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Static and dynamic parallel transmission (pTx) for human cardiac MRI at 14.0 T

Bilguun Nurzed¹, Thomas Wilhelm Eigentler^1,2, Christoph Stefan Aigner³, Sebastian Schmitter³, and Thoralf Niendorf^1,4,5
¹Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany, ²Technische Universität Berlin, Chair of Medical Engineering, Berlin, Germany, ³Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ⁴MRI.TOOLS GmbH, Berlin, Germany, ⁵Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine, Berlin, Germany

Synopsis

Transmission field inhomogeneities at ultrahigh and extreme field MRI can be offset by using static or dynamic pTx. Responding to the challenges and recognizing the opportunities of cardiac MRI, this abstract examines the feasibility of parallel transmission (pTx) using fractionated dipole (FRD) RF array configurations for static and dynamic B₁⁺ homogenization of the heart at 7.0T and 14.0T. Our results reveal that static pTx provides limited performance at 14.0 T but dynamic pTx enables uniform excitation of the heart at 14.0T. This finding is heartening and provides the technical foundation for explorations into cardiac MRI at 14.0T.

Introduction

MRI of the human torso and heart at ultrahigh magnetic fields (UHF, B₀ ≥ 7.0T) benefits from the sensitivity gain and spatial resolution.^1,2 This benefit is challenged by transmission field (B₁⁺) inhomogeneities due to RF wavelength shortening and destructive/constructive electromagnetic field (EMF) interferences. For cardiac UHF-MRI (CMRI) the highly heterogeneous tissue environment within the thorax constitutes an extra challenge. To mitigate spatial B₁⁺ variation local RF transceiver arrays^2,3 were established to support B₁⁺ shimming, through an adaptation of multichannel transmission.⁴ Pushing the boundaries and unlocking the potential of extreme field MR (EF, B0 ≥ 14.0T) requires unraveling and leveraging the electrodynamics of the short wavelength regime. To respond to the challenges and in recognition of the opportunities of CMRI at 14.0T, this abstract elucidates the electrodynamic constraints at high spin excitation frequencies, explores the benefits of multi-transmission using a fractionated dipole (FRD) RF array, and demonstrates the feasibility of static and dynamic parallel transmission (pTx) using optimized kT points.^4–6

Methods

The 7.0 T FRD cardiac array was reproduced according to previous reports of this antenna type for CMRI.^7,8 The meander elements were modeled by lumped elements and designed to enhance the antenna’s performance according to the design recommendations.⁷ With increasing static magnetic field strength the antenna dimensions were scaled respectively to the wavelength and electrically optimized. The inductivity was set to minimize the imaginary part of the antenna’s impedance and additionally represent a trade-off between superficial SAR and B₁⁺ efficiency (B₁⁺ scaled to √1 kW input power). At 7.0 T an 8-channel and at 14.0 T a 16-channel configuration were investigated (Figure 1). EMF simulations were performed in CST Microwave Studio (CST Studio Suite 2020, Dassault Systèmes, Vélizy-Villacoublay Cedex, France) using the human voxel model Duke⁹ at a resolution of 1.0x1.0x1.0 mm³. The mesh size of the voxel model was kept at 4.0x4.0x4.0 mm³. Post-processing was performed in Matlab 2019b (MathWorks, Natick, MA) including tuning and matching and channel-wise B₁⁺ calculation. For the static pTx approach (i.e. B₁⁺ shimming) a genetic algorithm (GA) of the Matlab global optimization toolbox was used. The static pTx approach consists of one rectangular RF pulse with 1ms pulse duration. The RF pulse was optimized to (i) minimize the spatial B₁⁺ variation (Coefficient of Variation, CoV(B₁⁺)=SD/mean) and (ii) to maximize the minimum B₁⁺ efficiency in the 3D volume of the heart (Region Of Interest, ROI). For dynamic pTx several RF sub-pulses were used for parallel transmission - separated by gradient blips – with the goal of 3D flip angle (FA) homogenization (CoV(FA)) targeting the whole heart. The pulse design^6,10,11 was performed in Matlab using the small-tip-angle approximation (STA)¹² for a nominal FA distribution of 10° in the heart. At 14.0 T 4, 8, and 12 kT points were optimized with a total pulse duration of 0.96 ms, 1.92 ms, and 2.88 ms respectively.

