Reduced dose uncertainty in MRI-based polymer gel dosimetryusing parallel RF transmission with multiple RF sources
Sang-Young Kim • Hyeon-Man Baek • Jung-Hoon Lee • Dae-Hyun Kim •
Jin-Young Jung • Do-Wan Lee • Jung-Whan Min • Ji-Yeon Park •
Seu-Ran Lee • Bo-Young Choe
Received: 19 March 2014
� Akademiai Kiado, Budapest, Hungary 2014
Abstract In this work, we present the feasibility of using a
parallel RF transmit with multiple RF sources imaging method
(MultiTransmit imaging) in polymer gel dosimetry. Image
quality and B1 field homogeneity was statistically better in
the MultiTransmit imaging method than in conventional sin-
gle source RF transmission imaging method. In particular,
the standard uncertainty of R2 was lower on the Multi-
Transmit images than on the conventional images. Further-
more, the MultiTransmit measurement showed improved dose
resolution. Improved image quality and B1 homogeneity results
in reduced dose uncertainty, thereby suggesting the feasibility
of MultiTransmit MR imaging in gel dosimetry.
Keywords Gel dosimetry � MultiTransmit MR imaging �B1 field homogeneity � Image quality � Standard
uncertainty � Dose resolution
Introduction
To date, the most commonly used method for dose verifi-
cation in a clinical radiotherapy setting is either to use
ionization chambers, which calibrate the point dose in one-
dimension (1D), or a film dosimeter for dose measurement
in two-dimensions (2D). However, such methods may not
be appropriate for the measurement of more complex dose
distributions in advanced radiotherapy techniques, such as
intensity-modulated radiotherapy and three-dimensional
conformal radiotherapy. The polymer gel dosimeter, which
utilizes the mechanism of radiation-induced polymeriza-
tion of the monomer, has a specific advantage in that it can
uniquely record the radiation dose distribution in three-
dimensions (3D) [1].
When the gel is irradiated, the co-monomers polymerize
to a cross-linked polyacrylamide, resulting in a change in
the spin–spin relaxation time (T2). It was found that the
relaxation rate (R2 = 1/T2) is increased as a function of the
absorbed dose by a decrease in the monomer concentration
and by the formation of the polymer. This was evidenced
by a model of the fast exchange of magnetization [2]. The
other MRI parameters, such as the spin–lattice relaxation
time (T1) and the magnetization transfer ratio [3, 4] also
changed upon irradiation. However, among these parame-
ters, the R2 is the most sensitive to dose variation; hence, in
Electronic supplementary material The online version of thisarticle (doi:10.1007/s10967-014-3232-9) contains supplementarymaterial, which is available to authorized users.
S.-Y. Kim � J.-H. Lee � J.-Y. Jung � D.-W. Lee � S.-R. Lee �B.-Y. Choe (&)
Department of Biomedical Engineering and Research Institute of
Biomedical Engineering, College of Medicine, The Catholic
University, Seoul, Republic of Korea
e-mail: [email protected]
H.-M. Baek
Center of Magnetic Resonance Research, Korea Basic Science
Institute, Ochang, Chungbuk, Republic of Korea
H.-M. Baek
Department of Bio-Analytical Science, Korea University of
Science and Technology, Daejeon, Republic of Korea
D.-H. Kim
Department of Radiation Oncology, Samsung Medical Center,
Seoul, Republic of Korea
J.-W. Min
Department of Radiological Science, The Shingu University
College of Korea, Seongnam, Republic of Korea
J.-Y. Park
Department of Pediatrics and Molecular Imaging Program at
Stanford, Standford University, Palo Alto, CA, USA
123
J Radioanal Nucl Chem
DOI 10.1007/s10967-014-3232-9
the majority of MRI-based polymer gel dosimetry studies,
the R2 value have been used for dose verification [5]. As
the dose distribution is directly derived from R2, the
accuracy and artifacts in the R2 maps is of major concern in
gel dosimetry.
