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Combined EM simulations and measurements of birdcage coil B1+ for designing a 3T multichannel TMS /MRI head coil array

Lucia I Navarro de Lara¹, Laleh Golestani Rad¹, Sergey Makarov^1,2, Jason P Stockmann¹, Lawrence L Wald¹, Thomas Witzel¹, and Aapo Nummenmaa¹

¹Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, ²Worcester Polytechnic Institute, Worcester, MA, United States

Synopsis

The multichannel TMS approach enables steering the stimulation “hot spot” electronically making the execution of concurrent TMS and fMRI experiments potentially more flexible. An integrated multichannel TMS/MRI system is currently under development and here we investigate how TMS coils influences the B₁⁺ field of a birdcage coil. We compare detailed electromagnetic simulations with empirical data using a single three-axis TMS coil element as well as generate simulated data for 4x4 TMS coil array. Both simulations and empirical results indicate that an array of TMS coils should not distort the B₁⁺ significantly at distances bigger than 2cm from the coil.

Purpose

To develop a new type of multichannel TMS/MRI stimulation/imaging array for 3T, we evaluated the effects of the TMS coil elements on the B₁⁺ of a birdcage coil.

Introduction

The combination of non-invasive neuromodulation and simultaneous imaging techniques constitutes a powerful approach for neuroscience and potentially treatments. For example, Transcranial Magnetic Stimulation (TMS) during fMRI could help to evaluate causal relationships between the activations of different cortical regions. This combination was presented for the first time by Bohning et al. 20 years ago (1). Dedicated coil arrays have been developed to increase the sensitivity of the MR imaging (2) for these experiments. However, to enable even broader range of potential applications, a multichannel TMS/MRI head coil array for 3 T utilizing a novel three-axis TMS coil design is currently under development (see Fig.1A). When bringing multiple TMS coils inside the MRI scanner it is important to assess the influence of these conducting elements on the RF excitation. Here, we studied the effects of such system on the transmit field based on EM simulations. In addition, a prototype TMS coil element has been fabricated (TristanTechnologies, San Diego, US) and it was used to compare the simulations with experimental data (see Fig.1B).

Methods

A Skyra body coil (Siemens, Erlangen, Germany) was modeled in HFFS (Ansys, USA) and tuned and matched with a gel phantom as load. The dimensions of the phantom were 60cmx41cmx6cm. Three models were built: (i) the birdcage coil loaded only with the phantom, (ii) the coil loaded with the phantom and a three-axis TMS coil element placed over the center of the phantom, and (iii) the coil loaded with the phantom and 16 three-axis TMS elements distributed in a cartesian grid over the center of the phantom (see Fig.2). To compare the simulations with experimental data, B₁⁺ measurements using the Skyra body coil were done. As load, an in-house built phantom (0.82% NaCl, 17% Agar-Agar) with the same dimensions was used. Two setups were employed: (i) only the phantom and (ii) the phantom and one TMS prototype over the center and placed in the isocenter. Gradient Echo (GRE) images with 6 different flip angles (FA) (TR=3000ms, TE=10ms, FA=10,20,30,50,70 and 90, 50 slices, 3mmx3mmx3mm, MA=96x54) were acquired for each setup to calculate B₁⁺ maps. The measured data was fitted pixel-wise to B1*sin(FA) to calculate the field. The reference voltage was set to be 300 V.

Results

Fig3A and Fig3B show the B₁⁺ simulated over the same field of view as the measurement. Fig3C and Fig3D show the corresponding results of the calculated B₁⁺ field based on the measured data. Profiles of the B₁⁺ at 3 different points on the phantom, for both the simulations and the measurements with and without the coil are shown in Fig4. The table on the figure summarizes the changes of the B₁⁺ when the coil is over the phantom at 3 different depths, for both simulated and measured fields. The simulated B₁⁺ obtained for the multichannel TMS array is shown in Fig5A. For the sake of a clear comparison, Fig5B shows the simulated B₁⁺ for the no coil case. The table on the figure summarizes the changes of simulated B₁⁺ when the array is over the phantom at 3 different depths, for 7 distinct positions.

Discussion

The simulated and measured B₁⁺ fields show in general similar characteristics. Both simulations and measurements show enhancement and decrease of the field close to the TMS coil. The profiles show that B₁⁺ enhancement at 4.2cm based on the simulation (between 4-12%) is slightly higher than the measured data (1-5%). This difference is larger for closer regions to the coil. The reason for this is the potential susceptibility effects due to presence of the housing. For the multichannel TMS case, the results show an enhancement of the B₁⁺ at the depth of 4.2cm between 7-19%, and for closer regions, between 15-25%. On the other hand, the field homogeneity at distances greater than 2cm is not significantly affected. No drastic artifacts are expected based on either the simulated or the experimental data. Our conclusion from this study is that an additional transmit coil will not be required even for a large-scale multichannel TMS array positioned inside the MRI environment. However, if using a standard body transmit coil the enhancement of the B₁⁺ must be considered. Further safety assessment must be done and also the effects of the TMS cables should be included in future analysis. The framework presented here may also be useful for other MRI compatible TMS coil design considerations.

Acknowledgements

This work was funded by NIH R00EB015445, R01MH111829 and NIH K99EB021349. We want to thank Dr. Douglas Paulson, Mr. Kevin Pratt, Mr. Paul Mascarenas, and Mr. Paul Miller from Tristan Technologies for their contribution in fabricating the coil. We also thank Dr. Yoshio Okada from Moment Technologies and Dr. Bastien Guerin from MGH Martinos Center for valuable discussions on the project.

References

(1) Bohning et al.,Invest Radiol,33(6):336-340,1998

(2) Navarro de Lara et al., Magn Reson Med, 74(5):1492-1501,2015

Figures

Figure 1. A) A CAD sketch of the multichannel TMS/MRI coil to be designed. B) A MR-compatible TMS coil prototype consisting of three orthogonal coil elements.

Figure 2. Simulation set up. A) Birdcage coil including a shield and the phantom. B) Birdcage coil including a shield, the described phantom and a three-axis TMS coil placed in the center of the coil. C) Birdcage coil including a shield, the described phantom and 16 three-axis TMS coils placed in the center of the coil in a Cartesian array of 4x4 elements. The edge-to-edge distance between the TMS coil elements is 0.5 cm.

Figure 3. A) Simulated B₁⁺ of the birdcage coil on the phantom over the field of view used for the measurements over the central XY plane. B) Simulated B₁⁺ on the phantom with the TMS coil in the isocenter over the field of view used for the measurement. C) Calculated B₁⁺ on the phantom based on the measured data over the central XY plane. D) Calculated B₁⁺ on the phantom with the TMS coil in the isocenter based on the measured data over the central XY plane.

Figure 4. Left) Profiles of the simulated and calculated B₁⁺ fields on the phantom with and without the three-axis TMS coil are shown (red and blue lines, respectively). The distinct positions are indicated in the top part of the figure with black dashed lines. Right) Summary table of the evaluated changes of the fields for different depths and positions for the simulations and measurements.

Figure 5. Left A) Simulated B₁⁺ on the phantom over the central XY plane with the multichannel TMS array over the center of the phantom. B) Simulated B₁⁺ on the phantom without any TMS coils. Right) Summary Table of the evaluated changes of the simulated fields for different depths and positions.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)

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