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Metamaterial Liner RF Head Coil for ²³Na and ¹H at 4.7 T

Adam Maunder¹, Ashwin Iyer², and Nicola De Zanche¹
¹Oncology, Medical Physics, Cross Cancer Institute, University of Alberta, Edmonton, AB, Canada, ²Electrical and Computer ENgineering, University of Alberta, Edmonton, AB, Canada

Synopsis

Well-established and emerging methods of ²³Na sodium imaging in the brain benefit from high field strength, but require imaging sequences that are constrained by specific absorption rate (SAR) limits. Additionally, concurrent proton imaging is desirable for complementary anatomical/structural information. Here, we present a novel metamaterial liner-based MR coil for ²³Na/¹H imaging at 4.7 T, consisting of longitudinally-spaced rings that alter the effective electromagnetic properties between the liner and outer RF shield (waveguide boundary). In simulation, the metamaterial liner was found to produce lower local 10g averaged SAR for the same mean transmit field relative to a conventional birdcage coil.

Background

Increasing B₀ field strength is desirable due to the increased SNR, especially for ²³Na imaging, which has approximately 3000 times lower MR sensitivity relative to ¹H in the body¹. However, at high field strengths the use of high flip angles (FA) and short repetition time (TR) sequences (emerging methods of ²³Na employing multi-pulse sequences^2,3) is constrained by the local specific absorption rate (SAR).. We propose a novel transmit resonator consisting of a stack of longitudinally-spaced ring structures, previously explored for high frequency microwave applications (3.2GHz)^4,5. This structure was subsequently described theoretically as producing an effective near zero radially-oriented permittivity in the annular region between the rings and a surrounding conductive shield. This engineered metamaterial property drastically lowers the cutoff frequency of the hybrid-electric (HE₁₁, which resembles TE₁₁) cylindrical waveguide mode, allowing propagation in the longitudinal direction similarly to travelling-wave MRI⁶. This structure distributes the electric fields more homogenously over the imaging subject compared to conventional resonators, thereby reducing SAR^7,8.

Purpose

To demonstrate a metamaterial liner for dual tuned ²³Na/¹H head imaging at 4.7 T that harnesses the improvement in SAR performance compared to a conventional birdcage coil.

Methods

Individual rings of the metamaterial structure consist of radially-oriented capacitors separated by arc-shaped sections of conductor; azimuthally-oriented capacitors are included in series with the segments for the ¹H tuned rings. The structure is similar to that of the annular ladder resonator⁹, but the metamaterial liner is a distributed structure that consists of many strongly-interacting, closely spaced rings (nine ²³Na and nine ¹H alternatingly tuned rings here) and is engineered using metamaterial methods. By theoretical analysis, the effective medium properties within the liner, as depicted in Figure 1, are represented as biaxial for the effective permittivity $$$\bar{\bar{ε_l}}=ε_0\bar{\bar{I}}(ε_r,ε_r,1)$$$ and uniaxial for the permeability $$$\bar{\bar{μ_l}}=μ_0\bar{\bar{I}}(1,1,μ_z)$$$, where $$$ε_r$$$ is negative and near zero and $$$μ_z$$$ is large and negative at the desired cut-off frequencies of the HE₁₁ mode. Furthermore, the metamaterial liner dispersion must produce the HE11 mode with a λ/2 longitudinal resonance over its length. The capacitor values that produce the required metamaterial properties and resonance were found by simulation (Ansys, HFSS) of the propagation-dispersion characteristics of the structure with the eigenmode solver using the methods employed in Reference 4. The metamaterial liner was constructed by wrapping two ROGERS 3006TM laminates (0.25mm thick, ε=6.15, tan𝛿=0.025, 18 μm thick copper) with conductor layout shown in Figure 2 around a 1-mm-thick polycarbonate cylinder. Parallel plate capacitors were realized by etching overlapping pads, including isolated pads that could be joined by conductive bridges for fine tuning on the bench. The transmit efficiency ( $$$B_1^+$$$ ) for 1 kW input power and 10g average local SAR normalized for a mean 1µT field were calculated within the same region of the head for both the metamaterial liner and a single-tuned shielded hybrid birdcage coil. Models of birdcage coil and metamaterial liner, and constructed metamaterial resonator, are shown in Figure 3. The birdcage coil capacitor tuning values were C_ring=282 pF, C_leg=150 pF for ²³Na and C_ring=16.58 pF, C_leg=8.2 pF for ¹H. All conductors were modelled with a surface impedance boundary (5.8$$$×10^7$$$S/m) and the VHP female human body model 2.0^10,11,12 was used as a load. ²³Na Imaging was carried out on a cylindrical phantom (20 cm height, 7 cm radius, 2.64g/L CuSO₄·H₂O and 5.59/L NaCl) with parameters: 3-plane, single 50mm thick slice, 2D gradient echo sequence, TR=400ms, TE=4.3 ms, 32x64 matrix, 32x32cm², N_avg=4, BW=8.043kHz. ¹H imaging was carried out on a bottle phantom with parameters: 3-plane, five 4mm slices, 2D gradient echo sequence, TR=130ms, TE=7ms, 256x100 matrix, 25.6x20cm², N_avg=1, BW=34.965kHz. Low-power imaging with small FA was carried out for this preliminary test.

