16
16
16
135
135
135
100
200
400
34.4 (1.9) 27.1 (0.3) 20.3 (0.8)
73.1 (2.6) 56.5 (3.2) 49.7 (0.5)
135.4 (4.6) 107.9 (1.5) 79.1 (4.6)
16
16
16
100
120
135
100
100
100
21.5 (0.5)
30.1 (0.8)
39.3 (1.3)
12.5 (1.3) 11.6 (0.5)
19.9 (0.9) 18.4 (0.9)
26.9 (0.4) 20.3 (0.8)
2
4
8
12
16
24
32
135
135
135
135
135
135
135
100
100
100
100
100
100
100
97.7
66.3
44.9
39.1
34.0
34.8
31.6
94.8
53.5
38.9
31.6
28.7
26.9
20.6
(2.0)
(1.4)
(0.9)
(0.8)
(0.7)
(0.7)
(0.7)
(1.7)
(0.9)
(0.5)
(0.4)
(0.5)
(0.1)
(0.3)
45.6
35.9
25.2
23.0
20.4
21.1
19.7
(0.9)
(0.8)
(0.6)
(0.5)
(0.4)
(0.5)
(0.4)
Polymer gel dosimetry on a multislice CT scanner
153
Table 3 CTDI(100C) linear fits for the PAGAT polymer gel across the range of phantoms as a function of tube current time and
nominal slice widths
Set CT parameters
Linear regression
Slice width (mm)
Tube voltage (kVp)
Current time (mAs)
Gradient
Intercept
Correlation co-efficient
16
16
16
135
135
135
100
200
400
1.2 0.2
2.4 1.0
5.5 0.5
38.8 2.0
85.5 10.8
161.1 5.2
0.99
0.92
0.99
2
4
8
12
16
24
32
135
135
135
135
135
135
135
100
100
100
100
100
100
100
1.8 0.3
1.2 0.1
1.8 0.3
1.6 0.1
1.3 0.1
1.3 0.4
1.2 0.1
54.4 3.3
38.2 0.4
54.4 3.4
46.7 1.3
40.6 0.9
40.5 4.3
38.2 0.4
0.99
0.99
0.99
0.99
0.99
0.96
0.99
MRI scanner and image processing
the gel filled phantom and used to convert the relative R2
map profile to a relative dose profile [36].
Polymer gel phantoms were scanned in Siemens Vision and
Avanto 1.5 T MRI whole body scanners using circularly polarised head coils between 12 and 24 h following irradiation. T2 weighted base images were acquired using a 64
echoes multiple slice multiple spin-echo pulse sequence
with the following parameters: TE 20 e 1280 ms, TR 4 s,
FOV 256 256 mm2 in the Siemens Vision and a 32 echoes
multiple slice multiple spin-echo pulse sequence with the
following parameters: TE 40e1280 ms, TR 4 s, FOV
256 256 mm2 in the Siemens Avanto. Acquisition time
was 25 min with 5 min set up time.
The T2 relaxation signal intensity base images of the coronal slices were transferred to a PC and the images were
processed with modified Matlab (Mathworks) software
to calculate T2 image maps [54]. The R2 image map was calculated and a profile of the slice was calculated and exported to Excel (Microsoft) and Origin (Microcal).
An R2 versus dose calibration curve was obtained from
the ionization chamber measurements at the centre of
CTDI calculations
CTDI100C for the PAGAT polymer gel dosimeter dose profile
was calculated by integrating under the dose profile, converting to dose in soft tissue (correction factor 0.94 (R/cGy))
and multiplying by 100 mm to take into account the length of
the dose profile and dividing this value by the nominal slice
width [36]. CTDI will henceforth be used to refer to CTDI100C.
CTDI for the TLD dose profile was also calculated by
integrating under the dose profile, converting to dose in
soft tissue and multiplying by 100 mm to take into account
the length of the dose profile and dividing this value by the
nominal slice width.
CTDI for the ionization chamber, which is CTDI for the
100 mm ionization chamber in the centre of each of the
phantoms and corrected for dose in soft tissue was determined according to the literature [38,55].
