唐德港，李紅闖，劉小玲，石 磊，李海東，葉朝輝，周 欣*

1.5T下高介電材料幾何結(jié)構(gòu)對發(fā)射場影響的仿真研究

唐德港1,2，李紅闖1,2，劉小玲1,2，石 磊1,2，李海東1,2，葉朝輝1,2，周 欣1,2*

1. 波譜與原子分子物理國家重點(diǎn)實(shí)驗(yàn)室，武漢磁共振中心（中國科學(xué)院精密測量科學(xué)與技術(shù)創(chuàng)新研究院），湖北 武漢 430071；2. 中國科學(xué)院大學(xué)，北京 100049

引 言

1 HPMs幾何結(jié)構(gòu)對發(fā)射場影響的仿真研究

1.1 HPMs提高射頻線圈發(fā)射效率的原理

將HPMs放置在成像物體ROI附近，在射頻脈沖激勵(lì)下，根據(jù)麥克斯韋全電流定律：

1.2 仿真模型及參數(shù)設(shè)置

圖1 帶有水模負(fù)載的鳥籠線圈仿真模型（對照組）．(a)仿真模型示意圖；(b)水模內(nèi)有限元網(wǎng)格剖分結(jié)果

在水模附近加入四種文獻(xiàn)報(bào)道過的不同幾何結(jié)構(gòu)的高介電襯墊作為實(shí)驗(yàn)組，包括：（1）四等分圓筒狀襯墊[30]；（2）對稱環(huán)繞水模的四塊方形襯墊[20]；（3）同側(cè)三塊方形襯墊[31]；（4）120°扇環(huán)柱狀襯墊[32]．采用的HPMs厚度均為13 mm、長度均為71 mm．其中四等分圓筒狀襯墊相鄰單元間隙為3 mm；環(huán)繞四方塊襯墊寬58 mm；同側(cè)三方塊襯墊寬34 mm，相鄰兩單元中心成60°夾角．襯墊材料為摻雜鎬和鈰的鈦酸鋇，首先將鈦酸鋇（Ba/Ti比為0.996）和高純度的ZrO2、CeO2研磨混合，再在1 340°下高溫?zé)Y(jié)為陶瓷襯墊．襯墊相對介電常數(shù)設(shè)為4 500[20]，電導(dǎo)率為0.44 S/m．實(shí)驗(yàn)組水模均采用和對照組相同的網(wǎng)格剖分，以保證參數(shù)和電磁場的精確度和一致性，實(shí)驗(yàn)組水模仿真模型示意圖如圖2所示．

圖2 不同幾何結(jié)構(gòu)的高介電襯墊（深灰色）的仿真模型示意圖（隱藏線圈）．(a)無襯墊；(b)四等分圓筒狀襯墊；(c)四塊方形襯墊對稱環(huán)繞水模；(d)同側(cè)三塊方形襯墊；(e) 120°扇環(huán)柱狀襯墊

2 結(jié)果與討論

2.1 HPMs幾何結(jié)構(gòu)對發(fā)射場的影響

圖3 不同幾何結(jié)構(gòu)的高介電襯墊仿真模型水模中心橫斷面的發(fā)射效率h分布．(a)無襯墊；(b)四等分圓筒狀襯墊；(c)四塊方形襯墊對稱環(huán)繞水模；(d)同側(cè)三塊方形襯墊；(e) 120°扇環(huán)柱狀襯墊

表1 不同幾何結(jié)構(gòu)的高介電襯墊仿真模型ROI內(nèi)的發(fā)射效率h均值與不均勻度（CV）

圖4 不同幾何結(jié)構(gòu)的高介電襯墊仿真模型ROI內(nèi)發(fā)射效率均值和不均勻度分析

2.2 理論分析

不同模型水模中心橫斷面沿軸方向中心線的發(fā)射效率分布如圖5所示，可以看出，加入四等分圓筒狀、環(huán)繞四方塊、同側(cè)三方塊襯墊后，發(fā)射效率沿軸方向中心線分布都較為均勻．

圖5 不同仿真模型水模中心橫斷面沿y軸方向中心線的發(fā)射效率分布（以中心為原點(diǎn)）

3 結(jié)論

無

[1] UGURBIL K. Imaging at ultrahigh magnetic fields: history, challenges, and solutions[J]. Neuroimage, 2018, 168: 7-32.

[2] YANG Q X, WANG J, ZHANG X, et al. Analysis of wave behavior in lossy dielectric samples at high field[J]. Magn Reson Med, 2002, 47(5): 982-989.

[3] SCHICK F. Whole-body MRI at high field: technical limits and clinical potential[J]. Eur Radiol, 2005, 15(5): 946-959.

