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A Metamaterial Liner Body Coil for Wide-bore 3T MRI

Leo Rémillard¹, Adam Mitchell Maunder¹, Fraser Robb², Ashwin K. Iyer³, and Nicola De Zanche¹
¹Oncology, Medical Physics, University of Alberta, Edmonton, AB, Canada, ²GE Healthcare, Aurora, OH, United States, ³Electrical and Computer Engineering, University of Alberta, Edmonton, AB, Canada

Synopsis

Keywords: Hybrid & Novel Systems Technology, Body, metamaterial

The transmit field suffers from standing wave inhomogeneities at field strengths ≥3T, which degrade image quality and create specific absorption rate (SAR) hotspots. We present the first experimental demonstration of a whole-body metamaterial liner (metaliner) that enables traveling wave excitation at a lower frequency than achievable otherwise. The simulated transmit efficiencies for a comparable birdcage and metaliner were 2.43μT⁄√kW±23.8% and 1.80μT⁄√kW±25.5% respectively, and the maximum 10g local SAR was reduced by 18% with the metaliner. This shows that the metaliner is an attractive alternative to the BC with greater safety thanks to the lower localized SAR.

Introduction

At higher field strengths (e.g., ≥3T), the MR excitation becomes inhomogeneous due to standing wave effects arising from the inverse relationship between B₀and RF wavelength in the body¹. Beyond concealing areas of disease, many MRI methods involve quantitative and dynamic studies of the MR signal dependent on a consistent excitation throughout the field of view². Furthermore, the birdcage coil (BC) produces localized hot spots of the specific absorption rate (SAR). Imaging parameters that affect SAR become greatly constrained at 3T by safety limits³, limiting the contrast and SNR obtained. Thus, many clinical MRI applications are more effective at <3T, which nonetheless has worse signal-to-noise, spectroscopic resolution and imaging speed. Traveling wave (TW)-MRI was proposed to eliminate the BC using the conductive lining of the bore to produce the RF magnetic field with electromagnetic waveguide concepts⁴. The more homogeneously distributed electrical and magnetic fields were expected to mitigate SAR challenges. The use of MTMs as thin MRI bore liners to artificially lower the cutoff frequency was proposed in Ref. (5), following studies showing their utility for circular electromagnetic waveguides^6–8. Since then, the MRI MTM-liner concept has been rigorously validated numerically⁹ and a robust analytical design framework has been developed^10,11. The benefit of reduced SAR in human body models compared to the BC was demonstrated¹². Here we investigate the real-world performance of a metaliner constructed for 3T imaging in a wide-bore magnet (70cm diameter) to determine its feasibility as an alternative to the stalwart BC.

Methods

The geometry, the equivalent circuit of a single unit-cell, a rendering of the full 12 metaliner rings and photo of the constructed metaliner with temporary shield used to emulate the gradient shield are shown in Figure 1. The lumped capacitors are formed from overlapping conductors on Rogers 3003 substrate (ε_r=3, tanδ=0.0007). The C_r capacitors are ~4.5pF, the 5/3C_ϕ capacitors are ~16.6 pF and the 5C_ϕ capacitors are ~51 pF. The 5/3C_ϕ capacitors have additional tuning strips that are connected by pins, and headers (jumpers) can be added to tune the capacitance and vary the frequency of operation. To compare to the metaliner and predict the relative safety performance, a body BC with approximately the same dimensions as those of the 3T GE 70cm bore Signa Premier was simulated (Ansys, HFSS) with geometry shown in Figure 2 (conductor strip width of 12mm) and tuning capacitor values of C_h=19pF, ESR = 0.035Ω, 2C_L=12pF, ESR = 0.045Ω. Lumped elements are used to represent the chip capacitors used in practice for the BC as well as the capacitances of the metaliner. An adult human body model is used in simulation^13,14. Copper conductors were modelled with conductivity=5.8×10⁷S/m, while the shield at a radius of 372.5mm is modelled as finite conductive boundary with conductivity=8×10⁶S/m. Three metrics are used to quantify the transmit performance: the mean transmit efficiency (B₁⁺ for 1kW accepted RMS input power - η_tx), the homogeneity within the body imaging region, and the maximum 10g averaged SAR normalized to a 1μT transmit field within the imaging field of view. This is the inverse square of the excitation produced per maximum 10g averaged SAR; the safety excitation efficiency (η_EFF). Images were acquired using linear transceive operation through one of the quadrature ports while the other was left open. 3D SPGR imaging was performed (124Hz/pixel, 64×64×32 matrix, 6.6×6.6×15mm³ resolution, TE=4.4ms, TR=75ms, 2 avg.) in a 34cm radius spherical phantom with T₁≈200ms. For comparison, the expected imaging intensity was simulated using the B₁^- as the receive sensitivity and Ernst equation with FA found by scaling the simulated B₁⁺ to the expected input power and RF pulse.

