This paper investigates the use of High Impedance Surfaces (HIS) to enhance the magnetic near-field within a dielectric phantom stimulated by a surface coil antenna resonating at 63.8 MHz for use in Magnetic Resonance Imaging (MRI) systems. Specifically, the optimization of the space between the surface of the coil and the HIS is presented here. The HIS incorporates interdigitated capacitive elements that produce an electrically small unit cell size. The magnetic field strength is shown to be improved by 42% as compared to a system which uses only an RF shield The associated Specific Absorption Rate (SAR) for the optimum design is simulated and compared to international standards. The work is aimed at 1.5T MRI applications

Ford, K.L.

Issa, I.

Rao, M.

Wild, J.

White Rose Research Online

This is a repository copy of Evaluation of High Impedance Surfaces for MRI RF Coil Applications  Simulations of RF Field and Specific Absorption Rate.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/121846/Version: Accepted VersionProceedings Paper:Issa, I., Ford, K.L. orcid.org/0000-0002-1080-6193, Rao, M. et al. (1 more author) (2016) Evaluation of High Impedance Surfaces for MRI RF Coil Applications  Simulations of RF Field and Specific Absorption Rate. In: 2016 10th European Conference on Antennas and Propagation (EuCAP). 2016 10th European Conference on Antennas and Propagation (EuCAP), 10-15 April 2016, Davos, Switzerland. IEEE . ISBN 978-8-8907-0186-3 https://doi.org/10.1109/EuCAP.2016.7481761© 2016 IEEE. Personal use of this material is permitted. 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Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Evaluation of High Impedance Surfaces for MRIRF Coil Applications - Simulations of RF Field andSpecific Absorption RateIsmail Issa1, Kenneth Lee Ford1, Madhwesha Rao2, James Wild21Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield, UK2Department of Academic Radiology, University of Sheffield, Sheffield, UKAbstract—This paper investigates the use of High ImpedanceSurfaces (HIS) to enhance the magnetic near-field within a dielec-tric phantom stimulated by a surface coil antenna resonating at63.8 MHz for use in Magnetic Resonance Imaging (MRI) systems.Specifically, the optimization of the space between the surface ofthe coil and the HIS is presented here. The HIS incorporatesinterdigitated capacitive elements that produce an electricallysmall unit cell size. The magnetic field strength is shown to beimproved by 42% as compared to a system which uses only anRF shield The associated Specific Absorption Rate (SAR) forthe optimum design is simulated and compared to internationalstandards. The work is aimed at 1.5T MRI applications.Index Terms—High Impedance Surface , Magnetic Resonance,Specific Absorption Rate.I. INTRODUCTIONMagnetic Resonance Imaging (MRI) is a diagnostic medicalimaging technique which yields images from biological tissuebased on Nuclear Magnetic Resonance (NMR) phenomenon.Radio Frequency (RF) coils which are tuned to the Larmorfrequency are used to excite the nuclear spins and detect thetransverse magnetization from their free induction decay. TheLarmor frequency can be calculated using (1), [1].f =γB02π(1)where γ is the gyromagnetic ratio (γ/2π = 42.58MHz/T forhydrogen nuclei, 1H) and B0 is the static magnetic field ofthe MRI scanner and the gyromagnetic ratio is dependent onthe nuclei of interest. In this paper the interest is in 1.5T MRIsystems which is typical for most clinical systems and givesa Larmor frequency of 63.87MHz.The RF coil is positioned such that the magnetic polarityis perpendicular to the axis of the cylindrical RF shield thatlines the bore of the MRI scanner. The RF shield is a high-passstructure which permits low frequency gradient magnetic fieldsand blocks high frequency components, at a typical Larmorfrequency of 1H , the RF shield thus it can be approximated toa Perfect Electrical Conductor (PEC), [1]. The radio frequencycurrent induced in the RF coil by the sample is typicallyreduced because of the out of phase currents caused by theRF shield, this leads to a reduction in Signal to Noise Ratio(SNR) of the received signal and thus, lowers the MRI imagequality. In recent years, there has been some interest in usingmetamaterials to increase the strength of the RF magnetic fieldin MRI systems, in order to improve image SNR and quality.A metamaterial is a periodic array of metallic or dielectricelements, which produce electromagnetic properties not foundin homogeneous materials. In recent metamaterial research,[2], the authors proposed using a Magneto Inductive (MI) lensto improve the SNR of images obtained with a 7T MRI systemusing a simple RF surface coil. However, performance at lowerresonant frequencies required for 1.5T and 3T MR systemswere not found to be as encouraging. A High ImpedanceSurface (HIS) is an Artificial Magnetic Conductor (AMC), [3],which has properties of in-phase reflection coefficient, whichcan be used to enhance the radio frequency magnetic field.In, [4], the use of HIS was investigated on a 7T MRI systemoperating at 300MHz