45
2 mm nominal slice width
4 mm nominal slice width
8 mm nominal slice width
12 mm nominal slice width
16 mm nominal slice width
24 mm nominal slice width
32 mm nominal slice width
90
CTDI (mGy)
80
70
60
35
30
25
50
20
40
15
30
Ion chamber in gel phantom 100 kVp
PAGAT phantom 100 kVp
TLD in gel phantom 100 kVp
Ion chamber in gel phantom 120 kVp
PAGAT phantom 120 kVp
TLD in gel phantom 120 kVp
Ion chamber in gel phantom 135 kVp
PAGAT phantom 135 kVp
TLD in gel phantom 135 kVp
40
CTDI (mGy)
100
10
20
6
6
8
10
12
14
16
Diameter (cm)
Figure 3 CTDI(100C) as a function of diameter at nominal slice
widths 2, 4, 8, 12, 16, 24 and 32 mm for PAGAT phantoms.
7
8
9
10
11
12
13
14
15
16
Diameter (cm)
Figure 4 CTDI(100C) as a function of diameter at voltages 100,
120 and 135 kVp for an ionization chamber, TLD and PAGAT.
Fitted lines are for PAGAT only.
154
Table 4
B. Hill et al.
CTDI(100C) linear fits for the PAGAT polymer gel across the range of phantoms as a function of tube voltage
Set CT parameters
Single exponential decay
Slice width (mm)
Tube voltage (kVp)
Current time (mAs)
Decay constant
Offset
Amplitude
Correlation co-efficient
16
16
16
100
120
135
100
100
100
1.0
1.7
2.7
11.0
17.2
20.7
3027.0
407.0
186.0
1.0
1.0
1.0
z-Efficiency
For each of the nominal slice widths a Catphan 500 (The
Phantom Laboratory, Cambridge NY, USA) was used to measure the imaged slice width of the X-ray detector system.
The Catphan 500 has angled lengths of wire that appear
on the CT image and through calculation the imaged slice
width could be obtained. The percentage difference of
the imaged slice width obtained from the Catphan 500
and measured SWDP obtained from the full-width half-maximum (FWHM) values of either the TLD or polymer gel were
used for comparison to obtain z-efficiency for each of the
nominal slice widths for the multislice CT scanner.
produce type A uncertainty for the polymer gel. Similar
type A uncertainty can be considered for TLD e.g. evaluation of the reader and temperature drift in the annealing
cycle [58]. The ionization chamber has much greater reproducibility and therefore, the type A uncertainty is reduced
significantly compared to both the polymer gel and TLD.
Type B uncertainty for all of the dosimeters was considered
to approximate a Gaussian distribution. The combined estimated uncertainty for each of the dosimeters was 0.5% for
the ionization chamber, 3% for TLD and 3% for the polymer
gel. The plotted uncertainty on each of the data points is
the absolute value of uncertainty for that data point.
Results and discussion
Uncertainty
Tube current time (mAs)
Uncertainty associated with the determination of dose for
each of the dosimeters was estimated following the general
guidelines of the International Organisation of Standardisation [56]. Standard uncertainties are classified into types A
or B. Type A uncertainties are associated with the distribution of measured values around the mean and type B uncertainties are such that they cannot be determined from
repeated measurements. The polymer gel uncertainties
were from several sources e.g. the method of calibration
from the ionization chamber, production of the polymer
gel, the output of the CT scanner varying at the time the
polymer gel data is recorded, evaluation variation from
the MRI such as temperature drift or non-uniformity of
the radiofrequency [14,57]. The combined uncertainty of
these sources of uncertainty and others were combined to
TLD
PAGAT
100
80
b
Normalised Dose
Normalised Dose
a
Fig. 2 shows CTDI as a function of diameter for each of the
dosimeters when exposed to varying mAs. The mean CTDI
values for the dosimeters and standard deviation of results
are in Table 2. The fitted lines in Fig. 2 are for the PAGAT
polymer gel as a function of phantom diameter with the linear fits summarized in Table 3. The slope of the linear fits
increases with decreasing mAs. The effects of mAs become
more significant for the smallest phantom diameter with an
increase in CTDI from 16 to 6 cm diameter phantom of 60%.
Optimisation of mAs based on weight or patient diameter
has been suggested by several authors in order to reduce
dose and maintain acceptable image quality [40,43,59].