The intended target accuracy in gel dosimetry for clin-
ical radiotherapy is about 5 % of the maximum dose in the
regions of homogeneous dose and a spatial error of less
than about 2 mm in the regions of high-dose gradient [6].
However, a number of factors including accuracy of cali-
bration curve [7], the ageing dynamics of the polymer gel
[8, 9], the B1 field inhomogeneity [10], and eddy currents
[11] influence the uncertainty of the dose determination.
While all these sources for the uncertainty need to be
addressed for optimal results, the current study mainly
focused on the effects of image quality (i.e., signal-to-noise
ratio, SNR and image uniformity) and/or B1 field inho-
mogeneity on the final dose profile.
In recent years, MRI-based polymer gel dosimetry
working at a field strength of 3 Tesla has become
increasingly more frequent since it allows improved SNR
compared to 1.5 Tesla. However, the spatial inhomogeneity
of the B1 field at 3 Tesla has become an issue in accurately
determining the R2 values for 3D dose distribution. It is
shown that at higher field strengths, the dielectric proper-
ties of the polymer gel dosimeters have a significant impact
on the B1 field homogeneity in the phantom and hence on
the dose maps. Due to the inhomogeneous transmitted RF
fields and inhomogeneous recording sensitivity of the
receiver coil, the magnitude in the MR images of a
homogeneous phantom may not be uniform. In addition,
the slice profile differs from an ideal rectangular shape due
to the limited duration of the slice-selective RF pulse.
When a multiple spin-echo sequence is used, the non-
rectangular slice profile induces stimulated echo compo-
nents that alter the measured R2 [10]. Multiple transmit
channels and RF sources can minimize this problem and
facilitate better control of the RF field. The technique of
parallel RF transmission allows for independent adjustment
of the phase and amplitude for different RF sources, thus
enabling the achievement of a more homogeneous excita-
tion and reduction or elimination of standing-wave artifacts
at higher field imaging [12–14].
In this study, we aimed to quantify the dose uncertainty
in MRI-based gel dosimetry when using conventional
single source RF transmission. The results of dose profiles
are compared with those obtained from a parallel RF
transmit with multiple RF sources. To evaluate the dose
uncertainty in the R2-derived dose images, we used the
concept of dose resolution proposed by Baldock et al. [15].
The dose resolution concept is defined as the minimal
detectable dose difference within a given level of
confidence. Furthermore, we assessed the temporal varia-
tion in the dose response for MRI-based gel dosimetry.
To make use of the gel as a tool for clinical dosimetry, it
may be more efficient to purchase the small amounts
needed rather than invest the time and resources required
for successful in-house manufacture. Hence, in the current
study, a commercially available polymer gel (BANG3�
gel, MGS Research Inc., Guilford, CT, USA) was used as a
tool for MRI-based dosimetry.
Materials and methods
Materials and equipments
BANG3� gel kit containing BANG gel, antioxidant, and
CuSO4 was purchased from MGS Research Inc. (Madison,
CT, USA) The radiation experiment was performed using 6
MV Varian Linear Accelerator (Varian Medical Systems,
Palo Alto, CA, USA). The MR experiments were per-
formed using a whole-body 3.0 T MRI system (Achieva 3.0
T TX; Philips Medical System) equipped with a 32-channel
head coil.
T2 measurement precision
The T2 measurement was performed by imaging a com-
mercially available liquid phantom (known T2 value:
330 ms) containing copper sulfate (0.4 % weight of water)
and 99.6 % of water. To evaluate the T2 measurement
precision of the MR system, the phantom was scanned five
times contiguously to give a value for the precision. The
experiments of the T2 measurement precision were per-
formed after selecting the optimal echo spacing time.
