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Payne K, Bhosale AA, Zhang X. Double cross magnetic wall decoupling for quadrature transceiver RF array coils using common-mode differential-mode resonators. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2023; 353:107498. [PMID: 37295282 PMCID: PMC10527004 DOI: 10.1016/j.jmr.2023.107498] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/17/2023] [Revised: 05/09/2023] [Accepted: 05/31/2023] [Indexed: 06/12/2023]
Abstract
In contrast to linearly polarized RF coil arrays, quadrature transceiver coil arrays are capable of improving signal-to-noise ratio (SNR), spatial resolution, and parallel imaging performance. Owing to a reduced excitation power, a low specific absorption rate can also be obtained using quadrature RF coils. However, due to the complex nature of their structure and their electromagnetic properties, it is challenging to achieve sufficient electromagnetic decoupling while designing multichannel quadrature RF coil arrays, particularly in ultra-high fields. In this work, we proposed a double-cross magnetic wall decoupling for quadrature transceiver RF arrays and implemented the decoupling method on common-mode differential mode quadrature (CMDM) quadrature transceiver arrays at an ultrahigh field of 7 T. The proposed magnetic decoupling wall, comprised of two intrinsically decoupled loops, is used to reduce the mutual coupling between all the multi-mode currents present in the quadrature CMDM array. The decoupling network has no physical connection with the CMDMs' resonators, which provides less design constraint over size-adjustable RF arrays. To validate the feasibility of the proposed cross-magnetic decoupling wall, systematic studies on the decoupling performance based on the impedance of two intrinsic loops are numerically performed. A pair of quadrature transceiver CMDMs is constructed along with the proposed decoupling network, and their scattering matrix is characterized using a network analyzer. The measured results indicate that all the current modes from coupling are simultaneously suppressed using the proposed cross-magnetic wall. Moreover, field distribution and local specific absorption rate (SAR) are numerically obtained for a well-decoupled 8-channel quadrature knee-coil array.
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Affiliation(s)
- Komlan Payne
- Department of Biomedical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA.
| | - Aditya Ashok Bhosale
- Department of Biomedical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA.
| | - Xiaoliang Zhang
- Department of Biomedical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA; Department of Electrical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA.
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Payne K, Ying LL, Zhang X. Hairpin RF resonators for MR imaging transceiver arrays with high inter-channel isolation and B 1 efficiency at ultrahigh field 7 T. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2022; 345:107321. [PMID: 36335877 DOI: 10.1016/j.jmr.2022.107321] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/06/2022] [Revised: 10/17/2022] [Accepted: 10/22/2022] [Indexed: 06/03/2023]
Abstract
Electromagnetic decoupling among a close-fitting or high-density transceiver RF array elements is required to maintain the integrity of the magnetic flux density from individual channel for enhanced performance in detection sensitivity and parallel imaging. High-impedance RF coils have demonstrated to be a prominent design method to circumvent these coupling issues. Yet, inherent characteristics of these coils have ramification on the B1 field efficiency and SNR. In this work, we propose a hairpin high impedance RF resonator design for highly decoupled multichannel transceiver arrays at ultrahigh magnetic fields. Due to the high impedance property of the hairpin resonators, the proposed transceiver array can provide high decoupling performance without using any dedicated decoupling circuit among the resonant elements. Because of elimination of lumped inductors in the resonator circuit, higher B1 field efficiency in imaging subjects can be expected. In order to validate the feasibility of the proposed hairpin RF coils, systematical studies on decoupling performance, field distribution, and SNR are performed, and the results are compared with those obtained from existing high-impedance RF coil, e.g., "self-decoupled RF coil". To further investigate its performance, an 8-channel head coil array using the proposed hairpin resonators loaded with a cylindrical phantom is designed, demonstrating a 19 % increase of the B1+ field intensity compared to the self-decoupled coils at 7 T. Furthermore, the characteristics of the hairpin RF coils are evaluated using a more realistic human head voxel model numerically. The proposed hairpin RF coil provides excellent decoupling performance and superior RF magnetic field efficiency compared to the "self-decoupled" high impedance coils. Bench test of a pair of fabricated hairpin coils prove to be in good accordance with numerical results.
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Affiliation(s)
- Komlan Payne
- Department of Biomedical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA.
| | - Leslie L Ying
- Department of Biomedical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA; Department of Electrical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA.
| | - Xiaoliang Zhang
- Department of Biomedical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA; Department of Electrical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA.