which used electrically small HIS unitcells of only 7.5% of the free space wavelength. By usingthis method the SNR was improved by 47%. A more recentstudy using a HIS demonstrated how the spacing between theRF coil and the phantom could be optimized to improve theRF magnetic field, [5]. By using a HIS as an RF shield at300MHz the magnetic flux density was improved for a smallseparation distance between the dipole coil and HIS however,the improvement at the higher separation distance was not asmarked. The challenge for successful implementation of HIS atlower frequencies (60-130 MHz), is the design of miniaturizedHIS unit cell dimensions. Our recent work, [6], proposed aminiaturization technique which was based on an interdigitalcapacitor approach, [7], and was demonstrated at 63.8MHz.In this paper we investigate, through numerical simulations,the optimum spacing between a HIS, based on our previousdesign approach, and a RF surface coil which is spaced closeto a dielectric phantom that mimics the properties of the humanbody. The aim of work is to increase the strength of the RFmagnetic field as compared to the case when a HIS is notemployed. Additionally, the Specific Absorption Rate (SAR)inside the phantom is simulated and verified against regulatorystandards such as IEC60604-2-33/2010, [8].II. MRI CONCEPT AND HIS DESIGNThe concept of interest is illustrated in Fig.1. A PEC RFshield is placed a distance of t=100mm away from a capacitivelayer which forms the HIS. The choice of the spacing, t, isRF Shield (PEC)Capacitive layerSurface coilDielectric Phantomt=100mmsd=5mmZXFig. 1. MRI conceptWDFig. 2. Unit cell of capacitive layercommensurate with the available space that may be found inthe bore of a clinical MRI scanner.A resonant RF surface coil was designed and simulated in[6] and is used here as a transceiver operating at a Larmorfrequency of 63.8MHz which is appropriate for imaging the1H nuclear magnetic reosnance on a 1.5T MRI system. As asimple model of the load of the human body at 63.8MHz,a homogeneous phantom with dimensions length=335mm,width=230mm, height=140mm, (ǫr = 65, σ = 0.4S/m), [9],[10] was placed 5mm above the loop coil which is similar tothose used for surface imaging on clinical MRI systems. Fig. 2,illustrates the proposed unit cell of the capacitive layer whichcomprises of inter-digital metallic elements digits where thedigit width is equal to the spacing between digits. This unitcell geometry was chosen as it provides a high capacitancedensity in order to maximize the capacitance density [7].The dimensions are illustrated in Fig 2, and it was assumedthat the capacitive layer has a 1.6mm thick FR4 substrate(ǫr = 4.3, tan δ = 0.025). The design of the HIS was carriedout using approximations for interdigital capacitance, [6], [7],shown in [11] by (2). This can then be combined with theequivalent inductance of the space between the RF shield andthe capacitive layer, L=t to find the resonant frequency of theHIS, given by (3).C =ǫeff ∗ 10−3(N − 1)D18πK(k)K ′(k)pF (2)f =12π√LC(3)Where K(k) is the complete elliptic integral and K ′(k) isits compliment, D is the digit length, N is the number of digitsand ǫeff =ǫr+12 . It can be shown thatK(k)K′(k) = 0.5 if the digitwidth and the spacing between the digits are equal.The proposed HIS dimensions are D=50mm, W=56mm witha 1mm gap between the end of the digits and the metallichorizontal strip. The number of digits, N, is 40, the widthof each digit is 0.7mm and the spacing between digits is0.7mm. In order to reduce simulation time for the scenarioshown in Fig. 1, the capacitive layer was implemented asan effective surface impedance. The surface impedance wasdetermined by simulating the S-parameters of the inter-digitalunit cell, including PEC, and then converting this to an inputimpedance using CST microwave studio. Following this theequivalent complex impedance of the capacitive layer may bede-embedded from the impedance of the PEC shield.III. NUMERICAL SIMULATIONSA. Radio Frequency Magnetic Field ImprovementCST simulations were carried out for the model in Fig. 1where the magnitude of the magnetic field inside the phantomwas monitored, along the x-axis (z=45mm). The effect of avariation of the separation distance, s, between the RF coiland the capacitive layer was investigated to provide the largestmagnetic field when the surface coil had a 1A sinusoidalcurrent source at 63.8MHz. Fig.3 shows that for an increasedor decreased separation value between the RF coil and thecapacitor layer greater than or less than 5mm, the strength ofthe magnetic field is reduced inside the phantom, whereas thehighest value occurs for s=5mm.00.511.522.533.544.50 50 100 150 200 250 300Magnetic field (A/m)x(mm)s=3mms=4.5mms=5mms=10mms=15mms=35mmFig. 3. Simulated H1-field at 45mm above the RF coil for varying separation,s, between the coil and HIS.Figs. 4 and 5 present the absolute value of the magnetic andelectric fields at the optimal coil separation value of s=5mmin the transverse cut-plane (x-axis). Data is presented for the0123450 50 100 150 200 250 300Magnetic field (A/m)x(mm)PECHISFig. 