The data from both PAGAT polymer gel and TLDs indicates
that optimisation of mAs is necessary and a function of
60
40
80
60
40
20
20
0
0
0
20
40
60
Distance (mm)
80
100
TLD
PAGAT
100
0
20
40
60
80
100
Distance (mm)
Figure 5
(a) A 16 mm Slice Width Dose Profile for TLD and PAGAT polymer gel in the centre of a 6 cm, and (b) 16 cm diameter
phantoms.
Polymer gel dosimetry on a multislice CT scanner
155
Table 5 For each of the nominal slices the z-efficiency
was determined from TLD and PAGAT FWHM compared to
the Catphan 500
Nominal
slice
width (mm)
TLD
FWHM
(mm)
PAGAT
FWHM
(mm)
Catphan
FWHM
(mm)
z-Efficiency
(%)
2
4
8
12
16
24
32
4.0
7.8
12.0
14.5
18.8
27.5
35.4
4.2
8.0
11.6
15.0
18.8
28.0
36.1
4.7
5.4
7.4
11.0
13.5
18.1
23.0
111.9
67.5
63.8
73.3
71.8
64.6
63.7
associated with the gel itself but with the evaluation
method used, in this case MRI. The image resolution used
for determining the R2 map and subsequent dose map was
from a 1 mm pixel resolution limiting the dose measurement to 1 mm, similar to that of the TLD. This can be overcome by using a finer pixel resolution in the MRI evaluation
or an alternative evaluation technique.
The fitted lines shown in Fig. 3 are for the PAGAT polymer gel as a function of phantom diameter with the linear
fits summarized in Table 3. The correlation co-efficient remain above 0.99 indicating a strong correlation for all but
the 24 mm slice thickness. The gradient increases for
smaller nominal slice widths and the CTDI increases by
45% for the smaller nominal slice widths and smallest
phantom.
Tube voltage
patient diameter in order to achieve lower CTDI. Polymer
gel dosimeters irradiated on a multislice CT scanner supports the previously demonstrated relationship of increasing CTDI as a result of increasing mAs shown with TLD
[40]. Generally for Fig. 2 it was noted that with fixed
tube voltage and increasing mAs, CTDI increases with decreasing phantom diameter as a result of less attenuating
material.
Nominal slice width
Fig. 3 shows CTDI as a function of diameter for only the PAGAT polymer gel dosimeter when exposed to varying slice
widths. The mean CTDI values and standard deviation for
ionization chamber, TLD and PAGAT polymer gels are in
Table 2. The greatest standard deviation occurs for the
2 mm slice width and at the smallest diameter. Considerable difference was noted between the ionization chamber,
PAGAT polymer gel and TLD results at the smallest slice
width for which PAGAT and TLD are both consistently lower
than the ionization chamber values. This occurs because
both TLD and polymer gel are limited in measuring dose
at the smaller slice widths. TLDs were stacked together
to obtain the dose profile with each TLD having a width of
0.9 mm that limits the overall dose measurement level to
0.9 mm or two stacked TLD across the FWHM of the 2 mm
slice width. Two points were not enough to achieve an adequate representation of the SWDP and thinner TLD would
have been more appropriate for this slice width. The polymer gel limitation with the 2 mm slice width is not
Fig. 4 shows CTDI as a function of diameter for each of the
dosimeters when exposed to varying tube voltages. The
mean CTDI values for the dosimeters and standard deviation of results are in Table 2. The logarithmic relationship
of CTDI to phantom diameter for tube voltage should hold
true for mono-energetic energies due to linear attenuation
[60]. As the CT scanner produces a spectrum of energy with
a mean energy of approximately 70 kV for example at
120 kVp the logarithmic relationship may result in a weaker
correlation. The logarithmic relationship shown in Fig. 4
and summarized in Table 4 was a good representation as
the correlation co-efficient was 1.0. The results are consistent with a previous study that found it was reasonable to
model the relationship of CTDI to phantom diameter as
a logarithmic function [40]. Both TLDs and PAGAT polymer
gels demonstrate this relationship with an increase in
CTDI with tube voltage. The increase in CTDI at 100 kVp
was 50% from the 16 to 6 cm phantom diameter and 100%
for 135 kVp. The effect of tube voltage becomes more significant with lower tube voltages due to increasing scatter
and has the greatest impact on the smallest diameter
phantom.
