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Lê TP, Gruetter R, Jorge J, Ipek Ö. Segmenting electroencephalography wires reduces radiofrequency shielding artifacts in simultaneous electroencephalography and functional magnetic resonance imaging at 7 T. Magn Reson Med 2022; 88:1450-1464. [PMID: 35575944 PMCID: PMC9323442 DOI: 10.1002/mrm.29298] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2021] [Revised: 04/16/2022] [Accepted: 04/18/2022] [Indexed: 12/05/2022]
Abstract
Purpose Simultaneous scalp electroencephalography and functional magnetic resonance imaging (EEG‐fMRI) enable noninvasive assessment of brain function with high spatial and temporal resolution. However, at ultra‐high field, the data quality of both modalities is degraded by mutual interactions. Here, we thoroughly investigated the radiofrequency (RF) shielding artifact of a state‐of‐the‐art EEG‐fMRI setup, at 7 T, and design a practical solution to limit this issue. Methods Electromagnetic field simulations and MR measurements assessed the shielding effect of the EEG setup, more specifically the EEG wiring. The effectiveness of segmenting the wiring with resistors to reduce the transmit field disruption was evaluated on a wire‐only EEG model and a simulation model of the EEG cap. Results The EEG wiring was found to exert a dominant effect on the disruption of the transmit field, whose intensity varied periodically as a function of the wire length. Breaking the electrical continuity of the EEG wires into segments shorter than one quarter RF wavelength in air (25 cm at 7 T) reduced significantly the RF shielding artifacts. Simulations of the EEG cap with segmented wires indicated similar improvements for a moderate increase of the power deposition. Conclusion We demonstrated that segmenting the EEG wiring into shorter lengths using commercially available nonmagnetic resistors is effective at reducing RF shielding artifacts in simultaneous EEG‐fMRI. This prevents the formation of RF‐induced standing waves, without substantial specific absorption rate (SAR) penalties, and thereby enables benefiting from the functional sensitivity boosts achievable at ultra‐high field.
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Affiliation(s)
- Thanh Phong Lê
- Laboratory of Functional and Metabolic Imaging, École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland.,Geneva School of Health Sciences, HES-SO University of Applied Sciences and Arts Western Switzerland, Geneva, Switzerland
| | - Rolf Gruetter
- Laboratory of Functional and Metabolic Imaging, École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
| | - João Jorge
- Laboratory of Functional and Metabolic Imaging, École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland.,CSEM - Swiss Center for Electronics and Microtechnology, Neuchâtel, Switzerland
| | - Özlem Ipek
- CIBM Center for Biomedical Imaging - Animal Imaging and Technology, École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland.,School of Biomedical Engineering and Imaging Sciences, King's College London, London, UK
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Safety and data quality of EEG recorded simultaneously with multi-band fMRI. PLoS One 2021; 16:e0238485. [PMID: 34214093 PMCID: PMC8253410 DOI: 10.1371/journal.pone.0238485] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2020] [Accepted: 05/04/2021] [Indexed: 11/19/2022] Open
Abstract
PURPOSE Simultaneously recorded electroencephalography and functional magnetic resonance imaging (EEG-fMRI) is highly informative yet technically challenging. Until recently, there has been little information about EEG data quality and safety when used with newer multi-band (MB) fMRI sequences. Here, we measure the relative heating of a MB protocol compared with a standard single-band (SB) protocol considered to be safe. We also evaluated EEG quality recorded concurrently with the MB protocol on humans. MATERIALS AND METHODS We compared radiofrequency (RF)-related heating at multiple electrodes and magnetic field magnitude, B1+RMS, of a MB fMRI sequence with whole-brain coverage (TR = 440 ms, MB factor = 4) against a previously recommended, safe SB sequence using a phantom outfitted with a 64-channel EEG cap. Next, 9 human subjects underwent eyes-closed resting state EEG-fMRI using the MB sequence. Additionally, in three of the subjects resting state EEG was recorded also during the SB sequence and in an fMRI-free condition to directly compare EEG data quality across scanning conditions. EEG data quality was assessed by the ability to remove gradient and cardioballistic artifacts along with a clean spectrogram. RESULTS The heating induced by the MB sequence was lower than that of the SB sequence by a factor of 0.73 ± 0.38. This is consistent with an expected heating ratio of 0.64, calculated from the square of the ratio of B1+RMS values of the sequences. In the resting state EEG data, gradient and cardioballistic artifacts were successfully removed using traditional template subtraction. All subjects showed an individual alpha peak in the spectrogram with a posterior topography characteristic of eyes-closed EEG. The success of artifact rejection for the MB sequence was comparable to that in traditional SB sequences. CONCLUSIONS Our study shows that B1+RMS is a useful indication of the relative heating of fMRI protocols. This observation indicates that simultaneous EEG-fMRI recordings using this MB sequence can be safe in terms of RF-related heating, and that EEG data recorded using this sequence is of acceptable quality after traditional artifact removal techniques.
