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Evaluation of the Interaction between RF Coils and Multiple TMS Coils at 9.4T1Institute of Biomedical Engineering, Peking University Shenzhen Graduate School, Shenzhen, China, 2School of Electronics, Peking University, Beijing, China, 3School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, China, 4College of Future Technology, Peking University, Beijing, China
Synopsis
Keywords: High-Field MRI, Modelling, 9.4T RF coil LFMS TMS
Motivation: Recently, The research methods of transcranial magnetic stimulation(TMS)/fMRI in ultra-high fields are receiving increasing attention.
Goal(s): However, as the magnetic field strength increases, the interference problem between the TMS coil and the radio frequency（RF） coil will also become increasingly apparent.
Approach: We designed two types of TMS coils and 9.4T human RF coil using the target field method, and evaluated the impact between the TMS coil and the RF coil using electromagnetic simulation based on the human head model.
Results: We have established a method for evaluating the interaction between RF coils and TMS coils in ultra-high fields.
Impact: Our work explores the combined use of TMS/fMRI in ultra-high fields, providing a non-invasive and precise approach for the microscopic study of brain nerve activity under TMS, and also providing a basis for the regulation of brain nerves.
Introduction
Repetitive transcranial magnetic stimulation (rTMS), as an important regulatory tool in the field of brain nerves, is increasingly widely used, with significant regulatory effects on diseases such as depression and Alzheimer's disease (AD)1.Low-field magnetic stimulation(LFMS), as a new rTMS technique, has good therapeutic effects on major depressive disorder (MDD) and bipolar disorder2.Obtaining brain functional images immediately after magnetic stimulation is an important way to study brain neural activity3.At present, due to the rapid development of ultra-high field MRI, the advantages of combining ultra-high field MRI with TMS are increasingly attracting attention.However, as the field strength and operating frequency increase, the interference between the TMS coil and the RF coil becomes increasingly complex.This work used the target field method to design the RF coil at 9.4T and the TMS coil. Based on the human head model, key parameters such as transmit B1+ field and specific absorption rate (SAR) value of the RF coil were evaluated using modeling electromagnetic simulation.Method
This work utilized methods and techniques such as target field method4 and electromagnetic field numerical calculation to establish a multi-channel monopole transmit coil simulation model based on the human head model at 9.4T, as shown in Fig. 1.Fig.2 shows the simulation model of a multi-channel monopole transmit coil based on the human head model and a LFMS coil covering the entire brain.
The simulation model of a multi-channel monopole trasmit coil based on the human head model and a TMS coil array with focusing function covering the entire brain is shown in Fig. 3.
Result
Fig. 4 (a), Fig.4 (b), and Fig.4 (c) respectively show the distribution of transmit B1+ field in the coronal, sagittal, and transverse planes of 9.4T RF coil, LFMS coil, and TMS coil array.From the figure, it can be seen that the LFMS coil and TMS coil array has little effect on the distribution of transmit B1+ field of the RF coil.The homogeneity of the transmit B1+ field remains basically unchanged.From Fig. 5,it can be seen that the distribution of SAR values for three types of coils in the human head model. The results of RF coil were displayed in Fig.5(a).
Fig. 5 (b) presents the results of the LFMS coil.
Fig.5(c) illustrates the SAR values distribution of TMS coil array.
Compared with the results of the RF coil, the SAR value distribution of the LFMS coil shows little change. However, the SAR value distribution of the TMS coil array has significantly increased compared to the RF coil and LFMS coil.The maximum SAR value is 1.1282 W/Kg.
Discussion
The LFMS coil can operate in both up and down modes, as well as left and right modes, both of which can generate low-field magnetic stimulation covering the entire brain, and its impact on the RF coil is basically the same. The TMS coil array has a significant impact on the SAR value distribution of RF coils, and in practical applications, the coil structure needs to be further optimized.Actual testing can be conducted on the 9.4T full body superconducting system in the later stage.Conclusion
This work compares and analyzes the effects of LFMS coil and TMS coil on the performance of RF coil at 9.4T using electromagnetic modeling and simulation based on human head model. Mainly evaluate the distribution of transmit B1+ field and SAR value of the RF coil. The LFMS coil has a small impact on the performance of the RF coil, while the TMS coil array has a significant impact on the SAR value distribution of the RF coil.Acknowledgements
This work was supported by the Research on Key Technologies of Human 14T Ultra-high Field Magnetic Resonance Imaging System of the Development and Reform Commission of Shenzhen Municipality, China (Grant No. ZDKJ20190305002)， National Natural Science Foundation of China (NSFC) under grant No.62271508References
1.Zhen, J., Qian, Y., Weng, X., Su, W., Zhang, J., Cai, L., Dong, L., An, H., Su, R., Wang, J., Zheng, Y. and Wang, X. (2017), Gamma rhythm low field magnetic stimulation alleviates neuropathologic changes and rescues memory and cognitive impairments in a mouse model of Alzheimer's disease. Alzheimer's & Dementia: Translational Research & Clinical Interventions, 3: 487-497.
2.Sekar S, Zhang Y, Miranzadeh Mahabadi H, Buettner B, Taghibiglou C. Low-Field Magnetic Stimulation Alleviates MPTP-Induced Alterations in Motor Function and Dopaminergic Neurons in Male Mice. Int J Mol Sci. 2023 Jun 19;24(12):10328.
3.Navarro de Lara, L.I., Windischberger, C., Kuehne, A., Woletz, M., Sieg, J., Bestmann, S., Weiskopf, N., Strasser, B., Moser, E. and Laistler, E. (2015), A novel coil array for combined TMS/fMRI experiments at 3 T. Magn. Reson. Med., 74: 1492-1501.
4.Turner R. 1986. A target field approach to optimal coil design. J Phys D: Appl Phys 8: L147–L151.
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