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Wireless Split Ring Resonator Metasurface Enhances Transmission Efficiency of Surface Loop Arrays at 7T
Nandita M. Saha1,2, Santosh Kumar Maurya3, Bilguun Nurzed1,4, Helmar Waiczies5, Rita Schmidt3, and Thoralf Niendorf1,2,5
1Berlin Ultra High Field Facility, Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany, 2Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany, 3Department of Brain Sciences, Department of Brain Sciences, Weizmann Institute of Science, Rehovot, Israel, 4Chair of Medical Engineering, Chair of Medical Engineering, Technische Universität Berlin, Berlin, Germany, 5MRI.TOOLS GmbH, Berlin, Germany

Synopsis

Keywords: Non-Array RF Coils, Antennas & Waveguides, High-Field MRI, Split Ring Resonator; Metamaterial surface; Loop Array

Motivation: Metasurfaces are conceptually appealing for enhancing RF coil performance due to added degrees of freedom for shaping electromagnetic fields.

Goal(s): This work focuses on development and validation of a novel split ring resonator (SRR) based metasurface for improving the performance of a two-channel surface loop array at 7.0 T.

Approach: Application of a magnetic field perpendicular to SRR induces electromotive force, forming an LC circuit with resonance frequency as currents circulate between the rings. This property benefits MRI transmission field enhancement.

Results: Our simulations and experimental results demonstrate that wireless metasurfaces enhance transmission efficiency of a two-channel surface loop coil array at 7T.

Impact: Our approach provides technical foundation for development, implementation and validation of novel metasurfaces for RF arrays customized for UHF-MRI. Our metasurface offers customizable resonance properties by adjusting unit cells, periodicity, or structure placement to enhance transmit field efficiency and uniformity.

PURPOSE

Ultra-high field (UHF, B0≥7T) MRI, offers superior signal-to-noise (SNR) ratios and local transceive (TxRx) RF coils enable enhanced tissue excitation1,2. TxRx surface loop coils provide higher SNR near the patient's surface, but at UHF due to wavelength shortening, they produce non-uniform transmit B1+ field distribution1-4. Metasurfaces (MS) involve planar versions of metamaterials. RF wave propagation manipulation using MS is conceptually appealing to enhance B1+ field uniformity, depth penetration and sensitivity of MRI5-10. Split ring resonators (SRRs) are commonly used in metamaterials. SRR possess a large magnetic polarizability and magnetic dipole moment therefore producing the desired magnetic response to applied electromagnetic fields and are beneficial for B1+ field enhancement in MRI11-13. Double SRRs are more effective as they reduce the electric dipole moment with two concentric rings, while preserving the magnetic dipole moment11. Recognizing this opportunity our work examines B1+ gain at 7T using an MS based on an array of SRR unit cells in conjunction with a two-channel TxRx loop array (2L). Our simulations and phantom experiments demonstrated a significant improvement in B1+ at 7T when using 2L array in combination with MS (2L+MS).

METHODS

The MS and 2L were designed in CST microwave studio and constructed on a copper-clad FR4 PCB (εr=4.5) using a PCB prototyping machine (PROTMATE44, LPKF). Bench measurements were done with an 8-channel vector network analyzer (VNA) (Rohde & Schwarz).
The MS (size:264x190x2mm) comprises a 5x7 array of SRR unit cells with 4 long strips [Fig. 1a]. The resonant eigenmode of MS was simulated in the eigenmode solver of CST. The frequency of the MS prototype was determined by measuring the return loss (S11) using a 40 mm-diameter pickup loop coil with VNA.
The two-channel rectangular surface TxRx loop array (200x100x1mm) incorporated four distributed fixed capacitors. The loop was tuned and matched using capacitive tuning (Ct) and matching (Cm) networks, and decoupled from each other with a shared decoupling capacitor (Cd) on a common conductor. The two feeding ports of the 2L were connected to the transmit path of the TxRX switch (Stark Contrast Erlangen, Germany) with a 1:2 power splitter without any phase shift. The loop was tuned and matched to 297.2MHz [Fig. 2].
For performance evaluation of MS, phantom images were acquired for 2L with and without MS. These images were obtained using a rectangular phantom (45x27x10cm) (εr=58; σ=0.77S/m) filled with water, PVP, and salt. MRI experiments were performed using Siemens 7.0 T (MAGNETOM, Siemens Healthineers, Germany) [Fig. 3]. A gradient echo sequence (FLASH) was used for MRI with spatial resolution = 3.5x3.5x3.5 mm3; TE/TR=2/440ms; Flip angle= 840; FoV= 450x43.8 mm; transmitter voltage =160V. Transmit B1+ mapping was conducted in the phantom using 3D actual flip angle imaging (AFI14,15) with spatial resolution = 4x4x4 mm3, TR1/TR2=24/104ms; TE=2.9ms; nominal flip angle =900; FoV= 320x68.8 mm; transmitter voltage=200V. Evaluation of the relative transmit B1+ field distribution were assessed for central transversal slice through the rectangular phantom.

