Journal of the Korean Physical Society, Vol. 63, No. 8, October 2013, pp. 1630∼1636

Dual-source Parallel Radiofrequency Excitation ACR Phantom MagneticResonance Imaging at 3 T: Assessment of the Effect of Image Quality on

High-contrast Spatial Resolution, Percent Signal Ghosting, and Low-contrastObject Detectability in Comparison with Conventional Single-source

Transmission

Kyung-Bae Lee

Department of Biomedical Engineering, Research Institute of Biomedical Engineering,College of Medicine, The Catholic University of Korea, Seoul 137-701, Korea and

Department of Radiology, Kyunghee Medical Center, Seoul 130-702, Korea

Yong-Sung Park

Department of Radiology, Kyunghee University Hospital at Gang-dong, Seoul 134-727, Korea

Bo-Young Choe∗

Department of Biomedical Engineering, Research Institute of Biomedical Engineering,College of Medicine, The Catholic University of Korea, Seoul 137-701, Korea

(Received 24 January 2013, in final form 8 July 2013)

The purpose of the present study was to assess dual-source parallel radiofrequency (RF) exci-tation American College of Radiology (ACR) phantom magnetic resonance (MR) imaging at 3Tcompared with conventional single-source RF transmission and compared with the standard ACRMRI phantom test. We used a 3T MR scanner equipped with dual-source parallel RF excitationand an 8-channel head phased array coil. We employed T1- and T2-weighted fast spin echo (FSE)pulse sequences for an assessment of the impact of image quality on high-contrast spatial resolution,percent signal ghosting and low-contrast object detectability following the ACR MRI quality control(QC) manual. With geometric accuracy and identical slice locations, dual RFs using dual-sourceparallel RF excitation MR showed an advantage over single RF using dual-source parallel RF exci-tation MR and conventional MR in terms of high-contrast spatial resolution (p < 0.010), percentsignal ghosting (p < 0.010), and low-contrast object detectability (p < 0.010). The quality of theimage from the dual-source parallel RF excitation MR equipment was superior to that of the imagefrom conventional MR equipment for the ACR phantom. We need to pursue dual-source parallelRF excitation MR studies involving various clinical cases.

PACS numbers: 87.57.Ce, 87.57.Nk, 87.61.-cKeywords: Quality control (QC), ACR MRI phantom, Dual-source parallel radiofrequency (RF) excitationDOI: 10.3938/jkps.63.1630

I. INTRODUCTION

Quality control (QC) is essential to obtain a good im-age in magnetic resonance imaging (MRI). QC often fol-lows performance evaluation with utter objectivity anda legitimate and well-prepared standard, which is thereason that the American College of Radiology (ACR)MRI phantom has become an accepted approach. MRI-associated QC was developed in the 1980’s [1]. It waslater deemed to be insufficient for clinical applications,

∗E-mail: bychoe@catholic.ac.kr; Fax: +82-2-2258-7760

which prompted development of the ACR MRI phantomin 1992 [2–4]. The advancement made feasible imageanalyses of geometric accuracy, high-contrast resolution,slice thickness accuracy, slice position accuracy, imageintensity uniformity, percent signal ghosting, and low-contrast object detectability within a limited scanningtime.

Beyond the conventional ACR phantom test for single-source radiofrequency (RF) excitation, which cannot fac-tor in dual-source parallel RF excitation, researchers be-gan discussing single-source RF and dual-source parallelRF excitation equipment following development of dual-

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Table 1.

ImageFOV TR TE FA Slice Thickness Slice Gap

ETL* NEX(mm2) (ms) (ms) (◦C) (mm) (mm)

T1 sagittal250 × 250 200 20 90 20 2 3 1

localizer

T1 FSE 250 × 250 500 20 90 5 1 3 1

T2 FSE 250 × 250 2000 80 90 5 1 10 1

* Echo train length (ETL)

source parallel RF excitation [5–10]. Dual-source paral-lel RF excitation is particularly useful when eliminatingthe banding artifacts often seen in high-field MRI when awider field of view (FOV) is necessary [7,11]. When a sin-gle RF source is used in conventional 3T equipment, de-pending on the size and type of patient, dielectric shad-ing may appear [5–10]. Dual-source parallel RF exci-tation MR equipment provides with multiple adjustableRF sources for every patient. Dual-source parallel RFexcitation also reduces local RF deposition, provides op-timized speed and enhances diagnostic confidence by im-proving image quality and consistency [5–10].

