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Simulated radiation patterns of MRI without a shielded room from 0.5 to 7 Tesla
Ehsan Kazemivalipour1,2, Bastien Guerin1,2, and Lawrence L. Wald1,2,3
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: Safety, Safety

Far-field electromagnetic radiation patterns and levels were simulated on the 10m radius regulatory sphere for conventional MRI scanners at 0.5T, 1.5T, 3T, and 7T operated without an RF shielded room. The levels and patterns were strongly affected by the symmetry of the load. With a body load, the peak E-fields on a 10m radius surface rose with roughly the square of the frequency and far exceeded regulatory limits even for 0.5T. With the body in the bore, the radiated patterns also take on a highly asymmetric pattern not present for more symmetric loads.

Introduction

Eliminating the traditional RF-shielded cabin (Faraday cage) used in almost all clinical MRI installations would significantly lower scanner installation costs and simplify siting in diverse locations such as emergency departments and intensive care units. The Faraday cage serves two purposes. First, it attenuates electromagnetic interference detected by the receive system which results in image artifacts (the “receive problem”)1. Second, it attenuates out-going electromagnetic radiation from the body Tx coil, which might interfere with other hospital equipment (the “transmit problem”)2. The receive problem can be addressed through external interference detection coils and post-processing algorithms to remove detected interference from the image3-8. The transmit problem is less well studied and is the focus of this work. Regulatory limits restrict the peak |E|-field on a 10m radius sphere to less than 1mV/m (= 60dBµV/m) for operational frequencies >30MHz. For proton frequencies between 20 and 30MHz (such as 0.5T MRI), the limit is expressed as a maximum magnetic field amplitude |B|<3.35pT (= 8.5dBµA/m) on the 10m sphere9.

In this study, we used EM simulations to model the far-field emission levels and patterns for a conventional CP birdcage body coil inside a solenoidal superconducting MRI system without a shielded room. We modeled 0.5T, 1.5T, 3T, and 7T systems with loads ranging from a uniform sphere (most symmetric) to a body model (least symmetric).

Methods

Figure 1 shows the simulated system consisting of the magnet, cylindrical RF shield, and 32-rung high-pass CP-driven birdcage coil. Figure 1C shows the five loads studied (arranged from the most symmetric to least symmetric): sphere, cylinder, elliptic cylinder, symmetrized body, and body. EM simulations were performed with ANSYS Electronics (ANSYS Inc., Canonsburg, PA) using iterative adaptive meshing. The mesh size was restricted to <4mm on the birdcage coil, <1mm on lumped ports, and <20mm on the RF shield and <10mm within the load volume. The maximum mesh size on the magnet and radiation box surfaces were <40mm and <250mm, respectively. For each load, the birdcage coil was tuned and matched to |S|<-20dB in order to remove imperfect tuning/matching as a source of variability of the results. We computed the far-field directivity, D, defined as the ratio of the radiation intensity (= Poynting vector amplitude) in a given direction to the averaged radiation intensity. The |E|-field pattern on the surface of the 10m radius sphere was calculated for a CW input waveform of either: (a) 1-volt RMS total input voltage or (b) a voltage value leading to a global SAR of 2W/kg in the load.

Results

Figure 2 shows the far-field directivity and Figure 3 the |E|-field patterns at 1.5T for the different loads. The body model produces the most anisotropic radiation pattern at 10m distance (D is 65% greater than in the sphere) and the greatest radiated |E|-field levels (75% (5dBµV/m) greater |E|-field than in the sphere).

Figure 4 shows |E|-field patterns at 10m distance for 0.5T, 1.5T, 3T, and 7T MRI systems with sphere or body loads (CP mode drive, 1-volt RMS input voltage). With increasing field strength, the asymmetry and the |E|-field level increase. For example, the 7T system has radiation pattern 3 times more directive and generated 393-fold greater |E| than the 0.5T MRI system. The asymmetry pattern also undergoes a qualitative change between 3T and 7T, likely when the bore supports waveguide propagation of a traveling wave.