SWDP and z-efficiency
Fig. 5 shows SWDP measured centrally with TLD and PAGAT
polymer gel in a 6 and 16 cm diameter phantom for the
16 mm nominal slice. The 16 cm diameter phantom shows
a greater scatter contribution relative to the 6 cm
Table 6 For each of the nominal slices the CTDI(100C) was determined from an ionization chamber and TLD in a 16 cm diameter
gel phantom and from PAGAT
Nominal slice width
(mm)
Uncertainty TLD CTDI(100C) Uncertainty PAGAT CTDI(100C) (mGy) Uncertainty
Ion chamber
(mGy)
CTDI(100C) (mGy)
2
4
8
12
16
24
32
52.7
35.0
25.2
23.5
21.2
21.3
20.0
0.3
0.2
0.1
0.1
0.1
0.1
0.1
44.5
36.1
24.7
24.1
20.1
20.9
20.1
1.3
1.0
0.7
0.7
0.6
0.6
0.6
39.5
36.6
25.7
21.3
19.7
21.0
19.1
1.2
1.1
0.8
0.6
0.6
0.6
0.6
156
B. Hill et al.
120
b 120
100
100
Normalised Dose
Normalised Dose
a
80
60
40
80
60
40
20
20
0
0
0
10
20
30
40
50
60
0
20
Figure 6
40
60
80
100
120
140
160
180
Distance (mm)
Distance (mm)
(a) PAGAT polymer gel dose profile across the 16 mm nominal slice width in a 6 cm, and (b) 16 cm diameter phantoms.
diameter phantom. Table 5 summarises the results for
both TLD and PAGAT polymer gel SWDP for all nominal
slice widths and shows the calculated z-efficiency of polymer gel to range between 63.7 and 73.3% for all but the
nominal 2 mm slice width. For the 2 mm nominal slice
width the z-efficiency was 111.9% with the polymer gel
measured slice width less than the imaged. The maximum
difference of the TLD and polymer gel FWHM was 5% for
the 2 mm nominal slice width and less than that for all
other nominal slice widths. Both TLD and polymer gel
overestimate the slice width for the 2 mm nominal slice
due to the size of the TLD used and the evaluation parameters of the polymer gel. The TLD and polymer gel overlie
each other in most instances. The benefit of the polymer
gel over the TLD is that the number of data points that
can be obtained is dependent on the evaluation image resolution not the number of physically stacked TLD. Some of
the nominal slice widths were identified as outside of the
manufacturers specifications and subsequently corrected
as a result of this work.
Table 6 shows the results of the calculated CTDI data obtained with TLD and polymer gel for each of the nominal
slice widths as determined from the SWDP and for comparison CTDI calculated for an ionization chamber in air is also
shown. The uncertainty approximately doubles for decreasing nominal slice for all dosimeters. The maximum difference of 33.4% between CTDI occurs for the smallest
nominal slice width. The CTDI values for the other nominal
slice widths vary between 2 and 8%.
Dose profile
Fig. 6 shows a dose profile measured centrally with PAGAT
polymer gel in a 6 and 16 cm diameter phantom for the
16 mm nominal slice across the axial slices in the xey
plane. Variation in relative dose occurs across both of the
phantoms. For the 16 cm diameter phantom the variation
increases from 3.0 to 10.0%, respectively, from 32 to
2 mm nominal slice width. This effect is reduced for the
smaller diameter phantom. For the 6 cm diameter phantom
the difference increases from 1.0 to 5.0%, respectively,
from the 32 to 2 mm nominal slice width.
Conclusion
The aim of this paper was to compare PAGAT polymer gel
dosimeters to conventional TLD and ionization chamber
measurements for the purpose of multislice CT scanner
dosimetry and to investigate CTDI as a function of chosen
CT parameters.
Polymer gel dosimeters were shown to support the
previously demonstrated relationship of increasing CTDI
with increasing mAs measured with TLD [40]. It has also
been shown to be suitable for measurement of dose and
subsequently CTDI for changing mAs. The effect of tube
voltage was shown to become more significant with lower
tube voltages. This was also consistent with TLD measurements under the same conditions.
