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Investigating the influence of dielectric pads in 7T magnetic resonance imaging – simulated and experimental assessment

Investigating the influence of dielectric pads in 7T magnetic resonance imaging – simulated and... DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203007 Maíra M. Garcia*, Maryam Vatanchi, Khallil T. Chaim, Maria C. G. Otaduy, Andreas Rennings, Daniel Erni, and Waldemar Zylka Investigating the influence of dielectric pads in 7T magnetic resonance imaging – simulated and experimental assessment https://doi.org/10.1515/cdbme-2020-3007 lations suggest that SAR distribution will change when using the pads. Abstract: Dipole radiofrequency (RF) elements have been successfully used to compose multi-channel RF coils for ultra- Keywords: 7T MRI, dielectric pads, experiment, electro- high fields (UHF) magnetic resonance imaging (MRI). As magnetic simulation, dipole RF coil, phantom, 𝐵 , homogene- magnetic components of RF fields (𝐵 ) can be very inhomo- ity, SAR, MRI safety. geneous at UHF (𝐵 ≥7T), dielectric pads with high dielectric constants were proposed to improve the 𝐵 efficiency and ho- 1 Introduction mogeneity [1]. Dielectric pads can be used as a passive 𝐵 shimmimg technique thanks to inducing a strong secondary In MRI both transmission and receiving of signal are done magnetic field in their vicinity. The use of such dielectric pads through RF elements (RF coils). At UHF the magnetic com- affect not only the 𝐵 field but also the electric field. This ponent 𝐵 of RF fields can be very inhomogeneous inside the in turn affects the specific absorption rate (SAR) and conse- body, since the wavelength becomes comparable to the dimen- quently the temperature distribution inside the patient’s body. sions of the human head. For this reason, the task to create spe- To study these effects, a 29 cm-long transmission dipole RF cific RF elements for the use at 7T scanner is challenging. RF coil element terminated by two meander was used for 7T MRI coil’s design includes a rigorous assessment of the electromag- [2]. Using a cylindrical agarose-gel phantom, numerical and netic (EM) fields generated and evaluation of patient/object’s experimental results were analyzed with respect to homogene- safety during MRI procedures [2, 3]. However, when a specific ity and amplitude of the magnetic and electric fields generated RF coil is designed, it is impossible to predict all the usages by the RF element in various configurations with and with- that the MRI operator can handle: when different sized and out dielectric pads. Calculated and measured 𝐵 results were shaped objects are analyzed using this coil, there is a detun- cross-checked and found to be in good agreement. When us- ing and impedance mismatch caused by the different load in ing dielectric pads 𝐵 homogeneity and magnitude increase in the coil, which already generates intrinsic 𝐵 inhomogeneity. regions where it was previously weak or insufficient. Calcu- This is a very common problem for patients geometry that dif- fers from the standard human model. One of the methods to correct the 𝐵 transmitted field *Corresponding author: Maíra M. Garcia, General and is using passive RF shimming, which can be done by plac- Theoretical Electrical Engineering (ATE), University of ing high permittivity materials, more commonly, between the Duisburg-Essen, and CENIDE – Center of Nanointegration patient/object and the coil. Previous studies show that using Duisburg-Essen, D-47048 Duisburg, Germany, and Faculty of Electrical Engineering and Applied Natural Sciences, Westphalian such dielectric padding also alters SAR distributions and mag- University, Campus Gelsenkirchen, Germany, e-mail: nitudes, presenting increase or decrease on its magnitude de- maira.martins-garcia@stud.uni-due.de pending on the aim of the measurement [1]. Khallil T. Chaim, Faculdade de Medicina FMUSP, Universidade Investigations of the influence of positioning dielectric de Sao Paulo, Sao Paulo, SP, Brazil pads are especially important