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SAR safety procedure for self-built pTx human head RF array coils at 9.4T

Felix Glang¹, Dario Bosch^1,2, Georgiy Solomakha¹, Jonas Bause¹, Nikolai I Avdievich¹, and Klaus Scheffler^1,2
¹Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, ²Department for Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany

Synopsis

Keywords: Safety, High-Field MRI, SAR, EM simulation

Motivation: Ensuring subject safety, in particular to limit tissue heating, is a critical aspect of self-developed pTx RF array coils for UHF applications.

Goal(s): Establishing a dependable workflow for accurate simulation, data processing and realization of online supervision of power deposition.

Approach: The workflow relies on cross-comparisons of EM simulation results and intermediate processing steps based on representative excitation modes, and on comparison of measured and simulated field maps.

Results: Consistency was achieved in all cross-comparison steps. Residual discrepancies between measured and simulated B₁⁺ maps require further investigation, but their safety implications can be addressed by an appropriate safety factor.

Impact: A reliable workflow for EM simulation, subsequent data processing, and realization of online SAR monitoring for home-built RF human head array coils at 9.4T is presented. This is an essential building block to ensure subject safety in experimental UHF studies.

Introduction

Parallel transmission^1,2 (pTx) is a central tool in ultra-high field MRI to address the inherent issues of B₁⁺ inhomogeneity and increased power deposition (SAR). Research in this context often relies on self-built pTx coil arrays, especially at field strengths above 7T. For these, safe operation in humans, especially compliance with regulatory SAR limits³ must be ensured. To this end, various coil validation strategies have been proposed, e.g.^4–7, usually based on extensive electromagnetic (EM) modelling. In the present work, our current workflow for SAR modelling of self-built RF coils at 9.4T is presented, which focuses on faithful generation of Q⁸- and VOP⁹-matrices for online SAR monitoring.

Methods

A flowchart of the workflow is shown in Figure 1. First, an EM simulation model of the RF coil under investigation is established in CST Studio Suite 2021 (Dassault Systèmes, Vélizy-Villacoublay, France). To generate Q-matrices, the approach described in ¹⁰ is applied based on two voxel models¹¹ (Duke and Ella), which consists of simulating 10g-averaged (CST Legacy averaging method) SAR in CST for a certain set of RF excitation modes and inferring the Q-matrix entries in Matlab (MathWorks, Natick, MA, USA). A set of 12 RF test modes is defined for all further evaluations:

1) no phase shift, equal amplitudes in all channels,
2) CP+ mode,
3) CP2+ mode,
4) worst-case SAR mode (obtained as the eigenvector to the largest eigenvalue of all Q-matrices¹²),
5) mode 4 but with inverted phase, and
6)-12) additional 7 random modes (magnitude and phase drawn from the uniform random distribution [0,1] and [-π,+π], respectively).

SAR_10g maps for these modes are both simulated directly in CST and computed from the extracted Q-matrices, and the resulting maps are compared for both voxel models. Similarly, B₁⁺ maps are generated in Matlab from single-channel H-field maps exported from CST, and compared to B₁⁺ maps simulated by applying the modes directly in CST. VOP compression^9,13 is performed with an additional safety factor SF=2.2 used to scale up the resulting VOP-matrices. The maximum local SAR_10g for the RF test modes calculated from the Q-matrices and the VOP-matrices are compared. In addition, the same comparison is performed for 1000 additional random test modes.
MR measurements are performed on a 9.4T scanner (Siemens Healthineers) using a homogeneous head-and-shoulder phantom (ε=58.6, σ=0.64 S/m). B₁⁺ maps are acquired using a 3D pre-saturated turboFLASH sequence¹⁴. Simulated B₁⁺ maps are obtained in CST from a voxel model mimicking the real head-shoulder phantom. For a quantitative comparison of simulated and measured B₁⁺ maps, co-registration onto the same voxel space is performed based on the respective object masks, allowing the generation of voxel-wise scatter plots and the Pearson correlation coefficient.