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Gregersen F, Göksu C, Schaefers G, Xue R, Thielscher A, Hanson LG. Safety evaluation of a new setup for transcranial electric stimulation during magnetic resonance imaging. Brain Stimul 2021; 14:488-497. [PMID: 33706007 DOI: 10.1016/j.brs.2021.02.019] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2020] [Revised: 01/07/2021] [Accepted: 02/26/2021] [Indexed: 01/22/2023] Open
Abstract
BACKGROUND Transcranial electric stimulation during MR imaging can introduce safety issues due to coupling of the RF field with the stimulation electrodes and leads. OBJECTIVE To optimize the stimulation setup for MR current density imaging (MRCDI) and increase maximum stimulation current, a new low-conductivity (σ = 29.4 S/m) lead wire is designed and tested. METHOD The antenna effect was simulated to investigate the effect of lead conductivity. Subsequently, specific absorption rate (SAR) simulations for realistic lead configurations with low-conductivity leads and two electrode types were performed at 128 MHz and 298 MHz being the Larmor frequencies of protons at 3T and 7T. Temperature measurements were performed during MRI using high power deposition sequences to ensure that the electrodes comply with MRI temperature regulations. RESULTS The antenna effect was found for copper leads at ¼ RF wavelength and could be reliably eliminated using low-conductivity leads. Realistic lead configurations increased the head SAR and the local head SAR at the electrodes only minimally. The highest temperatures were measured on the rings of center-surround electrodes, while circular electrodes showed little heating. No temperature increase above the safety limit of 39 °C was observed. CONCLUSION Coupling to the RF field can be reliably prevented by low-conductivity leads, enabling cable paths optimal for MRCDI. Compared to commercial copper leads with safety resistors, the low-conductivity leads had lower total impedance, enabling the application of higher currents without changing stimulator design. Attention must be paid to electrode pads.
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Affiliation(s)
- Fróði Gregersen
- Section for Magnetic Resonance, DTU Health Tech, Technical University of Denmark, Kgs Lyngby, Denmark; Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Hvidovre, Denmark; Sino-Danish Center for Education and Research, Aarhus, Denmark; University of Chinese Academic of Sciences, Beijing, 100049, China
| | - Cihan Göksu
- Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Hvidovre, Denmark; High-Field Magnetic Resonance Center, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany
| | - Gregor Schaefers
- MRI-STaR-Magnetic Resonance Institute for Safety, Technology and Research GmbH, Gelsenkirchen, Germany; MR:comp GmbH, MR Safety Testing Laboratory, Gelsenkirchen, Germany
| | - Rong Xue
- State Key Laboratory of Brain and Cognitive Science, Beijing MRI Center for Brain Research, Institute of Biophysics, Chinese Academy of Sciences, Beijing, 100101, China; University of Chinese Academic of Sciences, Beijing, 100049, China; Beijing Institute for Brain Disorders, Beijing, 100053, China
| | - Axel Thielscher
- Section for Magnetic Resonance, DTU Health Tech, Technical University of Denmark, Kgs Lyngby, Denmark; Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Hvidovre, Denmark
| | - Lars G Hanson
- Section for Magnetic Resonance, DTU Health Tech, Technical University of Denmark, Kgs Lyngby, Denmark; Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Hvidovre, Denmark.
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Modeling radio-frequency energy-induced heating due to the presence of transcranial electric stimulation setup at 3T. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2020; 33:793-807. [PMID: 32462558 PMCID: PMC7669803 DOI: 10.1007/s10334-020-00853-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/08/2020] [Revised: 04/22/2020] [Accepted: 05/12/2020] [Indexed: 11/02/2022]
Abstract
PURPOSE The purpose of the present study was to develop a numerical workflow for simulating temperature increase in a high-resolution human head and torso model positioned in a whole-body magnetic resonance imaging (MRI) radio-frequency (RF) coil in the presence of a transcranial electric stimulation (tES) setup. METHODS A customized human head and torso model was developed from medical image data. Power deposition and temperature rise (ΔT) were evaluated with the model positioned in a whole-body birdcage RF coil in the presence of a tES setup. Multiphysics modeling at 3T (123.2 MHz) on unstructured meshes was based on RF circuit, 3D electromagnetic, and thermal co-simulations. ΔT was obtained for (1) a set of electrical and thermal properties assigned to the scalp region, (2) a set of electrical properties of the gel used to ensure proper electrical contact between the tES electrodes and the scalp, (3) a set of electrical conductivity values of skin tissue, (4) four gel patch shapes, and (5) three electrode shapes. RESULTS Significant dependence of power deposition and ΔT on the skin's electrical properties and electrode and gel patch geometries was observed. Differences in maximum ΔT (> 100%) and its location were observed when comparing the results from a model using realistic human tissue properties and one with an external container made of acrylic material. The electrical and thermal properties of the phantom container material also significantly (> 250%) impacted the ΔT results. CONCLUSION Simulation results predicted that the electrode and gel geometries, skin electrical conductivity, and position of the temperature sensors have a significant impact on the estimated temperature rise. Therefore, these factors must be considered for reliable assessment of ΔT in subjects undergoing an MRI examination in the presence of a tES setup.