Polymer gel phantom
The BANG3� gel used in the current study consisted of
water, gelatin, and methacrylic acid. To prepare the gel
without the need for any efforts to minimize oxygen con-
tamination, we added 1 mol of ascorbic acid as an anti-
oxidant that scavenged the oxygen of the gel. 0.1 mol of
CuSO4 was also added as a catalyst for binding the oxygen
to the ascorbic acid. The polymer gel was contained in 16
small glass vials (2 cm inner diameter and 9 cm long
cylinders) and a 500 ml plastic container (9 cm inner
diameter and 10 cm long) for experiments. All of the gel
used in the experiments was manufactured in a single batch
to maximize consistency. The phantoms containing the
polymer gel were stored at 4 �C in a refrigerator for 2 days
before irradiation as recommended by the manufacturer
(MGS Research Inc.).
J Radioanal Nucl Chem
123
Irradiation
Before the experiments began, the radiation outputs (cGy
per monitor unit) of the linear accelerator were characterized
and verified. The calculated monitor units were adjusted to
give the correct doses. To confirm the effects of the photon
beam attenuation due to the 3 mm phantom wall, dose
measurement was performed twice using a 0.6 cc Farmer-
type ionization chamber placed inside and outside the
phantom. After confirming that there was no difference in
the measured dose value, the experiments were started.
The polymer gel phantoms were irradiated with a 6 MV
photon beam with a dose rate of 2 Gy min-1 from a Varian
linear accelerator. The small calibration vials were indi-
vidually irradiated at various doses (0–10 Gy in 1 Gy steps
and 10–20 Gy in 2 Gy steps) with a 15 9 15 cm2 field size
in a water bath. The long axes of the small vials were
placed perpendicularly to the beam axis. A 500 ml phan-
tom was irradiated using a pair of 5 cm 9 5 cm parallel
opposing lateral beams. The dose delivered at the over-
lapping region was 8 Gy.
MRI measurement
MRI scanning was carried out after the irradiated gel
phantoms were left overnight in an MRI scanner room to
equilibrate to an ambient temperature of 22 �C. All phan-
toms were scanned together. Sixteen calibration vials were
placed around the 500 ml phantom for scanning. A multi-
slice multi-echo sequence (TR/DTE = 3,000/20 ms, the
number of echoes = 32, FOV = 230 9 230 mm2, pixel
size = 0.45 mm 9 0.45 mm, slice thickness = 3 mm,
NEX = 1, number of slices = 12) was used for T2 mea-
surement. The scanning was performed twice, once with
and once without, turning on ‘‘Multi-Transmit’’ in the MRI
console. With MultiTransmit, the power, amplitude, phase
and waveform of all RF sources are automatically adjusted
for optimal uniformity in the phantom. Before Multi-
Transmit measurement, a B1 calibration scan was per-
formed to adaptively shim the RF field. To consider the
effects of the homogeneity of the active B1 field (B1?) on
the final dose profile, we compared a B1? map obtained
from a conventional single source RF transmission system
with that obtained using parallel RF transmission with
multiple RF sources. We used the double angle method for
B1? mapping [16]. This method requires two magnitude
images: I1 with prescribed tip angle a1 and I2 with pre-
scribed tip angle a2 = 2a1. We can then calculate the
actual tip angles as a function of the spatial position
according to the following equation:
aðrÞ ¼ arccosI2ðrÞ
2I1ðrÞ
����
����
� �
ð1Þ
A long repetition time (TR C 5T1) is typically used to
minimize T1 dependence in either I1 or I2. The gel phantom
was scanned using the following parameters: TR/
TE = 8,000/2.4 ms, NEX = 1, matrix size = 76 9 74,
number of slice = 5, a1 = 45�, and a2 = 90�.
In addition, to assess the temporal variation of the
polymer gel dosimeter, we conducted MR scanning with
the same protocol after 1 week.