Results and Discussion

The scattering parameters for the loaded metaliner with temporary shield are shown in Figure 3, demonstrating that the ports are decoupled and matched. The simulated and measured transmission efficiency maps in orthogonal slices are shown in Figure 4a for the BC and metaliner. Within the ellipsoidal region of interest (12.5cm×10cm×12.5cm radii) the mean simulated transmit efficiencies were: BC unloaded - 5.74μT⁄√kW±2%; BC loaded - 2.43μT⁄√kW±24%; metaliner unloaded - 3.43μT⁄√kW±4%; metaliner loaded - 1.80μT⁄√kW±26%. In previous simulations the transmit efficiency of the metaliner was closer to that of the BC, but here it was necessary to design the metaliner with an outer shield that is 6.5mm closer to its inner conductor than for the BC, reducing the efficiency. However, the local SAR metric (1/η_SEE²) is 18% lower for the metaliner, and is even lower for designs of the metaliner with smaller bore radii (30cm¹² and 28cm^9,11). The simulated image signal intensity overlaid with predicted FA contour map for a central axial slice is shown in Figure 5a. The corresponding image obtained is shown in Figure 5b, with good agreement between simulation and measurement. Imaging with a single port was performed for this preliminary investigation because of frequency splitting that was not observed in the lab but was present when installed in the system. Quadrature excitation will be performed in the future.

Conclusion

We have performed the first experimental demonstration of a metaliner used as a body coil for wide-bore MRI at 3T. Images match the simulations, which show the important benefit of reduced local SAR hot spots compared to the birdcage coil.

Acknowledgements

This work was supported by the Alberta Innovates postdoctoral fellowship in health innovation, the Office of the Provost and VP of the University of Alberta, the University of Alberta Undergraduate Researcher Stipend, and the Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grants program. We thank CMC Microsystems for software access and support by the University of Alberta Faculty of Engineering IT. We thank GE Healthcare for technical support including Lalit Rai, Ravi Jaiswal, David Lee and Dan Spence. Thanks to Rudi Kopp and Al Dean Davis (GE Healthcare) for measurement support.

References

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Figures

Figure 1: (a) Simulation model of single ring of Metaliner (b) 1/8th segment of ring with simplified circuit model shown below (c) full model of Metaliner rendered in Solidworks and constructed metaliner being tested in system.

Figure 2: Rendering of simulation models of (a) BC and (b) metaliner with human body model and (c) photo of set-up for measurement of matching of the constructed metaliner with temporary aluminum foil shield emulating the gradient shield.

Figure 3: Measured Reflection and isolation scattering parameters for metaliner when loaded and with the temporary shield. The presence of the different hybrid modes are indicated above and the resonance dips for the first three longitudinal resonances of the HE₁₁ mode indicated by arrows. The 1^st order longitudinal resonance mode produces the transverse H-field in the center conducive to MRI RF transmission at the 3T Larmor frequency of 128MHz.

Figure 4: (a) Simulated transmit field homogeneity maps for 1kW RMS input power. The transmission efficiency is scaled to the mean transmit field in the outline region. (b) Maximum intensity projection maps of the simulated SAR for a mean 1μT transmit field within the outlined region. The maximum SAR within the maps is labelled for each.

Figure 5: (a) Simulated image intensity for a central axial slice using the transmit and receive fields of one port of the metaliner with Ernst equation and (b) the measured image.

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)

0218

DOI: https://doi.org/10.58530/2023/0218