For objective and efficient QC of the newly-developedMRI equipment, performing an objective evaluationthrough an itemized examination evaluation using anACR MRI phantom is essential. Given the dearth of re-search on ACR MRI phantom image quality assessmentfor dual-source parallel RF excitation equipment, thisstudy was undertaken n to test the ACR MRI phantomfor dual-source parallel RF excitation in comparison tothe standard MR. The study aimed at assessing the ACRMRI phantom test evaluation of dual-source parallel RFexcitation equipment.

II. MATERIALS AND METHODS

1. Image Acquisition Protocol

This study performed MRI using a conventional3T scanner (Intera Achieva Philips Medical Systems,Netherlands) and dual-source parallel RF excitationequipment (Intera Achieva TX Philips Medical Systems,Netherlands) equipped with an 8-channel head phasedarray coil. The imaging parameters of the T1 sagittallocalizer were as follows: FOV = 250 × 250 mm2, ma-trix = 256 × 256 mm2, repetition time (TR) = 200 ms,echo time (TE) = 20 ms, flip angle = 90◦, slice thickness= 20 mm, slice gap = 2 mm, and number of excitation(NEX) = 1. The imaging parameters of the T1 fast spinecho (FSE) were as follows: FOV = 250 × 250 mm2,matrix = 256 × 256 mm2, TR = 500 ms, TE = 20 ms,flip angle = 90◦, slice thickness = 5 mm, slice gap = 1,and NEX = 1. The imaging parameters of the T2 FSE

were as follows: FOV = 250 × 250 mm2, matrix = 256× 256 mm2, TR = 2000 ms, TE = 80 ms, flip angle =90◦, slice thickness = 5 mm, slice gap = 1, and NEX =1 (Table 1).

Sagittal localizer images are represented as a set by theACR MRI QC manual for the ACR phantom pivoted onan 8-channel head coil and with horizontal control to col-lect 11 different axial images including T1 FSE and T2FSE images. The ACR MRI phantom is made of a 148-mm-long, open-ended acrylic plastic pipe of 190 mm indiameter filled with a solution of 75-mM NaCl and 10-mM NiCl2. ACR MRI phantom images were measured10 times without repositioning. Also obtained were im-ages of the T1 sagittal localizer, T1 FSE, and T2 FSEfrom both the conventional MRI equipment and dual-source parallel RF excitation equipment (Fig. 1).

2. Image Analysis

Geometric accuracy is a QC item for evaluatingwhether the length shown in images indicates the actuallength of an object. It measures the length from top tobottom in a T1 sagittal localizer. It also measures thelength from top to bottom and the length from left toright in the T1 FSE #1 and #5 images, respectively [3].

High-contrast spatial resolution is an item for evaluat-ing the degree to which a small object is distinguished inan image. It visually evaluates the distinction of adjoin-ing small and bright dots. There is a sequence of dotsin the bottom part of the 1st image of T1 FSE and T2FSE. As well, the size of dots are 0.9, 1.0 and 1.1 mm,respectively. We performed a left-and right-direction res-olution evaluation and an up- and down-direction reso-lution evaluation [3].

Slice thickness accuracy is an item for evaluatingwhether an image slice is obtained with the exact thick-ness. There is a pair of traversing bars in the middle ofthe 1st image of T1 FSE and T2 FSE. We measured thelength of the signal ramp, which is the bright part in themiddle of the bars. The slice position accuracy measuresthe difference between the left and the right lengths ofa pair of vertical bars in the first and the 11th imageof T1 FSE and T2 FSE. The image intensity uniformity

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Fig. 1. ACR MRI phantom images: (a) slice #1, (b) slice#5, (c) slice #7 and (d) slice #11.

was measured and evaluated after the signal intensityhad been used to find the highest and the lowest part ofwhile adjusting the window width and the window levelafter drawing a region of interest (ROI) with a size ofabout 195 - 205 mm2 in the middle part of image #7 ofT1 FSE and T2 FSE [3].