Figures 5A-B show the fraction of the total input power radiated in free space, as well as the maximum |E|-field for the different loads on the 10m radius sphere (1.5T, CP mode). Figures 5C-D show the normalized radiated power and maximum |E|-field for the 0.5T, 1.5T, 3T, and 7T MRI systems loaded with the sphere and realistic body model. At constant global SAR, the body-loaded 1.5T MRI system radiated 1.9 times more power and generated 5.1dBµV/m more |E|-field at 10m distance compared to the other loads. Increasing B0 from 0.5T to 7T increased the body-load radiated power 4.6×104 fold and the |E|-field by 43.6dBµV/m. For the spherical load, these ratios were 4.1×102 and 35.8dBµV/m.

Conclusion and Discussion

MRI scanners, as group II, class A devices, must adhere to IEC 60601-1-210, which in turn refers to CISPR 119 for regulatory control of electromagnetic radiations. Controls are specified in terms of B-field for frequencies below 30MHz, and maximum E-field amplitudes at higher frequencies. This requires that radiation at a 10m distance should not exceed |H|= 8.5dBµA/m (|B|= 3.35pT) for 0.5T, and |E|= 50dBµV/m at 1.5T, and 60dBµV/m at 3T and 7T.

The radiation patterns emerge from the sides of the magnet cryostat and develop a strong azimuthal asymmetry with a body load. Our simulations show that the maximum radiation by body-loaded systems without a Faraday room are 46.4dBµA/m (= 263pT), 112.6dBµV/m, 124.3dBµV/m, and 141.2dBµV/m at 0.5T, 1.5T, 3T, and 7T for a CW input and a normalized RF exposure resulting in a global SAR of 2W/kg; which is well over the regulatory limit. This over-radiation will be even greater in realistic MRI sequences/protocols. This indicates that, without a Faraday room, MRI radiation limits are likely much more limiting than SAR limits.

Acknowledgements

No acknowledgement found.

References

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2. 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.

3. 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.

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7. 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.

8. 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.

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

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Figures

FIGURE 1 – Overview of simulated MRI system (mimicking Siemens 3T Skyra scanner) without a shielded room. (A) Magnet, RF shield, and RF birdcage coil loaded with a uniform body model at head position. (B) Birdcage coil driven in the circularly-polarized (CP) mode. (C) The five loads analyzed: sphere, cylinder, elliptic cylinder, symmetrized body, and body, all filled with material mimicking average body electrical properties.

FIGURE 2 – Far-field directivity patterns for the different loads at 1.5T driven in CP mode; side and end-on view. Dmax is the maximum directivity of the scanner for each load (maximum/average radiated power on the 10m radius sphere) in the CP mode. The asymmetry is imposed by the asymmetry of the load and is highest for the body model.

FIGURE 3 – |E|-field patterns for the different loads on the surface of the 10m radius sphere, all derived for 1.5T CP mode excitation when total input power was adjusted to impose a total 1-volt RMS input voltage. Side view and end-on view are shown. |E|max is reported in units of mV/m and dBµV/m. The MRI system loaded with the body model showed both the highest |E| asymmetry patterns and the highest |E|max in CP mode.

FIGURE 4 – |E|-field on the surface of the 10m radius sphere for 0.5T, 1.5T, 3T, and 7T with either (A) sphere or (B) body load and CP mode excitation for a total 1-volt RMS input voltage. For each case, |E|max is reported, and the |E|-field pattern was shown in 3 different views: a side-on view, and views looking down either the service-end or patient-end. Both |E|max and asymmetry increase with field strength. The asymmetry pattern also changes qualitatively between 3T and 7T.

FIGURE 5 – (A) Normalized radiated power and (B) maximum |E|-field for different loads reported on the surface of the 10m radius sphere for the 1.5T MRI system driven in CP mode. (C) Fractional radiated power and (D) maximum |E|-field on the surface of the 10m radius sphere for the 0.5T, 1.5T, 3T, and 7T MRI systems loaded with the sphere and body and driven in the CP mode. The results in (B-D) were achieved for a cw waveform with a global SAR of 2W/kg in the irradiated load section.

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)
0598
DOI: https://doi.org/10.58530/2023/0598