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Hernandez D, Nam T, Jeong Y, Kim D, Kim KN. Study on the Effect of Non-Symmetrical Current Distribution Controlled by Capacitor Placement in Radio-Frequency Coils for 7T MRI. BIOSENSORS 2022; 12:867. [PMID: 36291004 PMCID: PMC9599509 DOI: 10.3390/bios12100867] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/14/2022] [Revised: 10/06/2022] [Accepted: 10/09/2022] [Indexed: 06/16/2023]
Abstract
In this paper, we present a study on the effects of varying the position of a single tuning capacitor in a circular loop coil as a mechanism to control and produce non-symmetric current distribution, such that could be used for magnetic resonance imaging (MRI) operating at ultra-high frequency (UHF). This study aims to demonstrate that the position of the tuning capacitor of a circular loop could improve the coupling between adjacent coils, used to optimize transmission field uniformity or intensity, improve signal-to-noise ratio (SNR) or specific absorption rate (SAR). A typical loop coil used in MRI consists of symmetrically distributed capacitors along the coil; this design is able to produce uniform current distributions inside the coil. However, in UHF conditions, the magnetic flux density (|B1+|) field produced by this setup may exhibit field distortion, requiring a method of controlling the field distribution and improving the field intensity of the circular loop coil. The control mechanism investigated in this study is based on the position of the tuning capacitor in the circular coil, the capacitor position was varied from 15° to 345°, in steps of 15°. We performed electromagnetic (EM) simulations, fabricated the coils, and performed MRI experiments at 7T, with each of the coils with capacitor position from 15° to 345° to determine the effects on field intensity, coupling between adjacent coils, SAR, and applications for field uniformity optimization. For the case of free space, a coil with capacitor position at 15° showed higher field intensity compared to the reference coil; while an improved decoupling was achieved when a coil had the capacitor placed at 180° and the other coil at 90°; in a similar matter, we discuss the results for SAR, field uniformity and an application with an array coil for the spinal cord.
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Affiliation(s)
- Daniel Hernandez
- Neuroscience Research Institute, Gachon University, Incheon 21988, Korea
| | - Taewoo Nam
- Department of Health Sciences and Technology, GAIHST, Gachon University, Incheon 21999, Korea
| | - Yonghwa Jeong
- Department of Health Sciences and Technology, GAIHST, Gachon University, Incheon 21999, Korea
| | - Donghyuk Kim
- Neuroscience Research Institute, Gachon University, Incheon 21988, Korea
| | - Kyoung-Nam Kim
- Department of Health Sciences and Technology, GAIHST, Gachon University, Incheon 21999, Korea
- Department of Biomedical Engineering, Gachon University, Seongnam 13120, Korea
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Choi CH, Hong SM, Felder J, Tellmann L, Scheins J, Kops ER, Lerche C, Shah NJ. A Novel J-Shape Antenna Array for Simultaneous MR-PET or MR-SPECT Imaging. IEEE TRANSACTIONS ON MEDICAL IMAGING 2022; 41:1104-1113. [PMID: 34860648 DOI: 10.1109/tmi.2021.3132576] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Simultaneous MR-PET/-SPECT is an emerging technology that capitalises on the invaluable advantages of both modalities, allowing access to numerous sensitive tracers and superior soft-tissue contrast alongside versatile functional imaging capabilities. However, to optimise these capabilities, concurrent acquisitions require the MRI antenna located inside the PET/SPECT field-of-view to be operated without compromising any aspects of system performance or image quality compared to the stand-alone instrumentation. Here, we report a novel gamma-radiation-transparent antenna concept. The end-fed J-shape antenna is particularly adept for hybrid ultra-high field MR-PET/-SPECT applications as it enables all highly attenuating materials to be placed outside the imaging field-of-view. Furthermore, this unique configuration also provides advantages in stand-alone MR applications by reducing the amount of coupling between the cables and the antenna elements, and by lowering the potential specific absorption rate burden. The use of this new design was experimentally verified according to the important features for both ultra-high field MRI and the 511 keV transmission scan. The reconstructed attenuation maps evidently showed much lower attenuation ( ∼ 15 %) for the proposed array when compared to the conventional dipole antenna array since there were no high-density components. In MR, it was observed that the signal-to-noise ratio from the whole volume obtained using the proposed array was comparable to that acquired by the conventional array which was also in agreement with the simulation results. The unique feature, J-shape array, would enable simultaneous MR-PET/-SPECT experiments to be conducted without unduly compromising any aspects of system performance and image quality compared to the stand-alone instrumentation.