4. Simulated H1-field at 45mm above the RF coil at s=5mm, with andwithout the HIS.05101520250 50 100 150 200 250 300Electric field (V/m)x(mm)PECHISFig. 5. Simulated magnitude of E-field at 45 mm above the RF coil at s=5mm with and without the HIS.cases both with and without the capacitive layer for compar-ison. Fig. 4 shows that there is an improvement in the H1field of approximately 42% when compared to the PEC caseat the center position, x=150mm. This improvement is dueto the in-phase reflection from the HIS ground plane. Fig. 5shows that the magnitude of the electric field is also increasedwhen compared to the PEC case by approximately 14% atthe maximum E-field position (x=100mm or x=200mm). Dueto this increase in E-field the effect on the SAR was theninvestigated further.B. Specific Absorption RateThe SAR is investigated inside the homogeneous dielectricphantom for the cases of with and without the capacitive layer.All values of SAR have been normalized to a 1W transmissionpower. Following the full field simulations the values forthe maximum 10g sample in the phantom and the wholebody SAR were calculated using the in-built facility in CST.The simulated SAR values for the maximum 10g case were5.6W/kg and 3.3W/kg for the capacitive layer and withoutcapacitive layer respectively. The maximum value allowed bythe IEC60604-2-33/2010 standard is 10W/kg. The whole bodySAR was 0.26W/kg and 0.11W/kg for the capacitive layer andwithout capacitive layer respectively which can be comparedto the IEC standard of 2W/kg.IV. CONCLUSIONIn this paper we demonstrated an approach to enhance themagnitude of the radio frequency magnetic field of a RF loopcoil above a high impedance surface for MRI applications.The results show that the HIS can improve the H1-field byapproximately 42% when compared to the PEC case at thecenter of a dielectric phantom. However, the HIS also causesa concomitant increase in the electric field by 14%, whichleads to an increase in SAR when compared to the PECcase. According to the aforementioned SAR guidelines, theSAR values for both the RF shield and HIS ground planesin this study are below the maximum allowed SAR values.A possible alternative for our approach would be in a receiveonly application which will be investigated in future work.REFERENCES[1] J. T. Vaughan and J. R. Griffiths, RF Coils for MRI. Wiley, 2012.[2] J. M. Algarı́n, M. J. Freire, F. Breuer, and V. C. Behr, “Metamaterialmagnetoinductive lens performance as a function of field strength,”Journal of Magnetic Resonance, vol. 247, pp. 9–14, 2014.[3] D. Sievenpiper, L. Zhang, R. F. J. Broas, N. G. Alexopolous, andE. Yablonovitch, “High-impedance electromagnetic surfaces with aforbidden frequency band,” IEEE Transactions on Microwave Theoryand Techniques, vol. 48, no. 4, p. 620, 2000.[4] G. Saleh, K. Solbach, and A. Rennings, “EBG structure to improvethe B1 efficiency of stripline coil for 7 Tesla MRI,” in Proceedings of6th European Conference on Antennas and Propagation, EuCAP 2012,2012, pp. 1399–1401.[5] Z. Chen, K. Solbach, D. Erni, and A. Rennings, “Improved B1 distribu-tion of an MRI RF coil element using a high-impedance-surface shield,”in Microwave Conference (GeMiC), 2015 German, 2015, pp. 111–114.[6] I. Issa, K. Ford, M. Rao, and J. Wild, “A High Impedance Surface forImproving the Radio Frequency Magnetic Field for a 1.5 Tesla Mag-netic Resonance System,” in Loughborough Antennas and PropagationConference, LAPC 2015, In press, 2015.[7] R. Saad and K. L. Ford, “A miniaturised dual band artificial magneticconductor using interdigital capacitance,” in Antennas and Propagation(EuCAP), 2014 8th European Conference on, 2014, pp. 25–26.[8] IEC 60601-2-33: Particular requirements for the basic safety andessential performance of magnetic resonance equipment for medicaldiagnosis, IEC Std., 2010.[9] M. Stuchly and S. Stuchly, “Dielectric properties of biologicalsubstances-tabulated,” Journal of Microwave Power, vol. 15, pp. 19–26,1980.[10] K. R. Foster and H. P. Schwan, “Dielectric properties of tissues andbiological materials: a critical review.” Critical Reviews in BiomedicalEngineering, vol. 17, no. 1, pp. 25–104, 1989.[11] I. J. Bahl, Lumped Elements for RF and Microwave Circuits. ArtechHouse, 2003.

Evaluation of High Impedance Surfaces for MRI RF Coil Applications ­ Simulations of RF Field and Specific Absorption Rate

Ismail Issa

Kenneth Lee Ford

Madhwesha Rao

James Wild

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Evaluation of High Impedance Surfaces for MRI RF coil applications - simulations of RF field and Specific Absorption Rate

http://eprints.whiterose.ac.uk/121846/1/EUCAP%20final%20version.pdf

Evaluation of High Impedance Surfaces for MRI RF Coil Applications Simulations of RF Field and Specific Absorption Rate

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Evaluation of High Impedance Surfaces for MRI RF Coil Applications ­ Simulations of RF Field and Specific Absorption Rate

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Evaluation of High Impedance Surfaces for MRI RF Coil Applications Simulations of RF Field and Specific Absorption Rate