R2 estimation method
We ignored any multiexponential consideration of the T2
decay curves and limited our analysis to a monoexponen-
tial approach. Thus, a monoexponential decay model
applied to a multiecho sequence is given by:
SðtÞ ¼ q0 � exp �R2 � tð Þ ð2Þ
where the pseudodensity q0 is the signal intensity at echo
time t = 0. All multiple echo signals were used for the R2
estimation. The parameters q0 and R2 in Eq. (2) can be
estimated by using the non-linear least squares regression
function ‘‘nlinfit’’ provided with the MATLAB Statistics
Toolbox (Version R2008a; The Mathworks Inc., MI,
USA). This function used a Gauss–Newton algorithm with
Levenberg–Marquardt modifications for converging to the
minimum least squares solution. For each pixel, the R2 and
q0 values were obtained along with a corresponding
covariance matrix for the fitted coefficients (q0 and R2).
The standard deviation of R2, r(R2), which was defined as
equal to the standard uncertainty of R2, was obtained from
the covariance matrix. We used the Image J program
(National Institute of Health, Bethesda, MD) (http://rsb.
info.nih.org/ij/) to measure the mean R2 values in the cir-
cular type of region of interest (ROI) for each calibration
vial.
Image analysis and B1 field homogeneity
To evaluate image quality, we calculated the SNR and
signal homogeneity (image uniformity) of images
obtained from the conventional and MultiTransmit
methods. The ROIs were placed in the image of each
calibration vial treated with different radiation doses.
Because parallel imaging prohibits the use of a conven-
tional method of measuring noise in which the ROI is
placed in the air, we used the standard deviation of signal
intensity measured in each ROI as an estimate of local
noise [17, 18]. To compare image uniformity, the coef-
ficients of variation (CV, equal to SD divided by the
mean) of each ROI were calculated. The CV values
regarded as the uniformity index [19] were compared
between the conventional method and the MultiTransmit
J Radioanal Nucl Chem
123
method. The B1 homogeneity was assessed using the
same method on calculated B1 map.
Dose resolution
After the R2 distribution was obtained, we could calculate
the dose distribution by using the linear relationship
between R2 and the dose:
R2 ¼ a0 þ a1 � D ð3Þ
where a0 and a1 are constants, the values of which can be
determined by linear curve fitting as implemented by
OriginPro (Version 8.0, OriginLab Corporation, MA,
USA).
The relative standard deviation in the dose is derived by
solving Eq. (3) for D and using the formulas for Gaussian
error propagation:
rD
D¼ 1
a1
�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
r2a1þ rR2
D
� �2
þ ra0
D
� �2r
ð4Þ
where rx represents the standard deviation of the variable
x. As the dose sensitivity (i.e., slope a1) increases, the error
in the dose will reduce. Dose resolution describes the
minimum difference in dose that can be detected at a
certain level of confidence p. For a linear dose response, it
is given by [15]:
DpD ¼ kp
ffiffiffi
2p
rD � kp
ffiffiffi
2p
rR2
a1
ð5Þ
where p represents the level of confidence and coverage
factor, kp, is the value given by the t distribution for
experimental degrees of freedom, e.g., k95 % is 1.96. The
rD given by Eq. (4) can be approximated by rR2=a1because
rR2is the largest contributor to rD [20]. The dose resolu-
tions (DD95 %) for conventional single source RF transmis-
sion were compared with those obtained from parallel RF
transmission with multiple RF sources.
Conversion of MR images to CT-like images
Since CT X-rays may potentially induce polymerization in
the gel, we converted the MR images of the gel phantom
into CT-like images by assigning reported bulk electron
density values (i.e., electron density: 0.991, CT number:
17.7) for the BANG gel [21]. We compared the measured
3D dose distribution with that obtained using the treatment
planning system. The dose computation was performed
with the collapsed cone convolution algorithm of the
Eclipse treatment planning system (Varian, CA, USA). The
CT-like images were constructed using an in-house written
MATLAB program.
Statistical analysis
The statistical analysis was performed with PASW 18.0
(Chicago, Ill, USA). A paired Wilcoxon test was used to
compare image quality (SNR, signal and B1 homogeneity)
between the conventional and MultiTransmit method. P
values less than 0.05 were considered statistically
significant.