RESULTS

The eigenmode of MS shows the lowest TE mode that provides deep penetration depth of the magnetic (H) field at 300MHz [Fig. 1b]. Our VNA measurements revealed a resonant mode at 297.2MHz with the MS placed on the phantom [Fig. 1d,e]. These results are in good agreement with our simulations and with the experimental S11 parameter.
Figure 4 shows phantom data obtained from 3D GRE MRI. The 2L+MS setup yielded 55%, 37%, 34% higher signal intensity for central axial, sagittal and coronal slices versus the 2L setup. Figure 5, summarizes the results obtained for B1+ mapping. The B1+ maps demonstrate 19% (axial) and 48% (sagittal) enhancement and improved FoV coverage for 2L+MS setup over the 2L setup. The 2L+MS setup facilitated 17% enhancement in B1+ uniformity for the central sagittal slice. To summarize, constructive redistribution of the magnetic field facilitated by the wireless MS benefits both B1+ and signal intensity enhancement.

DISCUSSION & CONCLUSION

Our results demonstrate that wireless split ring resonator metasurface enhances the transmission efficiency of a two-channel surface loop array at 7T. The benefit of our MS is that their eigenmode properties can be customized by adjusting unit cells, periodicity, or structure placement relative to the imaging location for enhanced transmit efficiency and field homogeneity. Also, an unbalanced loop coil approach can be combined with the MS’s resonance eigenmode tuning to further shape the E and H field. Our cost-effective, in-house production of thin MS on FR4 PCB eliminates the need for bulky and expensive dielectric substrates for the MS. Our preliminary results provide the technical foundation for a loop coil array tailored for lumber spine MRI at 7.0 T.

Acknowledgements

This project is a joint collaboration between the Max Delbrück Center for Molecular Medicine (MDC) and the Weizmann Institute of Science, Israel as part of the Helmholtz International Research School (HIRS) for Imaging and Data Science from the NAno to the MESo (iNAMES).

References

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Figures

Figure 1. (a) The MS (264x190x2 mm) comprises a 5x7 array of split ring resonator (SRR) unitcell size: l1=33.6mm; l2=19.2mm; s=g=w=3.6mm; d=3.15mm with 4 long strips (width is 1.15mm, length is 190mm). (b) The resonant eigenmode of MS was tuned at 300 MHz. (c) The designed MS was fabricated on 35μm copper-cladded FR4 PCB board. (d)The resonant frequency of the MS was determined by measuring the S11 using a 40 mm-diameter pickup loop with VNA. (e) MS shows a resonant mode at 297.2MHz with S11 of -12dB on the phantom.

Figure 2. a) Schematic of 2-channel loop (2L) array. The 2L was tuned and matched to a resonant frequency of 297.2MHz for 7T (S11= -15.4dB, S21=-15.2dB). b) Experimental setup of the 2L placed on a rectangular phantom (45x27x10 cm) (εr=58; σ=0.77S/m) filled with water, PVP and salt solution (mimicking muscle tissue) for bench measurement. Four fixed ceramic capacitors of (C) 3.9pF and tuning (Ct), matching (Cm), decoupled Cd trimmer capacitors (2–23 pF) were soldered directly onto the surface. The 2L was tuned and matched to resonant frequency of 297.2MHz (S11 = -27dB, S21 = -20dB) of 7T.

Figure 3. (a) Simulation setup of the 2L array placed on the phantom (top and full view). (b) Simulation setup of 2L array with the metasurface (MS) placed on the phantom (top and full view). (c) The MS is positioned 5mm away from the phantom, while the 2L array is located in a 20mm distance from the phantom. (d) All MRI experiments were conducted using a Siemens Magnetom 7.0T whole-body MRI system (MAGNETOM, Siemens Healthineers, Erlangen, Germany).

Figure 4. 3D representation of 80 slices through rectangular phantom acquired with a 3D gradient echo sequence (FLASH) (spatial resolution = 3.5x3.5x3.5 mm3; TE/TR=2/440ms; Flip angle= 840; FoV= 450x43.8 mm; transmitter voltage =160V; receiver bandwidth 330 Hz/px. Signal intensity were assessed for the central coronal, axial, sagittal slice through the phantom. (a) 3D phantom measurement with 2L array. (b) 3D phantom measurement with 2L array in conjunction with the MS (2L+MS). (c) Central slice view of the phantom. (d) Ratio map of (2L+MS)/2L obtained for the central slices.

Figure 5. B1+ mapping demonstrated that wireless SRR MS enhances transmission efficiency of 2L array at 7T. B1+ distribution was calculated without any phase shimming at 7.0 T and is normalized to the √kW. a) Measured B1+ map (axial slice) for 2L and 2L+MS and ratio map of (2L+MS)/2L. b) 1D plots of measured B1+ along the vertical (dashed) and horizontal line of the phantom’s central axial slice. c) Measured B1+ map (sagittal slice) for 2L and 2L+MS and ratio map of (2L+MS)/2L. d) 1D plots of measured B1+ along the horizontal (dashed) and vertical line of the phantom’s central sagittal slice.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
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DOI: https://doi.org/10.58530/2024/1563