Percent signal ghosting was evaluated by measuringthe signal intensity after having drawn a ROI with a sizeof about 195 - 205 mm2 in the middle part of by usingimage #7 of T1 FSE and measuring the signal intensityby drawing a ROI with a size of 10 cm2 in each of thetop, bottom, left, and right sides outside the phantom.The low contrast object detectability was evaluated bymeasuring the sum of the spokes shown in each of fourimages from images 8 - 11 of T1 FSE and T2 FSE. Im-ages were assessed by way of ‘m-view’ of Marotech byusing seven different criteria, and the images obtainedusing conventional MRI equipment and dual-source par-allel RF excitation equipment were compared [3].

3. Statistical Analyses

SPSS version 12.01 (SPSS, Chicago, IL, USA) wasused for statistical analyses. The three groups (dualRF, single RF of dual-source RFs, and single RF of theconventional 3T) were analyzed using the independentt-test. The data were tested for homogeneity of the vari-ance, and a one-way ANOVA with the Bonferroni post-hoc test was used to compare the factors of image qualityfor each group. Null hypotheses of no difference were re-jected if p-values were < 0.05.

Fig. 2. Comparison of high-contrast spatial resolutions forthe ACR MRI phantom: (a) T1 FSE and (b) T2 FSE.

III. RESULTS

No significant differences were observed in geometricaccuracy between the localizer of the phantom image andT1 FSE for a single RF of the conventional 3T, a singleRF of a dual-source RF 3T and dual RFs, with figuresaveraging out at 190.43 mm and 147.46 mm (p = 1.000)(Table 2). Dual RFs of the dual-source RF 3T equip-ment, however, appeared to have an advantage over thesingle RF equipment with a high-contrast spatial resolu-tion measured at 0.9 mm, which was slightly (0.1 mm)lower than the single RF’s 1.0 ± 0.1 mm (p < 0.010; base-line < 1.0 mm; Table 2; Fig. 2). Such dual RFs of thedual-source RF 3T equipment featured a slice-thicknessaccuracy of 4.77 mm, which was 0.41 mm greater thanthat of the single RF equipment (4.36 mm) but 0.16 mmless than that of the conventional 3T system (4.93 mm;p < 0.010; baseline 5.0 ± 0.7 mm; Table 2). Also, theT2 FSE image analysis showed the same features, with

Dual-source Parallel Radiofrequency Excitation ACR Phantom Magnetic· · · – Kyung-Bae Lee et al. -1633-

Table 2. Comparison of test items with ACR MRI phantom: conventional, single RF and dual RF of dual-source RF 3Twith N = 10.