Results
T2 measurement precision
In the polymer gel dosimetry, a number of factors can
introduce significant uncertainty in determining R2 quan-
titatively on clinical MRI scanners. We attempted to test T2
measurement precision of the MR system. The short-term
precision of the MR system for T2 measurement was found
to be below 0.75 %. The measured mean T2 values were
3.7 % higher than the known T2 value. In addition, the
number of echoes in the multi-echo spin echo sequence
should be sufficient to cover the exponential signal decay
until it reaches the baseline or noise so that the majority of
the decay is sampled. We compared the T2 decay curves
obtained from the echo spacing of 20 ms when 16 and 32
echoes were used. The results showed that the echo signals
decayed to the baseline when 32 echoes were used, at the
expense of a larger number of data collected (data not
shown).
Image analysis
The image quality (SNR and image uniformity) within
each calibration vial on the base image of multi echo
sequence was statistically better in MultiTransmit imaging
than that in conventional single-source RF transmission
imaging (P \ 0.005 for all calibration vials). For both
imaging techniques, the calibration vials positioned on the
outer layer (0, 2, 4, 6, 8, 10, 14, and 18 Gy) had lower SNR
than those positioned on the inner layer (Fig. 1).
As shown in Fig. 2, quantitative analysis of B1 homo-
geneity revealed that the actual flip angles of each cali-
bration vial and large phantom on the MultiTransmit B1
map were significantly higher than on the conventional
single-source B1 map (P \ 0.05 for all comparisons). The
CVs of the actual flip angles of each calibration vial and
large phantom on MultiTransmit B1 map were significantly
lower than on the conventional single-source B1 map
(P \ 0.05 except for vial irradiated to 10 Gy), suggesting
the actual flip angles and local signal intensity distribution
are more consistent on the MultiTransmit images.
J Radioanal Nucl Chem
123
BANG3 gel dosimetry
No significant difference was observed between the dose
measured with an ionization chamber alone in the beam
and that placed inside a test tube. We therefore concluded
that there is no visible impact on dose to the gel due to
attenuation by the test tube wall or reduced lateral scatter.
We used all of the multiple echo signals (i.e., 32
Fig. 1 Comparison of image quality between conventional single-
source and MultiTransmit imaging in calibration vials delivered with
different doses. The values were calculated from base images of
multi-slice multi-echo sequence. a SNR and b image uniformity index
(coefficients of variation, equal to SD divided by mean) are shown.
The asterisk (*) indicates statistically significant differences
(p \ 0.05) and the error bar indicates standard deviation for all slices
Fig. 2 Comparison of B1 field homogeneity between conventional
single-source RF transmission imaging and parallel RF transmit with
multiple RF sources imaging (MultiTransmit) method. B1 maps
obtained with conventional (a) and MultiTransmit imaging method
(b) are shown. (c) The second and third column shows the flip angle
(upper panel) and B1 uniformity (lower panel: coefficients of variation,
equal to SD divided by mean) for the calibration vials and large
phantom, respectively. The flip angle profile along the dotted blue line is
shown for large phantom (right upper panel). The asterisk (*) indicates
statistically significant differences (p \ 0.05). (Color figure online)
Table 1 Fitted parameters for the dose versus R2 relationship
Parameters First measurement Second measurement
Conventional MultiTransmit Conventional MultiTransmit
a0 (intercept) 2.484 ± 0.045 2.654 ± 0.034 3.599 ± 0.118 3.584 ± 0.113
a1 (slope) 0.993 ± 0.016 1.083 ± 0.022 1.019 ± 0.010 1.097 ± 0.019
The data are expressed with mean ± standard deviation
J Radioanal Nucl Chem
123
echoes 9 12 slices = 384 images) for the R2 estimation.
Measured mean signal intensities in circular ROIs (about
170 pixels) for each calibration vial showed a similar trend
that the echo signals decayed faster to the background
signal level as the dose increased. For both measurements,
there was a variation in the R2 response of BANG3 gel
between the scanning sessions (from week to week). In the
second measurement, slight increases in the R2 values were
observed compared to the first measurement (Table 1).