Polymer gel dosimeters were used for SWDP determination for use in z-efficiency calculations and for the determination of CTDI from SWDP. These results were consistent
with TLD except for less than 4 mm nominal slice widths
in which case the utilized MRI parameters for evaluation
of PAGAT polymer gels were found to be unsuitable.
In summary, PAGAT polymer gel can be used as an
alternative dosimeter to TLD for the determination of
SWDP. A concern about clinical implementation of this
type of dosimeter may be the skills and preparation,
however, it was found that the annealing, preparation,
exposure and readout time of TLD was not significantly
different than preparation, exposure and evaluation time
of the PAGAT polymer gel. The benefit of the use of PAGAT
polymer gel is its inherent 3D nature and future work with
multislice CT should focus on investigating dose distributions in anatomical phantoms.
References
[1] Alexander P, Charlesby A, Ross M. The degradation of solid
polymethylmethacrylate by ionizing radiations. Proc R Soc
1954;A223:392.
[2] Hoecker FE, Watkins IW. Radiation polymerization dosimetry.
Int J Appl Radiat. Isot 1958;3:31e5.
[3] Boni A. A polyacrylamide gamma dosimeter. Radiat Res 1961;
14:374e80.
Polymer gel dosimetry on a multislice CT scanner
[4] Maryanski MJ, Gore JC, Kennan RP, Schulz RJ. NMR relaxation
enhancement in gels polymerized and cross-linked by ionizing
radiation: a new approach to 3D dosimetry by MRI. Magn Reson
Imaging 1993;11:253e8.
[5] Maryanski MJ, Schulz RJ, Ibbott GS, Gatenby JC, Xie J,
Horton D, et al. Magnetic resonance imaging of radiation
dose distributions using a polymer-gel dosimeter. Phys Med
Biol 1994;39:1437e55.
[6] Baldock C, Burford RP, Billingham N, Wagner GS, Patval S,
Badawi RD, et al. Experimental procedure for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic
resonance imaging (MRI) radiation dosimetry. Phys Med Biol
1998;43:695e702.
[7] Venning AJ, Hill B, Brindha S, Healy BJ, Baldock C. Investigation of the PAGAT polymer gel dosimeter using magnetic
resonance imaging. Phys Med Biol 2005;50:3875e88.
[8] Lepage M, Jayasekera PM, Back SAJ, Baldock C. Dose
resolution optimization of polymer gel dosimeters using
different monomers. Phys Med Biol 2001;46:2665e80.
[9] De Deene Y, De Wagter C, De Neve W, Achten E. Verification of
three-dimensional BANG gel dosimetry by use of magnetic
resonance imaging (MRI) in clinical applications. Proc Int Soc
Magn Reson Med 1996.
[10] De Deene Y, De Wagter C, Van Duyse B, Derycke S, De Neve W,
Achten E. Three-dimensional dosimetry using polymer gel and
magnetic resonance imaging applied to the verification of
conformal radiation therapy in head-and-neck cancer.
Radiother Oncol 1998;48:283e91.
[11] Schreiner LJ, Audet C, editors. DOSGEL 1999 e Proceedings of
the 1st international workshop on radiation therapy gel
dosimetry; 1999.
[12] Baldock C, De Deene Y, editors. DOSGEL 2001 e Proceedings of
2nd international conference on radiotherapy gel dosimetry;
2001.
[13] De Deene Y, Baldock C, editors. DOSGEL 2004 e Proceedings of 3rd
international conference on radiotherapy gel dosimetry; 2004.
[14] Lepage M, Jirasek A, Schreiner LJ, editors. DOSGEL 2006 e
Fourth international conference on radiotherapy gel dosimetry; 2006.
[15] Fong PM, Keil DC, Does MD, Gore JC. Polymer gels for
magnetic resonance imaging of radiation dose distributions
at normal room atmosphere. Phys Med Biol 2001;46:3105e13.
[16] Baldock C. Historical review of the development of gel
dosimetry: a personal perspective. In: Lepage M, Jirasek A,
Schreiner LJ, editors. DOSGEL 2006 e Fourth international
conference on radiotherapy gel dosimetry; 2006.
