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Mitigation of radiated E-field from a 3T MRI system operated without a Faraday shielded room using parallel transmission

Ehsan Kazemivalipour^1,2, Bastien Guerin^1,2, and Lawrence L Wald^1,2,3
¹A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, ²Harvard Medical School, Boston, MA, United States, ³Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, United States

Synopsis

Keywords: RF Arrays & Systems, RF Arrays & Systems, Transmit radiation, Birdcage coil, Faraday cage

Motivation: Eliminating the Faraday cage would lower installation costs by ~2X and facilitate deployment of MRI in diverse settings, but requires reducing the Tx system’s electromagnetic (EM) radiation.

Goal(s): We employ a parallel transmit (pTx) array and EM absorbers to reduce RF-radiation from a 3T MRI operating without a shielded room, ensuring operation within regulatory limits.

Approach: We model a 3T MRI with a CP body coil and design a 16-channel pTx array and EM-absorber to reduce E-field radiation. Performance is assessed with pTx pulse optimization and L-curve analysis.

Results: We demonstrate a 2270-fold reduction in radiation compared to the birdcage coil.

Impact: By successfully mitigating RF-radiation from MRI systems operated without Faraday cages, this research advances cost-effective MRI installations in diverse clinical/research environments, encouraging further exploration of pTx technology for controlling electromagnetic fields both inside/outside of the body.

Introduction

Eliminating the traditional RF-shielded cabin (Faraday cage) in clinical MRI could reduce installation costs and simplify placement in diverse settings. The Faraday cage mitigates two key issues: the receive problem, where incoming electromagnetic (EM) interference increases image noise and artifact levels^1-7, and the transmit problem, whereby EM radiation exiting the scanner’s body Tx coil might interfere with hospital equipment⁸. To address the Tx problem, regulatory limits^9,10 (IEC-60601-1-2 & CISPR-11) limit peak E-field on a 10-m radius sphere (peak-E_10m) to below 1-mV/m during 3T MRI. Our previous modeling of EM radiation from conventional field-strength, body-loaded superconducting scanner geometries without a Faraday room demonstrated radiation levels significantly above this regulatory threshold¹¹. Without some replacement mitigation method, MRI radiation limits would severely limit Tx operation. In this study, we use EM simulations of far-field E-field levels (peak-E_10m) and patterns for a 3T scanner with a conventional CP birdcage body coil to propose and examine an active and passive mitigation strategy employing parallel transmit (pTx) array coil pulse optimization.

Methods

Figure 1 illustrates the conventional MRI system simulated, including superconducting magnet, cylindrical RF shield, and a high-pass CP body birdcage coil. Without mitigation (by a Faraday shield or other method), this geometry exceeds the regulatory limits by 4680-fold for a 300Vrms input. Figure 2 shows the two mitigation strategies tested: (1) a 25-channel loop pTx array positioned around the patient's feet, and an EM absorber placed at the bore’s service-end (Figure 2B) and (2) a 16-channel head/neck pTx array¹² replacing the body birdcage coil together with the same EM absorber at the service-end (Figure 2C). In the first mitigation strategy, the pTx loops were strategically positioned away from the head and primarily utilized to minimize peak-E_10m. The passive EM absorber used high-dielectric materials optimized for graded interface impedance matching, placed at the bore's service-end. All EM simulations were conducted using ANSYS Electronics with the co-simulation approach, which allowed efficient tuning, matching, and decoupling. All transmit channels (birdcage coil and pTx arrays) were tuned and matched to achieve |S|<-30dB. The pTx channels were ideally decoupled.

For all configurations, the radiated E-fields of each Tx channel were exported on 1° isotropic grids on the surface of a 10-m radius sphere. Additionally, B₁⁺-map of each channel was exported on 2-mm isotropic grids in the central head axial slice. Q-matrices¹³ corresponding to the magnitude of the radiated E-field were computed and compressed using the VOP algorithm¹⁴, with a 5% overestimation factor. We computed and reported peak-E_10m for a 1Vrms continuous-wave input waveform and for RF-shimming pulses designed with a target flip-angle of 10° across the imaging slice using a 5-lobe slice-selective sinc pulse, with 2-ms duration and 100% duty-cycle. To quantify the radiation mitigation performance of the 25-channel pTx array configuration, we generated an L-curve of the tradeoff between loops pTx power consumption and peak-E_10m. In the case of the 16-channel head/neck pTx array, we designed the RF pulses using a least-squares optimization^15-17 with a target phase profile of the birdcage mode of the array while constraining the peak-E_10m. An L-curve shows the performance tradeoffs between excitation uniformity and peak-E_10m.