for 7T MRI brain scanning, due Maria C. G. Otaduy, LIM44, Hospital das Clinicas HCFMUSP, Faculdade de Medicina, Universidade de Sao Paulo, Sao Paulo, to the comparable dimensions between the human head and SP, Brazil the RF wavelength, which is not an issue at lower 𝐵 . In this Andreas Rennings, Daniel Erni, General and Theoretical study, a transmission RF element for 7T MRI was used to ana- Electrical Engineering (ATE), University of Duisburg-Essen, and lyze the incident RF magnetic field distribution and the impact CENIDE – Center of Nanointegration Duisburg-Essen, D-47048 on it, when using dielectric pads. SAR predictions were done Duisburg, Germany to estimate energy absorption inside the object. Maryam Vatanchi, Waldemar Zylka, Faculty of Electrical Engi- neering and Applied Natural Sciences, Westphalian University, Campus Gelsenkirchen, Germany Open Access. © 2020 Maira M. Garcia et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. 2 Maíra M. Garcia et al., Investigating the influence of dielectric pads in 7T magnetic resonance imaging Different simulation scenarios were defined including the 2 Materials and methods loaded coil in conjunction with a virtual phantom and dielec- tric pads, both with similar characteristics to the phantom and 2.1 Dipole coil configuration the pads used in the experimental setup (described in 2.3). Local SAR predictions were done by post-processing the The dipole element used in this study was described in [2], magnitude of electric field 𝐸(⃗𝑟) and the local material prop- but 29cm in length, intending its use as an element in a erties (𝜎 is specific electrical conductivity, 𝜌 is material mass multi-channel coil for 7T MRI [4]. The element consists of a density, ⃗𝑟 is position vector and 𝑉 is volume), as in: metallic strip line of width of 15mm terminated by two me- ∫︁ 1 𝜎 (⃗𝑟) ander and a metal ground plate printed on Rogers RO4003 2 SAR = 𝐸 (⃗𝑟)𝑑𝑉. (1) 𝑉 2𝜌( ⃗𝑟) substrates (𝜖 =3.55; thickness 𝑡 =0.5mm; length 𝑙 =290mm; 𝑟 𝑠 𝑠 width 𝑤 =100mm), which are separated by 19mm of air At 298MHz the material parameters were assumed as: phan- and connected at each extremity by a copper strip (thickness tom (𝜎 =0.92S/m, 𝜖 =58.2, 𝜌 =1000kg/m ); dielectric pad 𝑝 𝑟 𝑝 𝑡 =0.5mm; width 𝑤 =15mm) and an end-capacitor (𝐶 =1pF). 𝑐 𝑐 𝑒 (𝜎 =0.08S/m, 𝜖 =110) [1, 4]. To ensure reliability in the sim- 𝑑 𝑟 The meander geometry is unchanged in comparison to [3]. The ulated results all the calculations were performed with a very matching network consists of two shunt capacitors 𝐶 =5.6pF, fine mesh, setting the convergence criterion for the adaptive a series capacitor 𝐶 =3pF and a balun with 180° delay coaxial solutions to 0.006. cable placed at metal ground plate center (Fig. 1). 2.3 Experimental setup Experiments were performed at the 7T MRI scanner (MAG- NETOM 7T, Siemens Healthcare, Germany) installed in the University of São Paulo, São Paulo, Brazil. In order to analyze the influence of dielectric pads, the SIEMENS SA2RAGE pulse sequence was used to obtain the 𝐵 field maps (TR=2400ms and TE=0.9ms) [5, 6] from a cylindri- Fig. 1: Left: upper view of the dipole coil element and the coaxial cal agarose-gel based phantom with 13.3cm of height and cable. Right: setup to perform one of the experimental measure- 7cm of diameter. The dielectric pads had a square shape ments: an agarose-gel phantom is placed at one extremity of the (11x11x1cm ) and were filled with a suspension of calcium dipole coil and the two dielectric pads are indicated by arrows. titanate and deuteruim oxide (D O) (mass-mass ratio of 3:1). Four different experimental configurations were analyzed: Config. 1: phantom is placed at the center of the dipole ele- 2.2 Simulation setup ment and 2cm under it; Config. 2: phantom is positioned at dipole element’s lateral (1.7cm outside) and remains 2cm sep- The design of the dipole coil was done using HFSS (high arated from element; Config. 3: same setup as in Config. 2 and frequency structure simulator) combined with the Circuit De- dielectric pads are inserted in the 2cm gap between phantom sign tool in the commercially available ANSYS package. Such and element; Config. 