Test/ Slice/Item Equipment Mean SD SE p-value

Geometric accuracy (mm)/ Conventional 147.46 0.00 0.00 1.000

Localizer/ Single RF 147.46 0.00 0.00

End-to-end Dual RF 147.46 0.00 0.00

Geometric accuracy (mm)/ Conventional 190.43 0.00 0.00 1.000

T1 FSE/Top-to-bottom, Single RF 190.43 0.00 0.00

Left-to-right Dual RF 190.43 0.00 0.00

High contrast spatial resolution (mm)/ Conventional 1.00* 0.00 0.00 P < 0.010

T1 FSE/Top-to-bottom, Single RF 1.00* 0.00 0.00

Left-to-right Dual RF 0.90† 0.00 0.00

High contrast spatial resolution (mm)/ Conventional 1.00* 0.00 0.00 P < 0.010

T2 FSE/ Top-to-bottom, Single RF 1.00* 0.00 0.00

Left-to-right Dual RF 0.90† 0.00 0.00

Slice thickness accuracy (mm)/ Conventional 4.93* 0.00 0.00 P < 0.010

T1 FSE/ Single RF 4.36† 0.09 0.03

Slice thickness Dual RF 4.77‡ 0.10 0.03

Slice thickness accuracy (mm)/ Conventional 5.22* 0.00 0.00 P < 0.010

T2 FSE/ Single RF 4.52† 0.10 0.03

Slice thickness Dual RF 4.88‡ 0.06 0.02

Slice position accuracy (mm)/ Conventional 2.93 0.00 0.00 1.000

T1 FSE/ Single RF 2.93 0.00 0.00

Length difference Dual RF 2.93 0.00 0.00

Slice position accuracy (mm)/ Conventional 0.98 0.00 0.00 1.000

T2 FSE/ Single RF 0.98 0.00 0.00

Length difference Dual RF 0.98 0.00 0.00

Image intensity Conventional 86.79* 0.59 0.19 P < 0.010

uniformity (%)/ Single RF 96.94† 0.27 0.09

T1 FSE/ Dual RF 86.85* 0.45 0.14

Image intensity Conventional 84.88* 0.29 0.09 P < 0.010

uniformity (%)/ Single RF 93.49† 0.92 0.29

T2 FSE/ Dual RF 84.56* 1.53 0.48

Percent signal ghosting (%)/ Conventional 0.37* 0.05 0.02 P < 0.010

T1 FSE/ Single RF 0.16† 0.01 0.01

Ghosting ratio Dual RF 0.00‡ 0.00 0.00

Low-contrast Conventional 39 0 0 -

object detectability/ Single RF 39 0 0

T1 FSE Dual RF 40 0 0

Low-contrast Conventional 39 0 0 -

object detectability/ Single RF 39 0 0

T2 FSE Dual RF 40 0 0

Post-hoc test (p-value < 0.050) equal group: *group A, †group B and ‡group C

dual-RF system measured at 4.88 mm, the single RF ofthe dual-source RF system at 4.52 mm, and the singleRF of the conventional 3T system at 5.22 mm. Dual RFsappeared superior to a single RF system by 0.36 mm, butwere inferior to the conventional 3T system by 0.34 mm

(p < 0.010; baseline 5.0 ± 0.7 mm; Table 2).The slice position accuracy for both T1 FSE and T2

FSE averaged to 2.93 ± 0.98 mm for all subjects of theconventional 3T, single RF, and dual RF of dual-sourceRF 3T systems, without respect to where the slice was

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situated (p = 1.000; baseline ≤ 5 mm) (Table 2). Thesingle-RF dual-source RF 3T system displayed advan-tages over the dual-RF and the conventional 3T systemin terms of image intensity uniformity, which was mea-sured at 96.94%, greater than the 86.79% and the 86.85%of the other two systems (p < 0.010). No differencesin image intensity uniformity was detected between thesingle RF of the conventional 3T and the dual RFs ofdual-source RF 3T system (p = 0.962; Table 2).

The single RF dual-source RF 3T exhibited a T1 FSEimage intensity uniformity of 96.94%, which was superiorto the value for the dual-RF (86.79%) and the conven-tional 3T (86.85%) (p < 0.010). No differences in imageintensity uniformity were detected between the single RFof the conventional 3T and the dual RFs of the dual-source RF 3T (p = 0.962; Table 2).

T2 FSE image intensity uniformity was also tested andproduced analogous results for each subject of the singleRF of dual-source RF 3T (93.50%), the single RF of theconventional 3T (84.88%), and the dual RFs of the dual-source RF 3T (86.56%), with the advantage belongingto the single RF of the dual-source RF 3T (p < 0.010;baseline ≥ 82%; Table 2). No percent signal ghostingfor T1 FSE was detected for the dual RFs of the dual-source RF 3T (0.000%), that of the single RF was 0.16%and that for the single RF of the conventional 3T was0.37% (p < 0.010; baseline ≤ 2.5%; Table 2, Fig. 3). Thetotal of 40 low-contrast objects for both T1 FSE and T2FSE detected for dual RFs of the dual-source RF 3T wasslightly greater than the 39 objects detected for both thesingle RF of the conventional 3T and the single RF ofthe dual-source RF 3T (baseline ≥ 37; Table 2).

IV. DISCUSSION

QC of a medical image device aims at identifying andcorrecting problems of equipment before a decline in theimage quality is caused by those problems, thereby main-taining a suitable image quality [12–16]. The field ofmedical imaging has been studied by scores of researchersand is in need of QC to offer quality images to cliniciansand patients [17–21].

