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Simulation and Comparison of Transmit Elements for 7T Head-Imaging with a Large Diameter Transmit Coil

Max Joris Hubmann^1,2, Robert Kowal ^2,3, Stephan Orzada⁴, Piet Wagner⁵, Frank Seifert⁵, Oliver Speck^2,3, and Holger Maune²
¹Siemens Healthineers GmbH, Magdeburg, Germany, ²Otto-von-Guericke University, Magdeburg, Germany, ³Research Campus STIMULATE, Magdeburg, Germany, ⁴Deutsches Krebsforschungszentrum Heidelberg, Heidelberg, Germany, ⁵Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany

Synopsis

Keywords: Non-Array RF Coils, Antennas & Waveguides, Non-Array RF Coils, Antennas & Waveguides

12 transmission coil designs for 7T head imaging are compared using EM simulations for a large diameter transmit coil. The highest power and SAR efficiency are achieved by loop coils, a passively-fed dipole shows the highest intrinsic decoupling and an IMARS the lowest load dependence. The results provide insight to each element's performance. To make a final decision on a coil element, the array performance must also be evaluated in the futur, as this may differ from the performance of the single element performance. Thus, the decision on a particular transmit element depends on the coil's application and configuration.

Introduction

Many different transmit element designs for 7T head imaging were introduced and even more have been proposed for body imaging^1-12. Most of these designs target specific applications and are placed close to the subject to ensure high transmit efficiency. This proximity, however, can cause claustrophobia⁶. Additionally, they provide limited space for further devices such as external stimulation or physiologic monitoring, which are frequently used in neuroimaging^13,14. Hence, a transmit coil with a larger distance between patient and coil would benefit patient comfort and access. This contribution compares different transmit element designs for 7T head imaging using EM simulations for a large diameter parallel transmit coil.

Material and Methods

For this study, 12 antenna designs^1-12 were evaluated. Figure 1 shows the designs and their dimensions. The simulations were performed in CST Microwave Studio 2022 at 297.2MHz. The antennas were loaded with the head of the body model Hugo, which is placed as depicted in figure 2a-b. All antennas are adjusted to |S₁₁|=-50dB input reflection. The power efficiency ($$$|\text B _1^+|/\sqrt {\text P}$$$) normalized to 1W of input power and SAR efficiency ($$$|\text B _1^+|/\sqrt {\text pSAR}$$$) as well as the intrinsic coupling and the load dependence were evaluated in simulation. The peak power and SAR efficiency were evaluated at the surface of the head and at 10cm depth. To determine the mutual coupling, a second element was placed at 10cm distance to the first one centered above the head (figure 2c) and |S₂₁| between the coils’ feed points was examined. Thereby, no further decoupling actions were performed. For the load dependence examination, the head was rotated by 90° (figure 2d) and |S₁₁| for the new configuration was evaluated without readjusting the coil to -50dB. To verify the simulations, the MS antenna was built, and the $$$|\text B _1^+|$$$-field as a center line plot was examined¹⁵ with a H-field probe (H1TDSx/MRI, SPEAG) and compared to the simulation. A polyvinylpyrrolidone phantom was used for coil loading. Its parameters and dimensions are depicted in figure 5a-b.

Results

Figure 3 depicts the results of the power efficiency examination which shows that at the surface of the head the loops perform best, while the IP, LW, and PF show the lowest power efficiency. At a depth of 10cm the dipole like elements gain power efficiency relative to the loops, whereby the LW, IP, and PF are still least power efficient. The results of the SAR efficiency show that at the surface and at a depth of 10cm the loops provide best results. All other antenna types perform similarly less efficient. The intrinsic decoupling shows that the PF with |S₂₁|=-22.1dB has the best intrinsic decoupling, followed by the LW and IP. For the RL |S₂₁| was set to 0dB since a tuning to |S₁₁|=-50dB was not possible without additional decoupling actions. The results for the decoupling examination as well as for the load dependence are depicted in figure 4. Here, it can be seen, that the IP shows the lowest dependence to load changes (|S₂₁|=-26.6dB), whereby the RL and CL (|S₂₁|=-10.8dB and |S₂₁|=-6.0dB respectively) show the highest dependence. The evaluation measurement shows general agreement between the simulation and the measurement (figure 5c). The error up to a distance of 190mm is below 10%, however, further inside the phantom the error increases.

Discussion and Conclusion

Considering efficiencies, the loops outperform the other antenna types. This is mainly due to the lower SAR, especially at a larger depth inside the head. Furthermore, the coils having a low power efficiency show a high intrinsic decoupling. The reason is that they generate high magnetic and electric fields inside the substrate between the conductors so that less power is emitted. It must be noted that, |S₂₁|, however, only shows coupling between the input ports, thus, |S₂₁| of PF only represents the coupling of the short dipoles but the coupling of the passive dipoles is not directly represented by |S₂₁|, hence the actual field distribution of the elements must be further evaluated. Considering the load dependence, it can be seen that it correlates with the SAR efficiency. The transmit elements (loops) with a high SAR efficiency have a higher load dependence and vice versa. This is based on a stronger coupling to the load, which results in a larger detuning by load changes. The variation of the validation results rest on the problem of correctly aligning the field probe to get the same entrance point for the line plots. At this point, it must be mentioned that these results are only valid for this specific setup, i.e. a large distance between transmit elements and object and that the efficiencies can vary in an array configuration due to inter-element coupling. They do, however, give insights into the coils’ performances even for other setups.
For 7T head imaging it can be concluded that the choice of transmit elements depends on the use case. When power and SAR efficiency is most important as well as for transceive coils, loops seem to be the best choice, whereas for coils used with various loads or as a transmit only coil, a dipole like element (e.g., IP) may be a better choice.

Acknowledgements

This work is funded by the European Regional Development Fund under the operation number ‘’ZS/2019/02/97145‘‘.

References

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Figures

Figure 1: Coil elements under examination with their geometries

Figure 2: Simulation setup exemplarily displayed for the SN antenna a) sideview, b) front view, c) decoupling, and d) load dependence

Figure 3: Power and SAR efficiency at the surface of the head and at a depth of 10cm

Figure 4: a) Decoupling (|S₂₁|) between two adjacent coils for a center-to-center distance of 10cm; b) Remaining coil adjustment (|S₁₁|), initially adjusted to -50dB, after rotating the head by 90° as a measure for the load dependence

Figure 5: Visualization of the test setup (a & b) and results (c) of the evaluation measurement using a H-field probe, the red lines indicate the locations of the line plots depicted in c

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

4583

DOI: https://doi.org/10.58530/2023/4583