4: same setup as in Configuration 3, ap- combination enables the user to perform rapid optimizations plying a different power to the dipole element. and design detailed electronic devices. The simulation model of the unloaded element in air, in- cluding matching network circuit, was continuously improved, 3 Results and discussion in order to obtain a homogeneous magnetic field distribution and a small input reflection coefficient (S11). The final model 3.1 𝐵 field distribution is very similar to the fabricated dipole coil, counting with a different matching circuit (a series resistor of 41Ω, a series Simulated results. To study the interaction of the phantom capacitor of 5.36pF, no balun). It is intended to improve the with the surrounding magnetic field and to analyze the in- numerical model in order to get as close as possible to the fab- fluence of positioning the dielectric pads (close to the region ricated element, which means, that some losses from the ma- where the 𝐵 field is not homogeneous), simulations for the terials, cables and others also need to be counted in the model. 1 Maíra M. Garcia et al., Investigating the influence of dielectric pads in 7T magnetic resonance imaging 3 different configurations were performed and are presented in Fig. 2. A voltage of 77.8V was applied on the element’s port. Fig. 3: Magnetic field strength distribution along 𝑥 line placed in phantom’s center (presented in Fig. 2) for Config. 1, 2 and 3. Fig. 2: Simulated results for the magnetic field 𝐵 strength in a plane 𝑥𝑧 along the dipole element and positioned at the half width of the element: (a) Config. 1, (b) Config. 2, and (c) Config. 3. The (b) Config. 2 - Tx=77.8V (a) Config. 1 - Tx=72.2V black box represents the phantom, and the central line inside it will be used in Fig. 3. The 𝐵 strength distribution in a line positioned in the center of the phantom along 𝑥 axis (Fig. 2) was compared for the three configurations (Fig. 3). It shows that using dielectric pads, higher 𝐵 values can be achieved in comparison to Con- fig. 1, presenting similar distribution for a region 𝑥 > 30cm. (c) Config. 3 - Tx=77.8V (d) Config. 4 - Tx=155.6V When compared with phantom lateral positioned (Config. 2) higher 𝐵 values are achieved in 67.7% of the line. Fig. 4: Magnetic field 𝐵 distributions measured inside the phan- tom in the coronal plane taken in the middle of the phantom. Val- Experimental results. Magnetic field distributions inside the ues are normalized to the same color scale. cylindrical phantom were also measured, as can be seen in Fig. 4. A calibration was done for each configuration to reach the best transmission voltage (Tx) necessary to apply in the dipole for this reason, a new calibration was performed and a higher element, aiming to get good images. For Config. 1 the voltage transmission voltage was applied, allowing us to obtain a more 72.2V was applied, the element’s excitation for Config. 2 and homogeneous 𝐵 field region inside the phantom (Fig. 4(d)). 3 was 77.8V, and Config. 4 received 155.6V. The same values Simulated versus measured results. Comparisons between were used for each corresponding simulation. calculated and experimental transmitted 𝐵 fields were car- It appears that when the phantom is positioned outside the 1 ried out for two lines in the central coronal plane: along 𝑥 and homogeneous region, the magnitude of 𝐵 is smaller at one 𝑦 axis, having as common point the phantom’s center (Fig. 5). side of the phantom and is not high enough to excite equally The comparison of the results shows that the 𝐵 distribution all the spins (see Fig. 4(b) and Fig. 2(b)). Trying to improve 1 along 𝑥 and 𝑦, as for 𝑧 axis (not presented here), are similar 𝐵 homogeneity in this region, two dielectric pads were in- in its extension, but differs slightly in amplitude. To achieve serted and the same Tx voltage as in Config. 