[17] De Deene Y, Venning A, Hurley C, Healy BJ, Baldock C. Dosee
reponse and spatial stability of various polymer gel dosimeters. Phys Med Biol 2002;47:2459e70.
[18] De Deene Y, Hurley C, Venning A, Vergote M, Mather M,
Healy BJ, et al. A basic study of some normoxic polymer gel
dosimeters. Phys Med Biol 2002;47:3441e63.
[19] Venning AJ, Nitschke K, Keal P, Baldock C. Radiological
properties of normoxic polymer gels. Med Phys 2005;32:
1047e53.
[20] Gore JC, Ranade M, Maryanski MJ, Schulz RJ. Radiation dose
distributions in three dimensions from tomographic optical
density scanning of polymer gels: I. Development of an optical
scanner. Phys Med Biol 1996;41:2695e704.
[21] Oldham M, Siewersden HH, Shetty A, Jaffray DA. High
resolution gel-dosimetry by optical-CT and MR scanning. Med
Phys 2001;23:699e705.
[22] Maryanski MJ, Zastavker YZ, Gore JC. Radiation dose distributions in three dimensions from tomographic optical density
scanning of polymer gels: II. Optical properties of the BANG
polymer gel. Phys Med Biol 1996;41:2705e17.
[23] Doran SJ, Koerkamp KK, Bero MA, Jenneson PM, Morton E,
Gilboy WB. A CCD-based optical CT scanner for high-resolution
157
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
3D imaging of radiation dose distributions: equipment
specifications, optical simulations and preliminary results.
Phys Med Biol 2001;46:3191e213.
Lepage M, Whittaker AK, Rintoul L, Back SAJ, Baldock C. The
relationship between radiation-induced chemical processes
and transverse relaxation times in polymer gel dosimeters.
Phys Med Biol 2001;46:1061e74.
Jirasek A, Duzenli C. Effects of crosslinker fraction in polymer
gel dosimeters using FT-Raman spectroscopy. Phys Med Biol
2001;46:1949e61.
Mather ML, Baldock C. Ultrasound tomography imaging of
radiation dose distributions in polymer gel dosimeters:
preliminary study. Med Phys 2003;30:2140e8.
Mather ML, Charles PH, Baldock C. Measurement of ultrasonic
attenuation coefficient in polymer gel dosimeters. Phys Med
Biol 2003;48:269e75.
Mather ML, Collings AF, Bajenov N, Whittaker AK, Baldock C.
Ultrasonic absorption in polymer gel dosimeters. Ultrasonics
2003;41:551e9.
Mather ML, De Deene Y, Whittaker AK, Simon GP, Rutgers R,
Baldock C. Investigation of ultrasonic properties of PAG and
MAGIC polymer gel dosimeters. Phys Med Biol 2002;47:
4379e409.
Mather ML, Whittaker AK, Baldock C. Ultrasound evaluation of
polymer gel dosimeters. Phys Med Biol 2002;47:1449e58.
Hilts M, Audet C, Duzenli C, Jirasek A. Polymer gel dosimetry
using X-ray computer tomography: a feasibility study. Phys
Med Biol 2000;45:2559e71.
Hilts M, Duzenli C. Image filtering for improved dose resolution
in CT polymer gel dosimetry. Med Phys 2004;31:39e49.
Trapp J, Back SAJ, Lepage M, Michael G, Baldock C. An
experimental study of the dose response of polymer gel
dosimeters imaged with X-ray computer tomography. Phys
Med Biol 2001;46:2939e51.
Brindha S, Venning A, Hill B, Baldock C. Experimental study of
attenuation properties of normoxic polymer gel dosimeters.
Phys Med Biol 2004;49:N353e61.
Hill B, Venning A, Baldock C. The dose response of normoxic
polymer gel dosimeters measured using X-ray CT. Br J Radiol
2005;78:623e30.
Hill B, Venning A, Baldock C. A preliminary study of the novel
application of normoxic polymer gel dosimeters for measurement of CTDI on diagnostic X-ray CT scanners. Med Phys 2005;
32:1589e97.