Results

B₁⁺distributions obtained for static pTx using the FRD RF array configurations at 7.0 T and 14.0 T are shown in Figure 2 and 3. For the default setting (equal amplitude and phase for each channel) the mean B₁⁺ efficiency obtained for the cardiac ROI at 14.0 T is reduced by 50% and the CoV(B₁⁺) is increased by 9% versus the 7.0 T reference. GA optimization based static pTx using the cost function CoV(B₁⁺) revealed for the cardiac ROI: CoV of 20.2% (7.0 T) and 29.2% (14.0 T). Min(B₁⁺) was 1.17 µT/√kW at 7.0 T and 0 µT/√kW at 14.0 T. For GA optimization using the cost function min(B₁⁺) CoV was 41.5% (7.0 T) and 77.8% (14.0 T). Min(B₁⁺) was 2.81 µT/√kW at 7.0 T and 0.56µT/√kW at 14.0 T. Figure 4 highlights the results obtained from dynamic pTx with 4, 8, and 12kT points. Increasing the kT point count resulted in an improved FA homogeneity which is expressed by a CoV(FA) of 21.8%, 14.6%, and 11.6%. The CoV deduced for the cardiac ROI is summarized in Table 1 for static and dynamic pTx.

Discussion and Conclusion

Our EMF simulations demonstrate the B₁⁺-uniformity and efficiency challenges of CMRI at 14.0 T. At 7.0 T these challenges can be addressed with the static pTx. When moving to B₀=14.0 T static pTx is no longer sufficient to overcome B₁⁺ inhomogeneities. Our findings demonstrate that kT-point pTx pulses are highly suitable for mitigating FA inhomogeneities for a 3D cardiac ROI at 14.0 T. For STA the FA scales linearly with B₁⁺ and therefore the CoV(FA) of the dynamic pTx can be benchmarked against the CoV(B₁⁺) of the static pTx. Dynamic pTx showed improved CoV compared to the static pTx at 14.0 T. The improved CoV with increasing kT points results in a more homogeneous FA distribution but will come at the cost of reduced B₁⁺ efficiency due to the longer pulse durations.
To conclude, static pTx provides limited performance at 14.0T. Dynamic pTx enables uniform excitation of the heart at 14.0T. This finding is heartening and provides the technical foundation for explorations into cardiac MRI at extreme field strength.

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program under grant agreement No 743077 (ThermalMR).

References

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8. Steensma BR, Voogt IJ, Leiner T, et al. An 8-channel Tx/Rx dipole array combined with 16 Rx loops for high-resolution functional cardiac imaging at 7 T. Magn Reson Mater Physics, Biol Med. 2018;31(1):7-18. doi:10.1007/s10334-017-0665-5

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Figures

Figure 1: The 7.0 T 8-channel and 14.0 T 16-channel fractionated dipole antenna configurations on the human voxel model Duke. The antennas are scaled to the wavelength of the magnetic field strength.

Figure 2: 7.0 T static pTx results for default channel settings, optimized f=CoV and f=Min. The axial B₁⁺ maps are shown with mean ± SD (minimum) B₁⁺ values in the cardiac ROI. The cardiac ROI is highlighted in red.

Figure 3: 14.0 T static pTx results for default channel settings, optimized f=CoV and f=Min. The axial B₁⁺ maps are shown with mean ± SD (minimum) B₁⁺ values in the cardiac ROI. The cardiac ROI is highlighted in red.

Figure 4: 14.0 T dynamic pTx results for 4, 8, 12 kT points. The axial FA maps for a nominal FA=10° are shown with mean ± SD (minimum) FA values in the cardiac ROI. The cardiac ROI is highlighted in red.

Table 1: B₁⁺ Coefficient of Variants (CoV) with the static pTx shimming approach (top) for the default channel setting, for optimized f=CoV and f=Min at 7.0 T and 14.0 T and flip-angle CoV with the dynamic pTx approach (bottom) with 4, 8, and 12 kT points.

Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)

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DOI: https://doi.org/10.58530/2022/1120