[4] DIETRICH O, REISER M F, SCHOENBERG S O. Artifacts in 3-T MRI: Physical background and reduction strategies[J]. Eur J Radiol, 2008, 65(1): 29-35.

[5] KANGARLU A, BAERTLEIN B A, LEE R, et al. Dielectric resonance phenomena in ultra high field MRI[J]. J Comput Assist Tomogr, 1999, 23(6): 821-831.

[6] HUANG Q H, GAO Y, XIN X G. Study on the law of B1field homogeneity and SAR inside human body varying with field strength at high and ultra-high field MR[J]. Chin J Biological Eng, 2013, 32(1): 21-27.

黃綺華, 高勇, 辛學(xué)剛. 高場和超高場MR下人體內(nèi)B1場均勻性及SAR隨場強(qiáng)變化規(guī)律的研究[J]. 中國生物醫(yī)學(xué)工程學(xué)報(bào), 2013, 32(1): 21-27.

[7] OSCH M J P V, WEBB A G. Safety of ultra-high field MRI: What are the specific risks?[J]. Curr Radiol Rep, 2014, 2(8): 1-8.

[8] DOTY F D, ENTZMINGER G, KULKARNI J, et al. Radio frequency coil technology for small-animal MRI[J]. NMR Biomed, 2007, 20(3): 304-325.

[9] GULSEN G, MUFTULER L T, NALCIOGLU O. A double end-cap birdcage RF coil for small animal whole body imaging[J]. J Magn Reson, 2002, 156(2): 309-312.

[10] DARDZINSKI B J, LI S H, COLLINS C M, et al. A birdcage coil tuned by RF shielding for application at 9.4 T[J]. J Magn Reson, 1998, 131(1): 32-38.

[11] LEE K H, CHENG M C, CHAN K C, et al. Performance of large-size superconducting coil in 0.21 T MRI system[J]. IEEE Trans Biomed Eng, 2004, 51(11): 2024-2030.

[12] LIN I T, YANG H C, HSIEH C W, et al. Human hand imaging using a 20 cm high-temperature superconducting coil in a 3 T magnetic resonance imaging system[J]. J Appl Phys, 2010, 107(12): 124701.

[13] LIAO Z W, CHEN J F, YANG C S, et al. A design scheme for1H/31P dual-nuclear parallel MRI coil[J]. Chinese J Magn Reson, 2020, 37(3): 273-282.

廖志文, 陳俊飛, 楊春升, 等.1H/31P雙核并行磁共振成像線圈的研究與設(shè)計(jì)[J]. 波譜學(xué)雜志, 2020, 37(3): 273-282.

[14] FENG T, CHEN J F, ZHANG Z, et al. A design of short dead-time RF coil and RF switch for low-field NMR[J]. Chinese J Magn Reson, 2021, 38(1): 1-11.

馮濤, 陳俊飛, 張震, 等. 低場核磁共振短死時(shí)間射頻線圈與射頻開關(guān)的設(shè)計(jì)[J]. 波譜學(xué)雜志, 2021, 38(1): 1-11.

[15] WEBB A G, VAN DE MOORTELE P F. The technological future of 7 T MRI hardware[J]. NMR Biomed, 2016, 29(9): 1305-1315.

[16] ANDREYCHENKO A, BLUEMINK J J, RAAIJMAKERS A J E, et al. Improved RF performance of travelling wave MR with a high permittivity dielectric lining of the bore[J]. Magn Reson Med, 2013, 70(3): 885-894.

[17] YANG Q X, MAO W, WANG J, et al. Manipulation of image intensity distribution at 7.0 T: Passive RF shimming and focusing with dielectric materials[J]. J Magn Reson Imaging, 2006, 24(1): 197-202.

[18] FRANKLIN K M, DALE B M, MERKLE E M. Improvement in B1-inhomogeneity artifacts in the abdomen at 3 T MR imaging using a radiofrequency cushion[J]. J Magn Reson Imaging, 2008, 27(6): 1443-1447.

[19] DE HEER P, BRINK W M, KOOIJ B J, et al. Increasing signal homogeneity and image quality in abdominal imaging at 3 T with very high permittivity materials[J]. Magn Reson Med, 2012, 68(4): 1317-1324.

[20] ZIVKOVIC I, TEEUWISSE W, SLOBOZHANYUK A, et al. High permittivity ceramics improve the transmit field and receive efficiency of a commercial extremity coil at 1.5 tesla[J]. J Magn Reson, 2019, 299: 59-65.

[21] SICA C T, RUPPRECHT S, HOU R J, et al. Toward whole-cortex enhancement with a ultrahigh dielectric constant helmet at 3 T[J]. Magn Reson Med, 2020, 83(3): 1123-1134.