Results

Figure 3 shows the 10-m E-patterns for the three configurations of Figure 2. The conventional system exhibited a peak-E_10m of 24dBµV/m higher than the regulation (60dBµV/m at 3T). The utilization of the 25-channel pTx system in conjunction with the EM absorber reduced this by 16-fold and thus below the regulation limit (for this RF drive voltage). Additionally, using the 16-channel head/neck pTx array with the EM absorber yielded a remarkable 2271-fold reduction in the peak-E_10m, keeping it 43dBµV/m below the regulation.

Figure 4 shows the L-curve tradeoff between peak-E_10m and maximum pTx voltage for the 25-channel pTx loop approach, showing that pTx effectively attenuates EM radiation by up to 16-fold without requiring high voltage input. Figure 5 shows the L-curve comparison between peak-E_10m and flip-angle uniformity for the 16ch RF-shimming approach. At a constant flip-angle RMSE level (9.9%), the 16-channel pTx system reduces EM radiation by 67.5dBµV/m (2383-fold) compared to the conventional system.

Conclusion and Discussion

Regulations dictate that radiated peak-E_10m should not exceed 60dBµV/m for 3T systems, significantly below that found when conventional systems are operated without a Faraday room. Here, we proposed and simulated the ability of pTx methods to reduce radiated peak-E_10m levels. The pTx pulses achieved excellent flip-angle excitations within the body while reducing far-field RF radiation by up to 67dBµV/m compared to the CP-driven birdcage coil radiation, ensuring that peak-E_10m remained below the regulatory threshold (for 10° flip-angle pulses).

Acknowledgements

The authors extend their gratitude to Markus W. May and Boris Keil for the 16-channel head/neck pTx array model, originally designed for 7T imaging, used in this study.

References

[1] Harberts DW, Helvoort MV. Sensitivity of a 1.5-T MRI system for electromagnetic fields. 2014 International Symposium on Electromagnetic Compatibility 2014:856-859.

[2] Rearick T, Charvat GL, Rosen MS, et al. Noise suppression methods and apparatus (US Patent No. 9,797,971 B2). U.S. Patent and Trademark Office. 2017.

[3] Huang X, Dong H, Qiu Y, et al. Adaptive suppression of power line interference in ultra-low field magnetic resonance imaging in an unshielded environment. J Magn Reson. 2018;286:52-59.

[4] O'Reilly T, Teeuwisse WM, de Gans D, et al. In vivo 3D brain and extremity MRI at 50 mT using a permanent magnet Halbach array. Magn Reson Med. 2021;85(1):495-505.

[5] Liu Y, Leong ATL, Zhao Y, et al. A low-cost and shielding-free ultra-low-field brain MRI scanner. Nat Commun. 2021;12(1):7238.

[6] Srinivas SA, Cauley SF, Stockmann JP, et al. External Dynamic InTerference Estimation and Removal (EDITER) for low field MRI. Magn Reson Med. 2022;87(2):614-628.

[7] Zhao Y, Xiao L, Liu Y, et al. Predict and Eliminate EMI Signals for RF Shielding-Free MRI via Simultaneous Sensing and Deep Learning. 2022 Asia-Pacific International Symposium on Electromagnetic Compatibility (APEMC) 2022:213-215.

[8] Harberts DW, Helvoort MV. Shielding requirements of a 3T MRI examination room to limit radiated emission. 2013 International Symposium on Electromagnetic Compatibility 2013:1053-1057.

[9] IEC 60601-1-2, Medical electrical equipment - Part 1-2: General requirements for basic safety and essential performance – Collateral standard: Electromagnetic disturbances - Requirements and tests. 2014.