2 was applied, the best agreement, the value of end-capacitors in the model however, this setup didn’t generate good images. Due to de- was altered to 𝐶 =0.3pF, which could represent not explicitly tuning and mismatching of the dipole element when inserting 𝑒 modeled losses. Using this capacitor value, the simulated co- dielectric pads, the peak of S11 curve has shifted to a lower efficient was S11=-48dB while S11=-20dB were measured. frequency (as posterior confirmed in a network analyzer) and 4 Maíra M. Garcia et al., Investigating the influence of dielectric pads in 7T magnetic resonance imaging (a) Config. 1. Fig. 6: Local SAR distribution in phantom’s central 𝑥𝑧 plane (𝑥 axis is pointing to the right) for (a) Config. 2, and (b) Config. 3. regions where it was previously not high enough. Additionally, simulations inform that local SAR distribution will change when using pads. The cross-check of results indicates that sim- ulation model and measurement agree very well. However, im- provement in the validation process are intended, notably with (b) Config. 4. regard to real losses of the experimental setup, detuning and impedance mismatch when using pads, and calculation of tem- perature, which can be validated by dedicated MRI sequences. Fig. 5: 𝐵 curves along 𝑥 and 𝑦 lines in the central coronal plane for simulated and experimental results (where 𝐵 values repre- Author Statement sent the percentage times 10 of the attempted flip angle [5]), for setups with (a) and without (b) dielectric pads. Research funding: MMG was supported by the Coordination for the Improvement of Higher Education Personnel (CAPES): Full PhD Program Abroad (Programa de Doutorado Pleno no 3.2 SAR predictions Exterior), process n. 88881.173609/2018-01. Conflict of inter- est: Authors state no conflict of interest. As seen in Fig. 2 and 4, dielectric pads positioned near to the phantom can improve 𝐵 homogeneity inside a region of in- terest. However, it is also important to analyze how the config- uration with pads (Config. 3) will influence the object’s energy References absorption using our hardware setup, i.e. one dipole coil, phan- tom and dielectric pads. Simulations in Fig. 6 show different [1] Teeuwisse W M, Brink W M, Webb A G. Quantitative assess- SAR distribution inside the phantom for Config. 3 in compar- ment of the effects of high-permittivity pads in 7 T MRI of the brain. Mag. Reson. Med. 2012;67:1285-1293. ision to Config. 2, especially in the region that had higher 𝐵 [2] Chen Z, Solbach K, Erni D, Rennings A. Dipole RF element improvement, presenting no hot-spots and only small changes for 7 Tesla magnetic resonance imaging with minimized SAR. in SAR. In accordance to [1], SAR variation when using di- 7th EUCAP 2013;1775-1778. electric pads depends on the aim of the measurement and is [3] Orzada S, Bahr A, Bolz T. A novel 7T microstrip element sequence dependent. Since the aim of this work was to homog- using meanders to enhance decoupling. 16th Proc. Intl. Soc. enize 𝐵 inside the whole phantom, an increase in SAR due to Mag. Reson. Med. 2008; 2979. [4] Chen Z, Solbach K, Erni D, Rennings A. Electromagnetic the extra loss introduced by the usage of pads was expected. field analysis of a dipole coil element with surface impedance characterized shielding plate for 7-Tesla MRI. IEEE Trans. Microw. Theory Techn. 2016;64(3):972-981. [5] Siemens Application Guide: "SA2RAGE - Work-in-progress 4 Conclusions package for fast 𝐵 mapping", Siemens AG, v. 1.0, 2011. [6] Marques JP, Kober T, Krueger G, Van der Zwaag W, Van de Numerical and experimental investigations show that using di- Moortele PF, Gruetter R. MP2RAGE, a self bias-field cor- electric pads can improve the 𝐵 homogeneity distribution in- 1 rected sequence for improved segmentation and 𝑇 -mapping side an object and can also address higher 𝐵 magnitude for at high field. NeuroImage 2010;49:1271-1281. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Current Directions in Biomedical Engineering de Gruyter

Investigating the influence of dielectric pads in 7T magnetic resonance imaging – simulated and experimental assessment

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de Gruyter