AAPM Task Group Report No. 39. Specification and acceptance
testing of computer tomography scanners. Wisconsin: Medical
Physics Publishing; 1993.
AAPM task group report No. 32. Measurement of the
performance characteristics of diagnostic X-ray systems used
in medicine, part III, computed tomography X-ray scanners;
2001.
European Commission. Quality criteria for computed tomography. EUR 16262 EN. Brussels; 1999.
Siegel MJ, Schmidt B, Bradley D, Suess C, Hildebolt C.
Radiation dose and image quality in pediatric CT: effect of
technical factors and phantom size and shape. Radiology
2004;233:515e22.
Boone JM, Geraghty EM, Seibert JA, Wootton-Gorges SL. Dose
reduction in pediatric CT: a rational approach. Radiology
2003;228:352e60.
Nickoloff EL, Dutta AK, Zheng FL. Influence of phantom
diameter, kVp and scan mode upon computed tomography
dose index. Med Phys 2003;30:395e402.
Huda W, Scalzetti EM, Levin G. Technique factors and image
quality as functions of patient weight at abdominal CT.
Radiology 2000;217:430e5.
ImPACT. CT scanner acceptance testing. Leaflet no. 1 2001:
ver. 1.02.
158
[45] Cheung T, Cheng Q, Feng D, Stokes MJ. Study on examinee’s
dose delivered in computed tomography. Phys Med Biol
2001;46:813e20.
[46] McNitt-Gray F, Cagnon CH, Solberg TD, Chetty I. Radiation
dose in spiral CT: the relative effects of collimation and pitch.
Med Phys 1999;26:409e14.
[47] Tsai HY, Tung CJ, Huang MH, Wan YL. Analyses and applications of single scan dose profiles in computed tomography.
Med Phys 2003;30:1982e9.
[48] Chapple C-L, Willis S, Frame J. Effective dose in pediatric
computer tomography. Phys Med Biol 2002;47:107e15.
[49] Cohnen M, Poll LW, Puettmeann C, Ewen K, Saleh A, Modder U.
Effective doses in standard protocols for multi-slice CT
scanning. Eur Radiol 2003;13:1148e53.
[50] Dixon RL. A new look at CT dose measurement: beyond CTDI.
Med Phys 2003;30:1272e80.
[51] Thomson FJ. Measurement of CT scanner dose profiles in
a filmless department. Australas Phys Eng Sci Med 2005;28:
179e83.
[52] Nakonechny KD, Fallone BG, Rathee S. Novel methods of
measuring single scan dose profiles and cumulative dose in
CT. Med Phys 2005;32:98e109.
[53] Davis SD, Ross CK, Mobit PN, Van Der Zwan L, Chase WJ,
Shortt KR. The response of LiF thermoluminescence
B. Hill et al.
[54]
[55]
[56]
[57]
[58]
[59]
[60]
dosimeters to photon beams in the energy range from 30 kV
X-rays to 60Co gamma rays. Radiat Prot Dosimetry 2003;106:
33e43.
Murry P, Baldock C. Research software for MRI radiotherapy
gel dosimetry. Australas Phys Eng Sci Med 2000;23:44e51.
Performance standards for ionizing radiation emitting products.
Diagnostic X-ray systems and their major components, computer
tomography (CT) equipment. U.S. Code of Federal Regulations
T, Section 1020.33, Govt. Printing Office; 1984.
ISO guide to the expression of uncertainty in measurement.
Geneva: International Organization for Standardization; 1995.
De Deene Y, De Wagter C, De Neve W, Achten E. Artefacts in
multi-echo T2 imaging for high-precision gel dosimetry: II.
Analysis of B1-field inhomogeneity. Phys Med Biol 2000;45:
1825e39.
Yu C, Luxton G. TLD dose measurement: a simplified accurate
technique for the dose range from 0.5 cGy to 1000 cGy. Med
Phys 1999;26:1010e6.
Shope T, Gange R, Johnson GA. Method of describing the doses
delivered by transmission X-ray computed tomography. Med
Phys 1981;8:488e95.
Bushberg JT, Siebert JA, Leidholdt EM, Boone JM. The
essential physics of medical imaging. Philadelpheia, PA:
Lippincott, Williams and Wilkins; 2002.