Proceedings of the International Conference DAYS on DIFFRACTION 2015, pp. 75–80
c IEEE 2015 978-1-4673-8630-2/15/$31.00
Electromagnetic bandgap metasurfaces for decoupling of elements of MRI body coil array at 7 Tesla Tatyana A. Derzhavskaya, Stanislav B. Glybovski, Irina V. Melchakova Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia; e-mail:
[email protected]
Alexander J.E. Raaijmakers, Cornelis A.T. van den Berg University Medical Center Utrecht, 3584 CX, Netherlands; e-mail:
[email protected] Electromagnetic bandgap (EBG) structures belong to an important class of metamaterials that prevent wave propagation at certain frequencies. One of promising application areas of such structures is magnetic resonance imaging (MRI). Nowadays MRI technology develops towards using higher static magnetic field strengths. Particularly, development of novel receive and transmit body arrays for prospective 7T machines operating at 298 MHz is of a great importance. Such devices allow achieving higher image resolution and better diagnostic accuracy. As an alternative to conventional magnetic coils electric dipole antennas were recently proposed in order to reduce SAR keeping a high penetration depth and field uniformity. One of challenges in using dipole antennas as array elements in MRI is their strong mutual coupling, which lowers the useful signal. In the present work we test EBG metasurfaces with highly miniaturized unit cells for suppression of the coupling between two closely spaced dipole antenna elements of a 7T MRI body coil array. Metasurface samples were optimized by means of numerical simulations and manufactured as multi-layered structures containing PCB-boards. The metasurface samples improve isolation of matched dipoles by up to −6 dB even for the dipole separation of λ/12. The isolation improvement provided by the manufactured metasurface samples was tested experimentally in the presence of a phantom (an equivalent of a human body).
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Introduction
Metasurfaces are thin periodic two-dimensional arrays composed of electrically small engineered inclusions with subwavelength periodicity. Such structures attract high attention due to their possibilities to control characteristics of the electromagnetic field distribution. Metasurfaces effectively behave as electrically thin uniform sheets, but nevertheless provide a number of useful functions such as wave front focusing [1], polarization transform [2], surface wave manipulation [3], etc. A special class of metasurfaces widely applicable to low-profile antennas is High-Impedance Surfaces
(HIS). One of important HIS features is Electromagnetic Bandgap (EBG) effect, which prevents any surface waves from propagation across the metasurface in a certain frequency band near the resonance of inclusions (unit cells). Another property of HIS, which can either occur at the same or different frequency is artificial magnetic conductor (AMC) behaviour. Due to very high surface impedance at the resonant frequency such metasurfaces act as magnetic walls, which do not invert the phase of a reflected plane wave. The practical printed-circuit board (PCB) realization of HIS was first suggested in [4]. Particularly in this work a mushroom-type geometry of inclusions was developed, where each inclusion consisted of a thin metal patch printed over a grounded dielectric substrate and connected to the common ground plane by a metal via. The unit cells can be considered as LC-circuit, where the capacity is determined by the electric field concentrated between adjacent patches and the inductance is explained by loops of current flowing between adjacent patches through their vias and the ground plane. It was shown that for this certain geometry the EBG effect occurs at the same frequency and the AMC effect [5]. Already in [4] it was proposed to use mushroomtype surfaces for decoupling of closely-spaced dipole antennas, which was later employed for various antenna-array elements e.g. of a patch type [6]. The corresponding experiments, where the metasurface unit cells were tuned to resonate at the antenna’s operating frequencies were successfully made in the microwave range. These experiments confirmed the isolation improvement in a couple of antennas over a common HIS substrate, which becomes possible due to exponential decay of any surface waves excited by one antenna towards another one.