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© 2020 by Walter de Gruyter Berlin/Boston
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10.1515/cdbme-2020-3007
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Abstract

DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203007 Maíra M. Garcia*, Maryam Vatanchi, Khallil T. Chaim, Maria C. G. Otaduy, Andreas Rennings, Daniel Erni, and Waldemar Zylka Investigating the influence of dielectric pads in 7T magnetic resonance imaging – simulated and experimental assessment https://doi.org/10.1515/cdbme-2020-3007 lations suggest that SAR distribution will change when using the pads. Abstract: Dipole radiofrequency (RF) elements have been successfully used to compose multi-channel RF coils for ultra- Keywords: 7T MRI, dielectric pads, experiment, electro- high fields (UHF) magnetic resonance imaging (MRI). As magnetic simulation, dipole RF coil, phantom, 𝐵 , homogene- magnetic components of RF fields (𝐵 ) can be very inhomo- ity, SAR, MRI safety. geneous at UHF (𝐵 ≥7T), dielectric pads with high dielectric constants were proposed to improve the 𝐵 efficiency and ho- 1 Introduction mogeneity [1]. Dielectric pads can be used as a passive 𝐵 shimmimg technique thanks to inducing a strong secondary In MRI both transmission and receiving of signal are done magnetic field in their vicinity. The use of such dielectric pads through RF elements (RF coils). At UHF the magnetic com- affect not only the 𝐵 field but also the electric field. This ponent 𝐵 of RF fields can be very inhomogeneous inside the in turn affects the specific absorption rate (SAR) and conse- body, since the wavelength becomes comparable to the dimen- quently the temperature distribution inside the patient’s body. sions of the human head. For this reason, the task to create spe- To study these effects, a 29 cm-long transmission dipole RF cific RF elements for the use at 7T scanner is challenging. RF coil element terminated by two meander was used for 7T MRI coil’s design includes a rigorous assessment of the electromag- [2]. Using a cylindrical agarose-gel phantom, numerical and netic (EM) fields generated and evaluation of patient/object’s experimental results were analyzed with respect to homogene- safety during MRI procedures [2, 3]. However, when a specific ity and amplitude of the magnetic and electric fields generated RF coil is designed, it is impossible to predict all the usages by the RF element in various configurations with and with- that the MRI operator can handle: when different sized and out dielectric pads. Calculated and measured 𝐵 results were shaped objects are analyzed using this coil, there is a detun- cross-checked and found to be in good agreement. When us- ing and impedance mismatch caused by the different load in ing dielectric pads 𝐵 homogeneity and magnitude increase in the coil, which already generates intrinsic 𝐵 inhomogeneity. regions where it was previously weak or insufficient. Calcu- This is a very common problem for patients geometry that dif- fers from the standard human model. One of the methods to correct the 𝐵 transmitted field *Corresponding author: Maíra M. Garcia, General and is using passive RF shimming, which can be done by plac- Theoretical Electrical Engineering (ATE), University of ing high permittivity materials, more commonly, between the Duisburg-Essen, and CENIDE – Center of Nanointegration patient/object and the coil. Previous studies show that using Duisburg-Essen, D-47048 Duisburg, Germany, and Faculty of Electrical Engineering and Applied Natural Sciences, Westphalian such dielectric padding also alters SAR distributions and mag- University, Campus Gelsenkirchen, Germany, e-mail: nitudes, presenting increase or decrease on its magnitude de- maira.martins-garcia@stud.uni-due.de pending on the aim of the measurement [1]. Khallil T. Chaim, Faculdade de Medicina FMUSP, Universidade Investigations of the influence of positioning dielectric de Sao Paulo, Sao Paulo, SP, Brazil pads are especially important for 7T MRI