Furthermore, it is important to note that r(R2), defined as
the standard uncertainty of R2, was lower on the Multi-
Transmit images than on the conventional single-source
images (Fig. 3).
Table 1 summarizes the fitted parameters in the dose-R2
response curve for all slices. The dose response curve
appeared to be linear over the 0–20 Gy region (R2 [ 0.99).
Slightly higher R2-dose sensitivity (i.e., slope) was
observed in all slices for MultiTransmit measurement as
compared to conventional method, and intercept and slope
of the R2-dose plot was increased in the second measure-
ment compared to the first measurement.
Using the estimated parameters (a0, a1, and R2), we
could obtain a 3D dose distribution. Figure 4 shows the
measured dose distribution in the center slice of a 500 ml
gel phantom in which the delivered dose was 8 Gy. The
surrounding calibration vials were masked out, and the
Fig. 3 The standard uncertainty
of R2, defined by r(R2), map for
conventional and MultiTransmit
imaging method are shown.
Note that there are noticeable
differences in high dose regions
(10–20 Gy) between two
measurements methods
Fig. 4 Comparison of measured dose distribution and profile
between conventional single-source and MultiTransmit imaging.
Top row shows dose distribution (a) and profile (c: along solid blue
line, e: along dotted blue line) obtained using conventional single-
source MR imaging. Bottom row shows dose distribution (b) and
profile (d: along solid blue line, f: along dotted blue line) obtained
using MultiTransmit MR imaging. (Color figure online)
J Radioanal Nucl Chem
123
phantom image was 29 zoomed. As can be seen in Fig. 4-
(C–F), the dose profiles between the two measurement
methods clearly differed. When using a conventional single
source measurement, the maximum dose reached approx-
imately 9 Gy, even though the delivered dose was 8 Gy.
The dose measured in the gel using MultiTransmit imaging
was similar to the calculated dose distribution (Supple-
mentary Information).
Comparison of dose resolution
Since rR2is the largest contributor to rD, we could expect
that the dose resolution would be improved in the Multi-
Transmit measurement. In Fig. 5, DD95 % is plotted as a
function of the absorbed dose for both measurement
methods. As expected, it can be seen that the MultiTrans-
mit measurement gives a lower DD95 % than that obtained
using the conventional single-source method. The differ-
ences were greater in the high dose region (10–20 Gy) than
in the low dose region (0–9 Gy).
Discussion
In this study, we demonstrated that the dose uncertainty can
be reduced by using parallel RF transmission with multiple
RF sources. To our knowledge, this is the first study using a
MultiTransmit method in MRI-based polymer gel dosimetry
to minimize B1 field inhomogeneity and dielectric effect-
induced image shading. In many studies, a perturbation of T2
related to the B1 field inhomogeneities has been described
[22–24]. Several compensation strategies can be utilized for
B1 field inhomogeneity. One strategy involves using an
analytical expression in relation to flip angles and R2 values
to correct R2 images after obtaining a B1 field map in a gel
phantom [10]. However, the expression cannot be derived
easily even though computer simulations solving Bloch
equations may help in deriving that correlation. A more
practical method is to measure the R2 distribution in a
homogeneous phantom (i.e., gel phantom before irradiation)
and using this image-set as a template to correct the resulting
R2 images. The method should be carefully applied to
accurately match the position of the phantom between the
scanning sessions. The first benefit of the MultiTransmit
approach stem from the improvement in the uniformity of the
B1? field within FOV, which largely eliminates dielectric
shading. The second benefit of using the MultiTransmit
method is that, compared to former methods of correcting the
B1 inhomogeneity in a complicated post-processing step, it is
easy to perform and less time-consuming. Compared with
conventional single-source RF transmission images, our
results showed that the actual flip angle and signal intensity
distribution tends to be more uniform on MultiTransmit
images. These benefits suggest that the parallel RF trans-
mission with multiple RF source could significantly improve
the B1 field homogeneity. However, it should be noted that
B1 inhomogeneity correction might still be necessary even
with MultiTransmit system as evidenced by the non-zero
values shown in Fig. 2d.