Results

The mean transmit efficiency inside the outlined region displayed in Figure 4 is lower for the metamaterial liner compared to the birdcage coil (12.6 vs. 17.5$$$μT/\sqrt{kW}$$$ at 200MHz, and 55.7 vs. 56.7$$$μT/\sqrt{kW}$$$ at 52.9MHz), and more inhomogeneous at 52.9MHz (6.8% vs. 2.7%), but equally homogenous at 200MHz (both 12.5%). However, for a normalized mean 1µT excitation the simulated local 10g averaged SAR is 22% lower at the ²³Na frequency for the metamaterial liner vs. birdcage coil (0.076 vs. 0.098 $$$\sqrt{W/kg}/μT$$$) and 32% lower at the ¹H frequency (0.89 vs. 1.30 $$$\sqrt{W/kg}/μT$$$). Imaging performed with ¹H and ²³Na (Figure 5) demonstrates the prototype metamaterial liner functions as a volume coil. For the low FA imaging employed SNR~$$$∝(B_1^+ )^2$$$, and thus the simulated $$$ (B_1^+ )^2$$$ and measured SNR along the z-axis shown in Figure 5 are in close agreement.

Discussion

The metamaterial liner represents a new class of volume transmit coil with desirable features and many potential design variations. The benefit of the metamaterial liner in this implementation was a more distributed current pattern with the metamaterial liner resulting in a 22% lower maximum local SAR for ²³Na and 32% lower for ¹H. Lower SAR is critical for high field ²³Na imaging to comply with regulatory limits while ensuring efficient scanning and avoiding lengthening the repetition time or reducing the number of acquired slices¹³. Future work will focus on detailed validation of the simulated SAR and efficiency metrics with comparison to birdcage coils.

Acknowledgements

This work was supported by the Alberta Innovates Postdoctoral Fellowship in Health Innovation and the National Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants program. Thanks to Dr. Robert Stobbe and Karim Damji at the Peter S. Allen MR research center.

References

1. Juras V, Mlynarik V, Szomolanyi P, Valkovič L, Trattnig S. Magnetic resonance imaging of the musculoskeletal system at 7T: Morphological imaging and beyond. Magn Reson Imaging 2019 Jun;28(3):125-135.

2. Hoesl MAU, Schad LR, Rapacchi S. Efficient ²³Na triple‐quantum signal imaging on clinical scanners: Cartesian imaging of single and triple‐quantum ²³Na (CRISTINA). Magn Reson Imaging 2020 Jun;84(5):2412-2428.

3. Gilles A, Nagel AM, Madelin G. Multipulse sodium magnetic resonance imaging for multicompartment quantification: Proof-of-concept. Sci Rep 2017 12;7(1):17435.

4. Pollock JG, Iyer AK, Pratap D, Anantha Ramakrishna S. A class of circular waveguiding structures containing cylindrically anisotropic metamaterials: Applications from radio frequency/microwave to optical frequencies. J Appl Phys 2016;119(8):083103.