brain scanning, due Maria C. G. Otaduy, LIM44, Hospital das Clinicas HCFMUSP, Faculdade de Medicina, Universidade de Sao Paulo, Sao Paulo, to the comparable dimensions between the human head and SP, Brazil the RF wavelength, which is not an issue at lower 𝐵 . In this Andreas Rennings, Daniel Erni, General and Theoretical study, a transmission RF element for 7T MRI was used to ana- Electrical Engineering (ATE), University of Duisburg-Essen, and lyze the incident RF magnetic field distribution and the impact CENIDE – Center of Nanointegration Duisburg-Essen, D-47048 on it, when using dielectric pads. SAR predictions were done Duisburg, Germany to estimate energy absorption inside the object. Maryam Vatanchi, Waldemar Zylka, Faculty of Electrical Engi- neering and Applied Natural Sciences, Westphalian University, Campus Gelsenkirchen, Germany Open Access. © 2020 Maira M. Garcia et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. 2 Maíra M. Garcia et al., Investigating the influence of dielectric pads in 7T magnetic resonance imaging Different simulation scenarios were defined including the 2 Materials and methods loaded coil in conjunction with a virtual phantom and dielec- tric pads, both with similar characteristics to the phantom and 2.1 Dipole coil configuration the pads used in the experimental setup (described in 2.3). Local SAR predictions were done by post-processing the The dipole element used in this study was described in [2], magnitude of electric field 𝐸(⃗𝑟) and the local material prop- but 29cm in length, intending its use as an element in a erties (𝜎 is specific electrical conductivity, 𝜌 is material mass multi-channel coil for 7T MRI [4]. The element consists of a density, ⃗𝑟 is position vector and 𝑉 is volume), as in: metallic strip line of width of 15mm terminated by two me- ∫︁ 1 𝜎 (⃗𝑟) ander and a metal ground plate printed on Rogers RO4003 2 SAR = 𝐸 (⃗𝑟)𝑑𝑉. (1) 𝑉 2𝜌( ⃗𝑟) substrates (𝜖 =3.55; thickness 𝑡 =0.5mm; length 𝑙 =290mm; 𝑟 𝑠 𝑠 width 𝑤 =100mm), which are separated by 19mm of air At 298MHz the material parameters were assumed as: phan- and connected at each extremity by a copper strip (thickness tom (𝜎 =0.92S/m, 𝜖 =58.2, 𝜌 =1000kg/m ); dielectric pad 𝑝 𝑟 𝑝 𝑡 =0.5mm; width 𝑤 =15mm) and an end-capacitor (𝐶 =1pF). 𝑐 𝑐 𝑒 (𝜎 =0.08S/m, 𝜖 =110) [1, 4]. To ensure reliability in the sim- 𝑑 𝑟 The meander geometry is unchanged in comparison to [3]. The ulated results all the calculations were performed with a very matching network consists of two shunt capacitors 𝐶 =5.6pF, fine mesh, setting the convergence criterion for the adaptive a series capacitor 𝐶 =3pF and a balun with 180° delay coaxial solutions to 0.006. cable placed at metal ground plate center (Fig. 1). 2.3 Experimental setup Experiments were performed at the 7T MRI scanner (MAG- NETOM 7T, Siemens Healthcare, Germany) installed in the University of São Paulo, São Paulo, Brazil. In order to analyze the influence of dielectric pads, the SIEMENS SA2RAGE pulse sequence was used to obtain the 𝐵 field maps (TR=2400ms and TE=0.9ms) [5, 6] from a cylindri- Fig. 1: Left: upper view of the dipole coil element and the coaxial cal agarose-gel based phantom with 13.3cm of height and cable. Right: setup to perform one of the experimental measure- 7cm of diameter. The dielectric pads had a square shape ments: an agarose-gel phantom is placed at one extremity of the (11x11x1cm ) and were filled with a suspension of calcium dipole coil and the two dielectric pads are indicated by arrows. titanate and deuteruim oxide (D O) (mass-mass ratio of 3:1). Four different experimental configurations were analyzed: Config. 1: phantom is placed at the center of the dipole ele- 2.2 Simulation setup ment and 2cm under it; Config. 2: phantom is positioned at dipole element’s lateral (1.7cm outside) and remains 2cm sep- The design of the dipole coil was done using HFSS (high arated from element; Config. 3: same setup as in Config. 2 and frequency structure simulator) combined with the Circuit De- dielectric pads are inserted in the 2cm gap between phantom sign tool in the commercially available ANSYS package. Such and element; Config. 