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Toward Human Head Imaging at 10.5T Using an Eight-Channel Transmit/Receive Array of Bumped Fractionated Dipoles
Alireza Sadeghi-Tarakameh1,2, Angel Torrado-Carvajal3, Russell L. Lagore4, Sean Moen4, Xiaoping Wu4, Gregor Adriany4, Gregory J. Metzger4, Lance DelaBarre4, Kamil Ugurbil4, Ergin Atalar1,2, and Yigitcan Eryaman4

1Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey, 2National Magnetic Resonance Research Center (UMRAM), Ankara, Turkey, 3Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, United States, 4Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Synopsis

Ultra-high field (UHF)-magnetic resonance imaging (MRI) provides numerous benefits such as significant increase in signal-to-noise ratio (SNR), however, increasing the field strength, the local specific absorption rate (SAR) starts to become a limiting factor. Utilizing a TxArray coil with improved SAR performance may provide a good solution for this issue. In this study, we designed and built an eight-channel transmit/receive array of bumped fractionated dipoles for head imaging at 10.5T. We established a good agreement between simulations and experiments in order for RF safety validations. In addition, a cadaver head was imaged using pTx pulses to evaluate the imaging performance.

Introduction

Ultra-High Field (MRI) provides numerous benefits1-3 including increased signal-to-noise ratio4,5. However, increased local specific absorption rate (SAR) is a limiting safety factor for many applications6,7. Therefore, designing transmit arrays (TxArray) that reduce local SAR without impacting overall transmit performance is critical8,9.

Recently, a whole-body 10.5T scanner has been installed in the Center for Magnetic Resonance Research (CMRR) that provides a unique opportunity to investigate human body and brain in detail. Along with other coil designs10-15 a bumped dipole coil array was proposed16 and was demonstrated to have improved SAR performance in simulations.

In this study, we designed and built an eight-channel transmit/receive (T/R) array of bumped fractionated dipoles16 for head imaging at 10.5T. To evaluate RF safety, EM simulations were performed and compared against experimental RF heating and B1+ measurements. Imaging performance was evaluated with a cadaver head imaging study.

Theory and Method

Each element of the array was constructed with an optimal bump height of 30mm at the feed point. The conductors were etched on an RO4003C (Rogers Corp.) laminate and mounted on a 3D printed elliptical holder composed of PETG (Fig. 1a).

In order to investigate RF safety, a series of RF heating and B1+ mapping experiments were performed using a jar-shaped gel phantom (Radius =75 mm, εr=78.3, σ=0.66 S/m). For the RF heating experiments, the array was excited using eight separate 500 W RF amplifiers (Communications Power Corp., Hauppauge, NY, USA) with arbitrary amplitudes and phases (i.e excitation scenarios 1 and 2). The temperature was measured using eight fiber optic probes (Lumasense Technologies, CA, USA) immersed in the phantom. We calculated the local SAR from the temperature data using the first order approximation. For the B1 measurements, we used actual flip angle imaging17 (AFI).

After we completed RF heating and B1 mapping experiments, we measured the scattering (S-) parameters of the array using a sixteen channel vector network analyzer (Rohde & Schwarz ZNBT8, Munich, Germany). Then CT images of the set-up were acquired (Fig 1 b-c) which were later used to model the setup in the simulation environment (HFSS, ANSYS, Canonsburg, PA, USA) as shown in Fig. 1d.

Co-simulation was employed to minimize the difference between the measured and simulated S-matrices. A circuit simulator (AWR Corp., El Segundo, CA, USA) was used for the optimization of the lumped element values. Once a match was achieved with co-simulation, the B1-maps and 10g-averaged SAR were simulated. (Experimental excitation patterns were repeated in the simulations). Finally, the experimental data were compared with EM simulations in order to validate the coil model.

To investigate the peak SAR levels that could be induced in a realistic human head, we performed additional EM simulations. The gel phantom was replaced with the Duke head model18 imported into CST Studio (CST, Darmstadt, Germany), as shown in Fig. 1e-f. As an example, a quadrature excitation was performed.

Finally, we conducted an imaging experiment and acquired 2D turbo spin-echo images of a fixed cadaver head (TR/TE= 5000/72 ms, FOV = 192 × 256 mm2, in-plane resolution = 0.5 mm, slice thickness = 1 mm, and acquisition time = 153 sec) For improved RF uniformity, an RF phase-only shimming solution19 was calculated based on a fast estimation of the multichannel transmit B1 mapping20.

Results

Fig. 2a-b shows the magnitude and phase of the measured S-matrix, while, Fig. 2c-d shows the corresponding S-matrix obtained using the EM simulation after optimizing the lumped elements.

Fig. 3a and 3b show the simulated 10g-averaged SAR-maps corresponding to the excitation 1 and 2. The local SAR values obtained from the simulations and measurements at the locations of the temperature probes’ tips are compared in Fig. 3c and 3d.

Measured and simulated B1-maps corresponding to the excitation 1, 2 and quadrature excitation are given in Fig. 4.

Using the quadrature excitation in the presence of the Duke model, Fig. 5a-b represent the B1- and SAR-maps normalized to 1W delivered power.

Finally, Fig. 5c-d show the cadaver head image at two different transverse slices.

Discussion and Conclusion

We designed and built an eight-channel T/R bumped fractionated dipole array and utilized it for cadaver head imaging at 10.5T. We validated our coil simulation model and established good agreement between the simulation and experimental results. The validated coil model can be used to determine safe power levels for future human head imaging studies. Currently, approval is being sought using the RF safety data presented to obtain FDA approval to use this coil design in vivo.

Acknowledgements

This work was supported by following grants: NIBIB P41 EB015894, NIH S10 RR029672, NIH- U01 EB025144.

References

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Figures

Figure 1. Coil structure. (a) A 3D printed eight-channel array of bumped fractionated dipoles. (b-c) Sagittal and axial view of the experimental setup including the coil, jar-shaped phantom, and thermal probes which are obtained by CT scan. (d) A view of the coil in presence of the jar-shaped uniform phantom in the simulation environment. (e-f) Sagittal and axial view of the coil and Duke model in the simulation environment.

Figure 2. S-parameters of the coil. (a) Magnitude (dB) and phase (deg) of the measured S-parameters corresponding to the experimental setup. (b) Magnitude (dB) and phase (deg) of the simulated S-parameters which were optimized using the co-simulation method to minimize the difference between the measured and simulated S-parameters.

Figure 3. 10g-averaged SAR data. (a-b) Simulated 10g-averaged SAR map corresponding to excitation 1 and 2. (c-d) Comparison between simulated 10g-averaged SAR values at the tip of the thermal probes and the measured temperature rises which are transformed to the SAR values usıng linear approximation.

Figure 4. Measured vs. simulated B1-maps on an axial slice using (a-c) excitation 1, (b-d) excitation 2, and (c-e) quadrature excitation.

Figure 5. Realistic human model and cadaver imaging. (a) 10g-averaged SAR map corresponding to the quadrature excitation with 1W delivered power in presence of the realistic human body model (Duke). The demonstrated axial slice contains the peak SAR location. (b) B1-map corresponding to the same excitation on the slice passing through the center of dipoles (slightly higher than the slice containing the peak SAR). (c-d) Two representative axial slices from a 2D turbo spin echo imaging of a cadaver head using RF phase-only shimming for improved RF uniformity.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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