Beginning from the primitive effort for MRI QC in the1980’s, researchers have focused on the operating stabil-ity by measuring the signal intensity and signal-to-noiseratio from a physical point of view [21–23], devoid of in-formation on image quality. That is why the EuropeanEconomic Community has encouraged image quality re-search in terms of, but not limited to, geometric distor-tion, spatial resolution, signal uniformity signal-to-noiseratio, contrast-to-noise ratio, and slice thickness by usingEurospin as a MRI phantom [23]. Because of its com-plexity and low cost-efficiency in clinical applications,however, a new, self-applicable means for a phantom testwas introduced in 1992 by the ACR [22,23]. The ACRMRI phantom can be used to perform quantitative anal-

Fig. 3. Comparison of percent signal ghosting for the ACRMRI phantom.

yses of geometric accuracy, slice thickness accuracy, im-age intensity uniformity, and percent-signal ghosting forseven different criteria and to perform qualitative anal-yses of high-contrast spatial resolution and low-contrastspecimen detection [23]. With its widespread applicabil-ity to clinical MRI equipment, the ACR MRI phantominvolves a single phantom to eliminate the intricacy inthe assessment process, to ease troubleshooting, and toyield the best image quality [22,23].

Just like the recent research results for cardiac, body,liver, breast, and knee MRI using dual-source parallel RFexcitation equipment, the dual-source parallel RF exci-tation equipment has an image quality superior to thatof the conventional MRI equipment [5–10]. Thus, thedual-source parallel RF excitation equipment has supe-rior diagnostic values over the conventional MR equip-ment not only for the phantom image but also for theclinical image.

The RF frequency of protons increases as the MRIequipment moves to high field strengths, which has apartial influence on the electrical properties of the hu-man body and on the physical scale. The RF excitationuniformity that can be obtained using a single-channelvolume transmit coil is insufficient for reliable clinicaldiagnosis in some patients [24,25]. In the general clini-cal systems used nowadays, whole-body electromagneticfield simulations limit significant RF dielectric shadingand scan speed, and indicate local SAR variations [26–28]. Multi-channel RF transmission using multiple RFtransmit/receive chains and coil elements in parallel in-dicates that improving the RF (B1 field) uniformity ofrelatively high-field MRI is possible [29]. It enables RFshimming while minimizing the local SAR, which is pro-vided by many transmit channels coils [29,30]. If multi-channel RF transmission is used in high-field MRI, it

Dual-source Parallel Radiofrequency Excitation ACR Phantom Magnetic· · · – Kyung-Bae Lee et al. -1635-

may lead to new imaging methods and applications [31,32]. In this regard, this study compared, with the ACRMRI phantom being a globally applicable standard, con-ventional MR to single RF and dual RFs of dual-sourceparallel RF excitation MR and found no differences inthe geometric accuracy and the slice location accuracyover ten consecutive trials. In turn, the dual RFs ofdual-source parallel RF excitation MR appear superiorto the conventional MR and the single RF of dual-sourceparallel RF excitation MR in terms of high-contrast spa-tial resolution, percent-signal ghosting and low-contrastobject detectability. The results may vary with the ap-plication of different MRI systems and RF coils [33,34].

The dual-RF image of dual-source parallel RF excita-tion MR equipment is superior to the conventional MRimage and the single-RF image of the dual-source paral-lel RF excitation MR equipment. Currently, many im-age comparison research efforts with dual-source paral-lel RF excitation MR equipment are being performed.Dual-source parallel RF excitation MR equipment fullyaddresses the most significant constraints on 3T clinicalperformances.

In conclusion, the image quality of dual-source parallelRF excitation MR equipment is superior to the imagequality of conventional MR equipment. Thus, we needto put more effort into achieving excellent MR imagesthrough further clinical research.

ACKNOWLEDGMENTS

This study was supported by a program of the Ba-sic Atomic Energy Research Institute (BAERI) (2009-0078390) and by a grant (2012-007883) from the Mid-career Researcher Program through the National Re-search Foundation (NRF) funded by the Ministry of Sci-ence, ICT & Future Planning (MSIP) of Korea.

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