4: same setup as in Configuration 3, ap- combination enables the user to perform rapid optimizations plying a different power to the dipole element. and design detailed electronic devices. The simulation model of the unloaded element in air, in- cluding matching network circuit, was continuously improved, 3 Results and discussion in order to obtain a homogeneous magnetic field distribution and a small input reflection coefficient (S11). The final model 3.1 𝐵 field distribution is very similar to the fabricated dipole coil, counting with a different matching circuit (a series resistor of 41Ω, a series Simulated results. To study the interaction of the phantom capacitor of 5.36pF, no balun). It is intended to improve the with the surrounding magnetic field and to analyze the in- numerical model in order to get as close as possible to the fab- fluence of positioning the dielectric pads (close to the region ricated element, which means, that some losses from the ma- where the 𝐵 field is not homogeneous), simulations for the terials, cables and others also need to be counted in the model. 1 Maíra M. Garcia et al., Investigating the influence of dielectric pads in 7T magnetic resonance imaging 3 different configurations were performed and are presented in Fig. 2. A voltage of 77.8V was applied on the element’s port. Fig. 3: Magnetic field strength distribution along 𝑥 line placed in phantom’s center (presented in Fig. 2) for Config. 1, 2 and 3. Fig. 2: Simulated results for the magnetic field 𝐵 strength in a plane 𝑥𝑧 along the dipole element and positioned at the half width of the element: (a) Config. 1, (b) Config. 2, and (c) Config. 3. The (b) Config. 2 - Tx=77.8V (a) Config. 1 - Tx=72.2V black box represents the phantom, and the central line inside it will be used in Fig. 3. The 𝐵 strength distribution in a line positioned in the center of the phantom along 𝑥 axis (Fig. 2) was compared for the three configurations (Fig. 3). It shows that using dielectric pads, higher 𝐵 values can be achieved in comparison to Con- fig. 1, presenting similar distribution for a region 𝑥 > 30cm. (c) Config. 3 - Tx=77.8V (d) Config. 4 - Tx=155.6V When compared with phantom lateral positioned (Config. 2) higher 𝐵 values are achieved in 67.7% of the line. Fig. 4: Magnetic field 𝐵 distributions measured inside the phan- tom in the coronal plane taken in the middle of the phantom. Val- Experimental results. Magnetic field distributions inside the ues are normalized to the same color scale. cylindrical phantom were also measured, as can be seen in Fig. 4. A calibration was done for each configuration to reach the best transmission voltage (Tx) necessary to apply in the dipole for this reason, a new calibration was performed and a higher element, aiming to get good images. For Config. 1 the voltage transmission voltage was applied, allowing us to obtain a more 72.2V was applied, the element’s excitation for Config. 2 and homogeneous 𝐵 field region inside the phantom (Fig. 4(d)). 3 was 77.8V, and Config. 4 received 155.6V. The same values Simulated versus measured results. Comparisons between were used for each corresponding simulation. calculated and experimental transmitted 𝐵 fields were car- It appears that when the phantom is positioned outside the 1 ried out for two lines in the central coronal plane: along 𝑥 and homogeneous region, the magnitude of 𝐵 is smaller at one 𝑦 axis, having as common point the phantom’s center (Fig. 5). side of the phantom and is not high enough to excite equally The comparison of the results shows that the 𝐵 distribution all the spins (see Fig. 4(b) and Fig. 2(b)). Trying to improve 1 along 𝑥 and 𝑦, as for 𝑧 axis (not presented here), are similar 𝐵 homogeneity in this region, two dielectric pads were in- in its extension, but differs slightly in amplitude. To achieve serted and the same Tx voltage as in Config. 