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For novel 7T MRI body coil arrays dipole-type antenna elements were found to be more promising for reduction of SAR and improvement of B1-field homogeneity [7] in comparison with conventional magnetic loop elements. Since the elements in such arrays are densely packed so that the distance between adjacent dipoles is very small compared to wavelength in air, the inter-element coupling becomes a harmful effect restricting the system efficiency. Therefore there is a strong need for efficient decoupling structures for further development of 7T MRI RF equipment. EBG metasurfaces with the mushroom-type unit-cell geometry were first proposed for 7T MRI applications (operating frequency of 298 MHz) in [8]. The same group of authors has also proposed to use EBG substrates to improve performance of 7T stripline coils [9, 10]. Recently periodic arrays of subwavelength resonant elements with magnetic polarization (Magnetic Wall Distributed Filters) were employed for decoupling of magnetic coil array elements in a 7T setup [11]. The purpose of this work is to experimentally revise the performance of mushroom-type EBG metasurfaces for decoupling of neighbouring dipole elements of body coil arrays under their operation conditions specific for 7T MRI, namely the dense dipole packing and the presence of a human body at close distances. For this purpose we manufacture two finite-sized metasurface samples, in which the unit-cells are highly capacitive overlapping square patches over a thick plexi-glass substrate and test them in the presence of a phantom (the equivalent of a human body) in order to study the isolation improvement capabilities. 2
Mushroom EBG metasurface design for 7T MRI
In order to obtain an EBG effect of a metasurface in vicinity of the 7T MRI machine’s operating frequency unit cells must be resonant at 298 MHz. On the other hand we intend to design the metasurface samples for the use in body arrays so that the samples should hold low-profile (no thicker than 30 mm). Also since the decoupling effect provided by EBG metasurfaces is related to a certain surfacewave propagation regime, the samples should contain as many unit cells in the transverse direction with respect to the antenna axes as possible. The above conditions impose very strong restrictions on the unit cell miniaturization. We consider
(a)
(b) Figure 1: Geometry of the employed mushroom metasurface with overlapping patches: a — side view; b — top view (the dashed line means the border of a single unit cell).
two values of the separation between a pair of identical straight symmetric dipole antennas of the length 300 mm: s1 = 60 mm ans s2 = 80 mm. The distance between antennas restricts the unitcell area, and, therefore the capacity of patches. Moreover, the thickness of the body array limits the unit-cell inductance. The first measure of miniaturization is to choose the substrate as thick as possible. We used a plexi-glass spacer of the thickness h2 = 23.5 mm also making the whole sample mechanically robust. Another measure is to enhance the patch capacity, which was done by using of overlapping patches [4] printed on the opposite sides
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Figure 2: Dispersion diagram for TM- and TE-polarized surface waves supported by the studied EBG metasurface with optimized unit cells.
Figure 3: Schematic of the experimental setup with two dipole antennas with the EBG sample in between (the 80-mm sample with 4 unit cells in the transverse direction is shown).
of the same thin dielectric FR4 substrate with the thickness of h3 = 0.5 mm. This array of patches was manufactured as a double-layered PCB board and put on top of the plexi-glass layer. Another metal surface — the ground plane on the bottom of the metasurface was also implemented as a singlesided PCB on h1 = 1.5 mm FR4 layer. All patches are connected to the ground plane by soldered wires of the radius r = 0.5 mm going through drilled holes in the plexi-glass. All the wires are soldered at the the top layer on the upper PCB and go through one plexi-glass layer and two FR4 layers as shown in Figure 1a. The properties of patches were chosen to ensure EBG operation at 298 MHz: the period of patches a = 19.5 mm, the patch side length b = 19 mm. In order to solder the patches of a bottom layer of the upper PCB to the wires additional metal plates with the size c = 3 mm were located between the corners of adjacent patches of the top layer of the upper PCB. The corresponding configuration of patches printed on the upper PCB is depicted in Figure 1b. The top layer is painted in light-gray, while the bottom layer — in dark-gray. To check the EBG behaviour of the metasurface the dispersion diagram was obtained by simulation of the unit cell in CST Microwave Studio. The results for TM- and TE-polarized surface waves are