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Hawsawi HB, Papadaki A, Thornton JS, Carmichael DW, Lemieux L. Temperature Measurements in the Vicinity of Human Intracranial EEG Electrodes Exposed to Body-Coil RF for MRI at 1.5T. Front Neurosci 2020; 14:429. [PMID: 32477052 PMCID: PMC7235361 DOI: 10.3389/fnins.2020.00429] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2019] [Accepted: 04/07/2020] [Indexed: 11/13/2022] Open
Abstract
The application of intracranial electroencephalography (icEEG) recording during functional magnetic resonance imaging (icEEG-fMRI) has allowed the study of the hemodynamic correlates of epileptic activity and of the neurophysiological basis of the blood oxygen level-dependent (BOLD) signal. However, the applicability of this technique is affected by data quality issues such as signal drop out in the vicinity of the implanted electrodes. In our center we have limited the technique to a quadrature head transmit and receive RF coil following the results of a safety evaluation. The purpose of this study is to gather further safety-related evidence for performing icEEG-fMRI using a body RF-transmit coil, to allow the greater flexibility afforded by the use of modern, high-density receive arrays, and therefore parallel imaging with benefits such as reduced signal drop-out and distortion artifact. Specifically, we performed a set of empirical temperature measurements on a 1.5T Siemens Avanto MRI scanner with the body RF-transmit coil in a range of electrode and connector cable configurations. The observed RF-induced heating during a high-SAR sequence was maximum in the immediate vicinity of a depth electrode located along the scanner's central axis (range: 0.2-2.4°C) and below 0.5°C at the other electrodes. Also for the high-SAR sequence, we observed excessive RF-related heating in connection cable configurations that deviate from our recommended setup. For the low-SAR sequence, the maximum observed temperature increase across all configurations was 0.3°C. This provides good evidence to allow simultaneous icEEG-fMRI to be performed utilizing the body transmit coil on the 1.5T Siemens Avanto MRI scanner at our center with acceptable additional risk by following a well-defined protocol.
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Affiliation(s)
- Hassan B. Hawsawi
- Department of Clinical and Experimental Epilepsy, UCL Queen Square Institute of Neurology, London, United Kingdom
- MRI Unit, Epilepsy Society, Buckinghamshire, United Kingdom
- Administartion of Medical Physics, King Abdullah Medical City, Makkah, Saudi Arabia
| | - Anastasia Papadaki
- Lysholm Department of Neuroradiology, National Hospital for Neurology and Neurosurgery, UCLH NHS Foundation Trust, Queen Square, London, United Kingdom
- Department of Brain Repair and Rehabilitation, UCL Queen Square Institute of Neurology, London, United Kingdom
| | - John S. Thornton
- Lysholm Department of Neuroradiology, National Hospital for Neurology and Neurosurgery, UCLH NHS Foundation Trust, Queen Square, London, United Kingdom
- Department of Brain Repair and Rehabilitation, UCL Queen Square Institute of Neurology, London, United Kingdom
| | - David W. Carmichael
- Developmental Imaging and Biophysics Section, UCL Great Ormond Street Institute of Child Health, London, United Kingdom
- Wellcome EPSRC Centre for Medical Engineering, King’s College London, St Thomas’ Hospital, London, United Kingdom
| | - Louis Lemieux
- Department of Clinical and Experimental Epilepsy, UCL Queen Square Institute of Neurology, London, United Kingdom
- MRI Unit, Epilepsy Society, Buckinghamshire, United Kingdom
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Panych LP, Madore B. The physics of MRI safety. J Magn Reson Imaging 2017; 47:28-43. [DOI: 10.1002/jmri.25761] [Citation(s) in RCA: 82] [Impact Index Per Article: 11.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2017] [Accepted: 04/24/2017] [Indexed: 01/25/2023] Open
Affiliation(s)
- Lawrence P. Panych
- Department of Radiology; Brigham and Women's Hospital; Boston Massachusetts USA
- Harvard Medical School; Boston Massachusetts USA
| | - Bruno Madore
- Department of Radiology; Brigham and Women's Hospital; Boston Massachusetts USA
- Harvard Medical School; Boston Massachusetts USA
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