5. Pollock JG, Iyer AK. Below-cutoff propagation in metamaterial-lined circular waveguides. IEEE Transactions on Microwave Theory and Techniques 2013;61(9):3169-3178.

6. Brunner DO, Paška J, Froehlich J, Pruessmann KP. Traveling-wave RF shimming and parallel MRI. Magn Reson Med 2011 Jul;66(1):290-300.

7. Hosseini N, Pollock J, Iyer A, De Zanche N. Metamaterial bore liners as high-field body transmit coils: Advantages over standard birdcage coils. Proc. Int. Soc. Mag. Reson. Med 2018;27:4298.

8. Pollock J, De Zanche N, Iyer A. Traveling-wave MRI at lower B0 field strengths using metamaterial liners. Proc. Intl. Soc. Mag. Reson. Med. 2012;20:2792.

9. Wiggins G, Zhang B, Lattanzi R, Sodickson D. Mid-bore excitation of traveling waves with an annular ladder resonator for 7T body imaging with reduced SAR. 2010.

10. Ratiu P, Hillen B, Glaser J, Jenkins DP. Visible human 2.0--the next generation. Stud Health Technol Inform 2003;94:275-281.

11. Yanamadala J, Noetscher GM, Rathi VK, Maliye S, Win HA, Tran AL, et al. New VHP-female v. 2.0 full-body computational phantom and its performance metrics using FEM simulator ANSYS HFSS. Conf Proc IEEE Eng Med Biol Soc 2015;2015:3237-3241.

12. Low-frequency electromagnetic modeling for electrical and biological systems using MATLAB. : John Wiley & Sons; 2015. 13. Ladd ME, Bachert P, Meyerspeer M, Moser E, Nagel AM, Norris DG, et al. Pros and cons of ultra-high-field MRI/MRS for human application. Prog Nucl Magn Reson Spectrosc 2018;109:1-50.

Figures

Figure 1: (Top) sketch of Metamaterial lined waveguide, truncated in length to operate as a standing wave resonator for application as an MRI transceiver. (Bottom) Simplified lumped circuit model of alternating ²³Na-(52.9 MHz) and ¹H-(200 MHz) tuned rings spaced 2cm apart in the metametarial liner (18-rings total) to produce effective excitation at both frequencies. The inner ring of the lumped circuit model corresponds to the etched ROGERS substrate, while the outer ring corresponds to the aluminum. The intrinsic inductance of the ring segments is not explicitly shown in the model.

Figure 2: Conductor layout of the metamaterial liner that is placed on the inner cylinder formed of 1 mm thick polycarbonate. In each ring, sections of overlapping top and bottom conductors on the ROGERS 3006TM substrate form the radially-oriented (C^r) and azimuthally-oriented (C^ϕ) capacitors (Figure 2).

Figure 3: (Left) Simulation model of the birdcage coil (with tuning capacitors labelled) used to compare performance of the (Middle) simulation model of the metamaterial resonator with human body model. (Right) Photograph of the constructed metamaterial liner resonator. Small wires (0.6 mm diameter) join the C^r capacitor pads to the aluminum mesh through contact between conductive rails that press against aluminum tape on the inner side of the polycarbonate shell.

Figure 4: (Left) Simulated transmit performance of the metamaterial liner and single tuned birdcage coil for ¹H – 200 MHz and ²³Na – 52.9 MHz imaging. (right) Maximum intensity projections of the local 10g average SAR normalized to a mean 1μT transmit field in the region outlined in the transmit efficiency maps for ¹H–(200 MHz) and ²³Na–(52.9 MHz).

Figure 5: Central slices in head-sized phantoms for (top) ²³Na and (bottom) ¹H gradient echo imaging. The difference in field distribution at the two frequencies is highlighted by (right) the signal profile measured in phantoms with low FA gradient echo imaging compared to simulation of the in the human phantom. An offset is applied in the z direction so that the maxima occur at the same location in measurement and simulation.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)

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