depicted in Figure 2. From Figure 2 one can see that for the chosen unit-cell parameters there is a frequency band from 290 to 340 MHz in which there is no real solutions for the surface wave propagation factor (EBG regime). In this band all surface waves have imaginary propagation factors and decay exponentially. Therefore one should expect an isolation capability even from a finite-size sample of the EBG metasurface. For the chosen parameters the sample can contain 3 unit cells between the antennas with the separation s1 = 60 mm and 4 unit cells with the separation s2 = 80 mm. The metasurface samples with the corresponding unit cell numbers in the transverse direction have been manufactured. For both samples the number of unit cells in the longitudinal direction was 14. 3
Measurements
In this section we measure the isolation between two identical symmetric dipole antennas of the length 300 mm equipped with the manufactured EBG metasurface sample separating the dipoles. First we made the measurements for the antennas and the sample located in free space (in the absence of the phantom — the equivalent of a human body). Both antennas are fed by 100-Ohm symmetric fixtures each one composed of two coaxial probes
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Figure 4: Measured S-parameters of the antennas with the 60-mm EBG-sample (3 unit cells) in between. Solid lines — in free space; dashed lines — in the presence of the phantom at the distance d = 26 mm.
Figure 5: Measured S-parameters of the antennas with the 80-mm EBG-sample (4 unit cells) in between. Solid lines — in free space; dashed lines — in the presence of the phantom at the distance d = 26 mm.
of the vector network analyser Rohde & Schwarz ZVB20. Such a feed type is preferred since no balun is required for accurate S-parameter measurements. The schematic of the experiment is shown in Figure 3. The antennas and the sample are mounted on the same foam holder, which does not affect their properties. The VNA was used to extract the full S-matrix, which was then recalculated into isolation by post-processing. Similar measurements were made in the presence of the phantom, which was located on top of the setup shown in Figure 3 with an additional separating foam layer. The thickness of the separating layer was varied from 2 to 60 mm. A bottle-shaped neck-coil MRI phantom GE with dimensions of 35 × 15 × 15 mm was employed. The measured S-parameter magnitudes in the frequency range from 200 to 400 MHz are shown in Figures 4 (60-mm sample between the antennas) and 5 (80-mm sample between the antennas). In both figures solid lines describe the antennas with the EBG sample in free space, while the dashed ones — the same setup in the presence of the phantom at the distance of d = 26 mm. From Figures 4 and 5 one can check that there is a resonance in the vicinity of 298 MHz, where a sharp minimum of the S12 magnitude exists corresponding to the EBG effect. From the results comparison one can conclude that for the 80-mm sample with 4 unit cells in between of two antennas the value of |S12 | is by 8 dB lower than for the 60 mm sample with 3 unit cells.
For both the samples the minimal value of |S12 | is lower in the presence of the phantom as compared to the free-space setup. This can be explained by the additional resistive coupling between the antennas due to the currents induced in the bulk of the phantom. In fact, the closer the antennas are to the phantom, the lower is their mutual coupling. Therefore, in 7T coil arrays the inter-element coupling is weak enough when dipoles are very close to the patient. However, if the spacing between the patient and the dipoles is larger than 20–30 mm, the coupling can be unacceptably high for small inter-element separations. On the other hand, the distance to the patient should be chosen taking into account such MRI requirements as B1-field homogeneity, low losses and high sensitivity. Therefore, for practical array designs it is important to answer the question how the manufactured metasurface samples improve the isolation of dipoles depending on the distance to the phantom. We note that the measured |S12 | does not accurately represent the isolation between the dipoles since the dipoles occur to have some impedance mismatch at the desired frequency. However, the isolation level can be recalculated from the measured S-matrix of two dipoles by simulation of an idealized 2-port matching network in CST Microwave Studio Schematic. We used the networks consisting of a parallel short-circuited stub and a series inductor. The network parameters are chosen to obtain |S11 | < −30 dB at 298 MHz. Then
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Figure 7: Measured isolation for antennas with separation of 60 mm as function of the distance d to the phantom (solid curve — antennas without EBG, dashed curve — antennas with EBG).