2 was applied, the best agreement, the value of end-capacitors in the model however, this setup didn’t generate good images. Due to de- was altered to 𝐶 =0.3pF, which could represent not explicitly tuning and mismatching of the dipole element when inserting 𝑒 modeled losses. Using this capacitor value, the simulated co- dielectric pads, the peak of S11 curve has shifted to a lower efficient was S11=-48dB while S11=-20dB were measured. frequency (as posterior confirmed in a network analyzer) and 4 Maíra M. Garcia et al., Investigating the influence of dielectric pads in 7T magnetic resonance imaging (a) Config. 1. Fig. 6: Local SAR distribution in phantom’s central 𝑥𝑧 plane (𝑥 axis is pointing to the right) for (a) Config. 2, and (b) Config. 3. regions where it was previously not high enough. Additionally, simulations inform that local SAR distribution will change when using pads. The cross-check of results indicates that sim- ulation model and measurement agree very well. However, im- provement in the validation process are intended, notably with (b) Config. 4. regard to real losses of the experimental setup, detuning and impedance mismatch when using pads, and calculation of tem- perature, which can be validated by dedicated MRI sequences. Fig. 5: 𝐵 curves along 𝑥 and 𝑦 lines in the central coronal plane for simulated and experimental results (where 𝐵 values repre- Author Statement sent the percentage times 10 of the attempted flip angle [5]), for setups with (a) and without (b) dielectric pads. Research funding: MMG was supported by the Coordination for the Improvement of Higher Education Personnel (CAPES): Full PhD Program Abroad (Programa de Doutorado Pleno no 3.2 SAR predictions Exterior), process n. 88881.173609/2018-01. Conflict of inter- est: Authors state no conflict of interest. As seen in Fig. 2 and 4, dielectric pads positioned near to the phantom can improve 𝐵 homogeneity inside a region of in- terest. However, it is also important to analyze how the config- uration with pads (Config. 3) will influence the object’s energy References absorption using our hardware setup, i.e. one dipole coil, phan- tom and dielectric pads. Simulations in Fig. 6 show different [1] Teeuwisse W M, Brink W M, Webb A G. Quantitative assess- SAR distribution inside the phantom for Config. 3 in compar- ment of the effects of high-permittivity pads in 7 T MRI of the brain. Mag. Reson. Med. 2012;67:1285-1293. ision to Config. 2, especially in the region that had higher 𝐵 [2] Chen Z, Solbach K, Erni D, Rennings A. Dipole RF element improvement, presenting no hot-spots and only small changes for 7 Tesla magnetic resonance imaging with minimized SAR. in SAR. In accordance to [1], SAR variation when using di- 7th EUCAP 2013;1775-1778. electric pads depends on the aim of the measurement and is [3] Orzada S, Bahr A, Bolz T. A novel 7T microstrip element sequence dependent. Since the aim of this work was to homog- using meanders to enhance decoupling. 16th Proc. Intl. Soc. enize 𝐵 inside the whole phantom, an increase in SAR due to Mag. Reson. Med. 2008; 2979. [4] Chen Z, Solbach K, Erni D, Rennings A. Electromagnetic the extra loss introduced by the usage of pads was expected. field analysis of a dipole coil element with surface impedance characterized shielding plate for 7-Tesla MRI. IEEE Trans. Microw. Theory Techn. 2016;64(3):972-981. [5] Siemens Application Guide: "SA2RAGE - Work-in-progress 4 Conclusions package for fast 𝐵 mapping", Siemens AG, v. 1.0, 2011. [6] Marques JP, Kober T, Krueger G, Van der Zwaag W, Van de Numerical and experimental investigations show that using di- Moortele PF, Gruetter R. MP2RAGE, a self bias-field cor- electric pads can improve the 𝐵 homogeneity distribution in- 1 rected sequence for improved segmentation and 𝑇 -mapping side an object and can also address higher 𝐵 magnitude for at high field. NeuroImage 2010;49:1271-1281.

Journal

Current Directions in Biomedical Engineering – de Gruyter

Published: Sep 1, 2020

Keywords: 7T MRI; dielectric pads; experiment; electromagnetic simulation; dipole RF coil; phantom; B1; homogeneity; SAR; MRI safety

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