[22] LEE B Y, ZHU X H, RUPPRECHT S, et al. Large improvement of RF transmission efficiency and reception sensitivity for human in vivo P-31 MRS imaging using ultrahigh dielectric constant materials at 7 T[J]. Magn Reson Imaging, 2017, 42: 158-163.

[23] RUPPRECHT S, SICA C T, CHEN W, et al. Improvements of transmit efficiency and receive sensitivity with ultrahigh dielectric constant (uHDC) ceramics at 1.5 T and 3 T[J]. Magn Reson Med, 2018, 79(5): 2842-2851.

[24] VAN GEMERT J, BRINK W, REMIS R, et al. A simulation study on the effect of optimized high permittivity materials on fetal imaging at 3 T[J]. Magn Reson Med, 2019, 82(5): 1822-1831.

[25] BRINK W M, WEBB A G. High permittivity pads reduce specific absorption rate, improve B-1 homogeneity, and increase contrast-to-noise ratio for functional cardiac MRI at 3 T[J]. Magn Reson Med, 2014, 71(4): 1632-1640.

[26] SCHMIDT R, WEBB A. A new approach for electrical properties estimation using a global integral equation and improvements using high permittivity materials[J]. J Magn Reson, 2016, 262: 814.

[27] VAN GEMERT J, BRINK W, WEBB A, et al. High-permittivity pad design tool for 7 T neuroimaging and 3 T body imaging[J]. Magn Reson Med, 2019, 81(5): 3370-3378.

[28] BRINK W M, REMIS R F, WEBB A G. A theoretical approach based on electromagnetic scattering for analysing dielectric shimming in high-field MRI[J]. Magn Reson Med, 2016, 75(5): 2185-2194.

[29] LUO M, HU C, ZHUANG Y, et al. Numerical assessment of the reduction of specific absorption rate by adding high dielectric materials for fetus MRI at 3 T[J]. Biomed Eng-Biomed Tech, 2016, 61(4): 455-461.

[30] SEO J H, HAN S D, KIM K N. Improvements in magnetic field intensity and uniformity for small-animal MRI through a high-permittivity material attachment[J]. Electron Lett, 2016, 52(11): 898-899.

[31] RUYTENBERG T, O’REILLY T P, WEBB A G. Design and characterization of receive-only surface coil arrays at 3 T with integrated solid high permittivity materials[J]. J Magn Reson, 2020, 311: 106681.

[32] CHEN W, LEE B Y, ZHU X H, et al. Tunable ultrahigh dielectric constant (TuHDC) ceramic technique to largely improve RF coil efficiency and MR imaging performance[J]. IEEE Trans Med Imaging, 2020, 39(10): 3187-3197.

[33] 方俊鑫, 殷之文. 電介質(zhì)物理學(xué)[M]. 北京: 科學(xué)出版社, 1989.

[34] WEBB A G. Dielectric materials in magnetic resonance[J]. Concepts Magn Reson Part A, 2011, 38A(4): 148-184.

[35] HOULT D I. The principle of reciprocity in signal strength calculations—A mathematical guide[J]. Concepts Magn Reson, 2000, 12(4): 173-187.

[36] 羅超. 基于超材料的3 T磁共振射頻接收線圈性能研究[D]. 重慶: 重慶理工大學(xué), 2016.

[37] 張巍巍. 基于1.5 T磁共振系統(tǒng)體線圈電磁參數(shù)分析及共振頻率算法實(shí)現(xiàn)[D]. 成都: 西南交通大學(xué), 2016.

[38] XIN S X, HUANG Q, GAO Y, et al. Fetus MRI at 7 T: B1shimming strategy and SAR safety implications[J]. IEEE Trans Microw Theory Tech, 2013, 61(5): 2146-2152.

1,2，1,2，1,2，1,2，1,2，1,2，1,2*

1. State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China

O482.53

A

10.11938/cjmr20212904

2021-04-01;

2021-05-15

國家重點(diǎn)研發(fā)計(jì)劃(2016YFC1304702)；國家自然科學(xué)基金資助項(xiàng)目(82127802,81227902)；中國科學(xué)院戰(zhàn)略性先導(dǎo)科技專項(xiàng)(XDB25000000)；廣東省重點(diǎn)領(lǐng)域研發(fā)計(jì)劃(2018B030333001)；湖北省科技重大專項(xiàng)(2021ACA013)；中國科學(xué)院磁共振技術(shù)聯(lián)盟資助項(xiàng)目(2020GZL002).

* Tel: 027-87198802, E-mail: xinzhou@wipm.ac.cn.