[10] CISPR 11, Industrial, scientific and medical equipment – Radio-frequency disturbance characteristics - Limits and methods of measurement. 2016.

[11] Kazemivalipour E, Guerin B, Wald LL. Simulated radiation patterns of MRI without a shielded room from 0.5 to 7 Tesla. Proc Intl Soc Mag Reson Med 31 2023:0598.

[12] May MW, Hansen SJD, Mahmutovic M, et al. A patient-friendly 16-channel transmit/64-channel receive coil array for combined head-neck MRI at 7 Tesla. Magn Reson Med. 2022;88(3):1419-1433.

[13] Guerin B, Gebhardt M, Cauley S, et al. Local specific absorption rate (SAR), global SAR, transmitter power, and excitation accuracy trade-offs in low flip-angle parallel transmit pulse design. Magn Reson Med. 2014;71(4):1446-1457.

[14] Eichfelder G, Gebhardt M. Local specific absorption rate control for parallel transmission by virtual observation points. Magn Reson Med. 2011;66(5):1468-1476.

[15] Zhu Y. Parallel excitation with an array of transmit coils. Magn Reson Med. 2004;51(4):775-784.

[16] Setsompop K, Wald LL, Alagappan V, et al. Parallel RF transmission with eight channels at 3 Tesla. Magn Reson Med. 2006;56(5):1163-1171.

[17] Cloos MA, Luong M, Ferrand G, et al. Local SAR reduction in parallel excitation based on channel-dependent Tikhonov parameters. J Magn Reson Imaging. 2010;32(5):1209-1216.

Figures

FIGURE 1 – Overview of a simulated MRI system emulation of a Siemens 3T Skyra scanner without a Faraday shielded room. (A) Magnet, RF shield, and RF birdcage (BC) coil loaded with a uniform body model at head position. (B) BC coil is driven in quadrature mode at two ports. Mesh sizes: <4 mm on the BC coil, <1 mm on lumped ports, <20 mm on the RF shield, <10 mm within the load volume, <40 mm on the magnet surface, and <250 mm on the radiation box surface.

FIGURE 2 – Mitigation configurations tested at 3T without a shielded room. (A) Shielded birdcage (BC) coil, (B) shielded BC coil with a 25-channel loop parallel transmit (pTx) array at the patient-end and a 5-layer high-dielectric EM absorber at the service-end, and (C) a 16-channel head-neck pTx array with a 5-layer high-dielectric EM absorber at the service-end, all loaded with a uniform ANSYS body model.

FIGURE 3 - E-patterns of a 3T MRI scanner without a shielded room equipped with: (A) shielded birdcage (BC) coil, (B) shielded BC coil with a 25-channel loop pTx array and high-dielectric EM absorber, and (C) 16-channel head/neck pTx array with EM absorber. The CP-driven BC coils had a total input of 1Vrms. The 25-channel pTx system used 0.58Vrms to minimize E-field radiation. RF pulses in 16-channel pTx were designed to match the “mean ± SD” of B₁⁺ in the head axial slice with the BC coils, with a total voltage of 4.2Vrms. E-patterns are depicted from side and service/patient-end views.

FIGURE 4 - L-curve illustrating the tradeoff between peak E-field at 10-m distance and the maximum pTx voltage in an MRI system without Faraday shielded room equipped with 25-channel pTx loops, terminated with a 5-layers high-dielectric EM absorber in the bore’s service-end direction. The peak radiated E-field from an unshielded conventional MRI system is also reported in the L-curve. The CP-driven birdcage coils are adjusted to a total input voltage of 1Vrms.

FIGURE 5 - L-curves quantifying the tradeoff between peak 10-m E-field and flip-angle (FA) excitation error for least-squares (ideal decoupled birdcage [BC] mode phase) RF-shimming for an unshielded MRI system equipped with 16-channel head/neck pTx array and EM absorber. RF pulses are designed to achieve an average FA of 10° in the axial head slice. The peak radiated E-field from an MRI system with BC coil and an MRI system with BC and 25-channel pTx loops and EM absorber are also reported. Additionally, the performance of the 16-channel pTx and EM absorber in the BC mode is presented.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)

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DOI: https://doi.org/10.58530/2024/4087