Results

Results are shown for an in-house developed 16-loop TxRx / 32-loop Rx array¹⁵ (Figure 2A) as an example. Figure 3 shows simulated B₁⁺ and SAR_10g maps from CST for the defined RF test modes. Typical patterns of CP and CP2 modes can be observed in the B₁⁺ maps, and it can be verified that the worst-case mode indeed results in the highest local SAR_10g values. The corresponding SAR_10g maps obtained from the Q-matrices in Matlab match up to numerical precision with the CST results, confirming correct generation of Q-matrices. Same holds for the comparison of B₁⁺ maps from CST and Matlab. Figure 4 shows a comparison of maximum local SAR_10g predicted by Q-matrices and VOP-matrices, where it can be verified that, in addition to the safety factor, the VOPs still overestimate local SAR_10g by 4.78% at minimum. Figure 5 compares simulated and measured B₁⁺ maps. Correlation coefficients range from 0.79 (CP mode) to 0.93 (worst-case mode), reflecting a good overall qualitative agreement, but also some visible discrepancies.

Discussion and Conclusion

The proposed workflow has so far been applied for four self-built pTx coils^15–18 (Fig. 2). The degree of agreement between simulated and measured B₁⁺ was found similar for all of these coils. Possible reasons for remaining discrepancies include

interaction with structures affecting the RF field not included in the simulations (e.g., Rx coils)
discrepancies between the tissue properties assumed for simulation and the real phantom
inaccuracies of the experimental B₁⁺ mapping procedure especially in low-intensity regions
imperfect coregistration of experimental and simulated data.

However, given the overall agreement and considerable safety factor applied in the VOP compression on which the SAR monitoring of the scanner system is based, compliance with SAR regulations can be ensured. Further steps to improve the workflow will be the inclusion of a larger class of (potentially self-constructed¹⁹) voxel models, as well as additional SAR validation using MR thermometry²⁰.

Acknowledgements

Financial support of the Max-Planck-Society, ERC Advanced Grant “SpreadMRI”, No 834940 and DFG Grant SCHE 658/12 is gratefully acknowledged.

References

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13. Orzada S, Fiedler TM, Quick HH, Ladd ME. Local SAR compression algorithm with improved compression, speed, and flexibility. Magn Reson Med. 2021;86(1):561-568. doi:10.1002/mrm.28739.

14. Bosch D, Bause J, Geldschläger O, Scheffler K. Optimized ultrahigh field parallel transmission workflow using rapid presaturated TurboFLASH transmit field mapping with a three-dimensional centric single-shot readout. Magn Reson Med. 2023;89(1):322-330. doi:10.1002/mrm.29459.

15. Avdievich NI, Giapitzakis IA, Bause J, Shajan G, Scheffler K, Henning A. Double-row 18-loop transceive–32-loop receive tight-fit array provides for whole-brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T. Magn Reson Med. 2019;81(5):3392-3405. doi:10.1002/mrm.27602.

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Figures

Figure 1. Flowchart of the proposed workflow, which includes electromagnetic simulation of the investigated coil, generation of Q-matrices and VOP compression in Matlab, both validated by respective cross-checks, and comparison between simulated and measured B₁⁺ maps. To ensure correct safety parameters at the scanner, logging outputs based on directional coupler measurements are assessed.

Figure 2. Photos and schematics of the RF array coils that have been so far evaluated by the proposed workflow. (A) 16-loop TxRx / 32-loop Rx array¹⁵, (B) 16Tx / 31Rx loop array¹⁶, (C) 16Tx / 32Rx ToRo loop array that includes a layer for B₀ fieldprobes¹⁸, (D) double-row 16TxRx dipole array¹⁷. Results in this abstract are presented for the coil shown in (A) as an example. Figures adapted from the respective publication.

Figure 3. RF test modes used for the validation cross-checks. (A) Polar plots of the respective mode (CP: circularly polarized, wc: worst-case, wc-: worst-case with inverted phase, rnd: random). (B) Corresponding B₁⁺ maps and (C) SAR maps from EM simulation in a voxel model (Duke), the latter calculated for a total input power of 1 W.

Figure 4. (A) Comparison of maximum local SAR_10g predicted by Q-matrices (solid blue) and VOP-matrices (dashed blue) for the defined RF test modes (total input power of 1 W), as well as their ratio (orange), i.e., overestimation. (B) Comparison of SAR_VOP and SAR_Q for additional 1000 random RF test modes. Note that all SAR_VOP values have been divided by the chosen safety factor of SF=2.2 to focus on the residual overestimation due to VOP compression.

Figure 5. Comparison of measured and simulated B₁⁺ maps. (A) Different views of measured and simulated B₁⁺ maps in the homogeneous head-shoulder phantom for some of the defined RF test modes (see Fig.3). (B) Voxel-wise scatter plots of measured and simulated B₁⁺ with legends indicating the Pearson correlation coefficient.

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

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DOI: https://doi.org/10.58530/2024/0480