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: bychoe@catholic.ac.kr

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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