Furthermore, achieving high SNR with optimal echo
sampling is essential for accurate T2 estimation in MRI-
based gel dosimetry. Higher SNR can be achieved as the
main magnetic field strength is increased and/or by using
multi-channel RF coils. In this study, higher SNR were
evaluated using a combination of more sensitive RF coil
elements (32 channel head coil) with parallel RF transmis-
sion at a field strength of 3 T. The MR signal is expected to
increase in a linear manner as the field strength increases.
However, because of the increased dielectric effect, which
causes greater B1 inhomogeneity and increased power
deposition [25], it may affect the ability of the trains of 180�pulses to refocus magnetization, which may result in
increased error for T2 estimation. Although many studies
were performed using a single spin-echo sequence with
reasonable measurement times (the number of slices is
limited to 1–3) in the past [1, 26–28], the multiple echo
sequence is preferable to the standard single spin-echo
sequence due to higher SNR. It should be noted that con-
siderably more slices need to be acquired with optimized
parameters with respect to SNR in order to use the gel as a
clinical 3D dosimetry method. Much more literature with
different methodologies has been explored to determine T2
quantitatively on a clinical MRI scanner [7, 29–34].
Lastly, we demonstrated that the dose resolutions at
95 % confidence interval (DD95 %) in MultiTransmit imag-
ing were improved compared to those in conventional
Fig. 5 Comparison of dose resolution of BANG3 gel dosimeter as a
function of absorbed dose. DD95 % indicates the dose resolution at
95 % confidence level
J Radioanal Nucl Chem
123
single-source imaging. In this work, we neglected the
contribution of r(a1) and r(a0) for calculating dose reso-
lution because r(R2) is the largest contributor to r(D). In
this study, we obtained a good dose resolution over a dose
range of up to 20 Gy with MultiTransmit approach.
Several factors can influence the measured dose out-
come and introduce uncertainty. Errors can be introduced
when gels from different batches are used, which have
slightly different physical properties, such as R2. While
most of the irradiation induced polymerization takes place
within a few hours [35], changes in R2 still occur after-
wards, but on a much smaller scale [36]. Our results also
showed temporal variation in dose response (i.e., changes
in slope and intercept in dose-R2 plot). Changes in dose
response due to temperature gradients during MR scanning
have also been reported [37, 38].
It is assumed in clinical radiotherapy that the dose
delivered to the patient is within 5 % of the prescribed
value. We found that absolute dosimetry measured using
the MultiTransmit MR imaging method is in good general
agreement with the calculated dose distribution within the
suggested dose tolerance. On the other hand, conventional
single-source MR imaging can cause a dose error of up to
12 %; this further suggests the need for the correction of
the B1 field and for the acquisition of high SNR images.
Conclusion
In this work, we demonstrated for the first time the feasi-
bility of MultiTransmit MR imaging for introducing gel
dosimetry into clinical routine. The image quality (SNR
and uniformity) and B1 homogeneity were considerably
improved in MultiTransmit imaging compared to conven-
tional single-source MR imaging. The improved image
quality and B1 homogeneity resulted in reduced dose
uncertainty (i.e., r(R2) and DD95 %) in MRI-based polymer
gel dosimetry. Future work will aim to compare the dose
distributions obtained from MultiTransmit imaging with
the B1 field corrected images in the post-processing step.
Acknowledgments This study was supported by the program of
Basic Atomic Energy Research Institute (BAERI) (2009-0078390)
and a grant (2012-007883) from the Mid-career Researcher Program
through the National Research Foundation (NRF) funded by the
Ministry of Science, ICT & Future Planning (MSIP) of Korea. This
study was conducted using a whole-body 3T MRI scanner at the
Ochang Center of the Korea Basic Science Institute (KBSI-#E34600).
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