提高B_1场效率和信噪比的7.0 T MRI磁感应透镜研究
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摘要
设计了一种可用于超高场磁共振成像(MRI)的负磁导率超材料磁感应透镜(MIL),可以有效提高MRI的信噪比.MIL是由6×6个加载电容的金属开口谐振环组成的二维周期结构,并将其加工在一层0.6 mm厚度的FR-4介质基板上.将MIL放置于射频线圈和样品之间,能够实现等效的磁表面等离激元效应.MIL单元电磁场分布、散射参数和等效磁导率的仿真结果表明:所设计的MIL在拉莫尔进动频率297.2 MHz产生负的磁导率,并能增强由射频线圈产生的射频场的倏逝波分量.两个不同的标准表面射频线圈的MRI扫描实验表明,小水膜仿体在施加MIL后可以将矢状面图像信噪比提高约200%,大水膜仿体在施加MIL后可以将冠状面图像信噪比提高约58%.仿真和MRI图像结果均表明了所设计的MIL具有聚焦表面射频线圈磁场强度的能力,有益于提高MRI的信噪比、空间分辨率和探测深度.
A metamaterial Magneticinductive Lens( MIL) with negative permeability was designed to improve the Signal-to-Noise Ratio( SNR) for ultra-high field Magnetic Resonance Imaging( MRI). The proposed MIL,fabricated on a FR-4 substrate with a thickness of 0. 6 mm,consists of a two-dimensional periodic array of 6 ×6 capacitor-loaded metallic rings. The equivalent magnetic surface plasmons effect would be achieved in the MIL when it is placed between the RF coil and sample. In terms of the electromagnetic field distribution,the scattering parameters and the equivalent permeability,simulated results reveal that the evanescent harmonics could be heightened. In addition,the evanescent harmonics was excited by the RF coils at the Larmor frequency 297. 2 MHz. By loading the MIL,the SNR of MRI images is enhanced about 200% in the sagittal images with a small water phantom,and approximately 58% in the coronal images with a large water phantom.Both simulations and MRI experiments clearly demonstrate that the magnetic field of RF coils could be focused by the metamaterial MIL,which is beneficial to improve the SNR,spatial resolution and detecting depth ofMRI.
A metamaterial Magneticinductive Lens( MIL) with negative permeability was designed to improve the Signal-to-Noise Ratio( SNR) for ultra-high field Magnetic Resonance Imaging( MRI). The proposed MIL,fabricated on a FR-4 substrate with a thickness of 0. 6 mm,consists of a two-dimensional periodic array of 6 ×6 capacitor-loaded metallic rings. The equivalent magnetic surface plasmons effect would be achieved in the MIL when it is placed between the RF coil and sample. In terms of the electromagnetic field distribution,the scattering parameters and the equivalent permeability,simulated results reveal that the evanescent harmonics could be heightened. In addition,the evanescent harmonics was excited by the RF coils at the Larmor frequency 297. 2 MHz. By loading the MIL,the SNR of MRI images is enhanced about 200% in the sagittal images with a small water phantom,and approximately 58% in the coronal images with a large water phantom.Both simulations and MRI experiments clearly demonstrate that the magnetic field of RF coils could be focused by the metamaterial MIL,which is beneficial to improve the SNR,spatial resolution and detecting depth ofMRI.
引文
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[3]ANDERS P,MORENO E,PENDRY J B,et al.Localized spoof plasmons arise while texturing closed surfaces[J].Physical Review Letters,2012,108(22):223905.
[4]NIU Kai-kun,WANG Li-hua,HUANG Zhi-xiang,et al.Band structure of triangular lattices photonic crystals with lossy and dispersive materials[J].Acta Photonica Sinica,2016,45(3):0319002.牛凯坤,王丽华,黄志祥,等.三角晶格有耗色散光子晶体的能带结构分析[J].光子学报,2016,45(3):0319002.
[5]MA H F,SHEN X,CHENG Q,et al.Broadband and high-efficiency conversion from guided waves to spoof surface plasmon polaritons[J].Laser Photonics Review,2014,8(1):146-151.
[6]FANG M,HUANG Z X,SHA W,et al.Maxwell-hydrodynamic model for simulating nonlinear terahertz generation from plasmonic metasurfaces[J].IEEE Journal on Multiscale and Multiphysics Computational Techniques,2017,2(1):194-201.
[7]BARNES W L,DEREUX A,EBBESEN T W.Surface plasmon subwavelength optical[J].Nature,2003,424(1):824-860.
[8]GOLLUB J N,SMITH D R,VIER D C,et al.Experimental characterization of magnetic surface plasmons on metamaterials with negative permeability[J].Physical Review B,2005,71(19):195402.
[9]GAO Z,GAO F,ZHANG Y M,et al.Deep-subwavelength magnetic-coupling-dominant interaction among magnetic localized surface plasmons[J].Physical Review B,2016,93(19):195410.
[10]SHELBY R A,SMITH D R,SCHULTZ S.Experimental verification of negative index of refraction[J].Science,2001,292(5541):77-79.
[11]FREIRE M J,JELINEK L,MARQUES R,et al.On the applications ofμ=-1 metamaterial lenses for magnetic resonance imaging[J].Journal of Magnetic Resonance,2010,203(1):81-90.
[12]FREIRE M J,LOPEZ M A,MEISE F,et al.A broadside-split-ring resonator-based coil for MRI at 7 T[J].IEEE Transactions on Medical Imaging,2013,32(6):1081-1084.
[13]ALGARIN J M,BREUER F,BEHRV C,et al.Analysis of the noise correlation in MRI coil arrays loaded with metamaterial magnetoinductive lenses[J].IEEE Transactions on Medical Imaging,2015,34(5):1148-1154.
[14]SLOBOZHANYUK A P,PODDUBNY A N,RAAIJMAKERS A J E,et al.Enhancement of magnetic resonance imaging with metasurfaces[J].Advance Materials,2016,28(9):1832-1838.
[15]SCHMIDT R,SLOBOZHANYUK A P,BELOV P,et al.Flexible and compact hybrid metasurfaces for enhanced ultra high field in vivo magnetic resonance imaging[J].Scientific Reports,2017,7(1):1678.
[16]SHCHELOKOVA A V,SLOBOZHANYUK A P,BELOV P,et al.Experimental investigation of a metasurface resonator for in vivo imaging at 1.5T[J].Journal of Magnetic Resonance,2018,286(1):78-81.
[17]PENDRY J B.Negative refraction makes a perfect lens[J].Physical Review Letters,2000,85(18):3966-3969.
[18]SMITH D R,VIER D C,KOSCHNY T,et al.Electromagnetic parameter retrieval from inhomogeneous metamaterials[J].Physical Review E,2005,71(3):036617.