Figure 6: S-parameters of the antennas in free space with the 80-mm EBG-sample (4 unit cells) and matching networks optimized for 298 MHz.
the resulting |S12 | at the same frequency can be considered as the isolation level. The example of the effective S-parameters taking into account the idealized matching networks is presented in Figure 6 for the 80-mm sample in free space, where the solid curves represent the measured data, and the dashed ones — the results of a full-wave frequency-domain simulations in CST. The results depicted in Figure 6 show good agreement between the simulation and the experiment. It can be seen that the reachable isolation for the 300-mm dipoles separated by the 80-mm EBG metasurface sample is equal to −20 dB. The isolation level obtained this way (|S12 | for the antennas matched at 298 MHz) is presented in Figure 7 for the 60-mm EBG sample and in Figure 8 for the 80-mm EBG sample as a function of the distance to the phantom. In Figures 7–8 one can observe that even for such electrically small distances between the antennas (from 0.06 to 0.08 of the wavelength) the EBG metasurface samples improve their isolation. The decoupling effect is almost twice stronger for the 80-mm sample as compared to the 60-mm one due to a higher unit-cell number. It is also clear that the closer is the phantom to the antennas, the weaker is the isolation capability of EBG samples. This is due to an additional coupling between the antennas through the phantom, which cannot be avoided by surface-wave suppression. The 80-mm sample containing 4 unit cells in between of the dipoles improves the isolation by
Figure 8: Measured isolation for antennas with separation of 80 as function of the distance d to the phantom (solid curve — antennas without EBG, dashed curve — antennas with EBG). −6 dB when the phantom is far enough. The 60-mm sample with 3 unit cells improves the isolation only by −3 dB under the same conditions. 4
Conclusion
In this work two EBG metasurface samples of different unit-cell numbers were manufactured and measured in order to test the isolation improvement capability with respect to a pair of identical straight
80
dipole antennas operating at 298 MHz. The antennas separated by a distance of 60 mm can be additionally decoupled by −3 dB, while the same antennas with a separation of 80 mm — by −6 dB. Also the isolation capability was measured as a function of a distance to a phantom — an equivalent of a human body. The isolation improvement provided by the studied EBG samples will allow to suppress inter-element coupling of multi-channel MRI coil arrays operating at 7T and, therefore, improve their efficiency. Acknowledgements The present work was supported by the Government of the Russian Federation (Grant 074-U01) and the Russian Foundation for Basic Research (Project No. 15-32-20665) References [1] Pozar, D. M., 2007, Wideband reflectarrays using artificial impedance surfaces, Electronics Letters, Vol. 43(3), pp. 148–149.
DAYS on DIFFRACTION 2015 [5] Capolino, F., 2009, Theory and Phenomena of Metamaterials, ser. Metamaterials Handbook, CRC Press. [6] Yang, F., Rahmat-Samii, Y., 2003, Microstrip antennas integrated with electromagnetic band-gap (EBG) structures: a low mutual coupling design for array applications, IEEE Transactions on Antennas and Propagation, Vol. 51(10), pp. 2936–2946. [7] Raaijmakers, A., Ipek, O., Klomp, D., Possanzini, C., Harvey, P., Lagendijk, J., van den Berg, C., 2011, Design of a radiative surface coil array element at 7 T: The single-side adapted dipole antenna, Magnetic Resonance in Medicine, Vol. 66(5), pp. 1488–1497. [8] Saleh, G., Solbach, K., Rennings, A., 2012, EBG structure for low frequency applications, The 7th German Microwave Conference (GeMiC), pp. 1–4. [9] Saleh, G., Solbach, K., Rennings, A., 2012, EBG structure to improve the B1 efficiency of stripline coil for 7 Tesla MRI, 6th European Conference on Antennas and Propagation (EUCAP), pp. 1399–1401.
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