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Brizi D, Monorchio A. Magnetic metasurfaces properties in the near field regions. Sci Rep 2022; 12:3258. [PMID: 35228640 PMCID: PMC8885705 DOI: 10.1038/s41598-022-07378-y] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2021] [Accepted: 02/16/2022] [Indexed: 11/09/2022] Open
Abstract
In this paper, we present a general equivalent-circuit interpretation of finite magnetic metasurfaces interacting with an arbitrary arrangement of RF coils operating in near-field regime. The developed model allows to derive a physical interpretation of the interactions between the metasurface and the surrounding RF coils, both transmitting and receiving. Indeed, especially for near-field applications, the metasurface presence modifies the behavior of each RF coil differently, due to the specific reciprocal interactions. Hence, the proposed approach introduces a source-related complex magnetic permeability matrix, overcoming the traditional bulk definition. To prove the model validity against full-wave simulations, we present two significant test cases, commonly used in practical applications. The former is represented by the simple metasurface-coil arrangement from which important and fundamental considerations can be drawn. The latter system is composed by a transmitting and a receiving coil with a metasurface in between; detailed explanations on the metasurface interactions with both the RF coils are developed. Finally, we also achieved an excellent agreement between the numerical results and the measurements obtained through fabricated prototypes. In summary, the circuit interpretation herein presented, in addition to the rigorous electromagnetic theoretical approaches already appeared in the open literature, reveals useful in providing quantitative, practical, and easy-to-handle guidelines for the design and physical understanding of finite magnetic metasurfaces interacting with arbitrary RF coils arrangements in the near-field regime.
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Wang X, Guo H, Zhou C, Bai J. High-resolution probe design for measuring the dielectric properties of human tissues. Biomed Eng Online 2021; 20:86. [PMID: 34454484 PMCID: PMC8403451 DOI: 10.1186/s12938-021-00924-1] [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: 01/29/2021] [Accepted: 08/18/2021] [Indexed: 11/21/2022] Open
Abstract
Background In order to use the microwave to measure the dielectric constant of the human body and improve the measurement resolution, a small near-field probe working at 915 MHz is designed in this paper. Method Based on the electric small loop antenna model loaded by the spiral resonator (SR), a small near-field probe was designed. The probe model is designed and optimized by the HFSS (high frequency structure simulator) software. The human tissues were tested by the manufactured probe and the relationship between the S11 parameters of the probe and the human tissues was analyzed. Results and conclusions A probe with small size was designed and fabricated, with the overall size of 10.0 mm × 12.0 mm × 0.8 mm. The probe has a good performance with a 30.7 dB return loss, a 20 MHz bandwidth at the resonance point, and a distance resolution of 10 mm. Due to the small size and good resolution of the probe, it can be used in the measurement of human tissues.
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Affiliation(s)
- Xinran Wang
- School of Physics and Optoelectronic Engineering, Xidian University, Xi'an, China
| | - Hongfu Guo
- School of Physics and Optoelectronic Engineering, Xidian University, Xi'an, China.
| | - Chen Zhou
- School of Physics and Optoelectronic Engineering, Xidian University, Xi'an, China
| | - Junkai Bai
- School of Physics and Optoelectronic Engineering, Xidian University, Xi'an, China
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Kazemivalipour E, Sadeghi-Tarakameh A, Atalar E. Eigenmode analysis of the scattering matrix for the design of MRI transmit array coils. Magn Reson Med 2020; 85:1727-1741. [PMID: 33034125 DOI: 10.1002/mrm.28533] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2020] [Revised: 09/03/2020] [Accepted: 09/04/2020] [Indexed: 02/02/2023]
Abstract
PURPOSE To obtain efficient operation modes of transmit array (TxArray) coils using a general design technique based on the eigenmode analysis of the scattering matrix. METHODS We introduce the concept of modal reflected power and excitation eigenmodes, which are calculated as the eigenvalues and eigenvectors of SH S, where the superscript H denotes the Hermitian transpose. We formulate the normalized reflected power, which is the ratio of the total reflected power to the total incident power of TxArray coils for a given excitation signal as the weighted sum of the modal reflected power. By minimizing the modal reflected power of TxArray coils, we increase the excitation space with a low total reflection. The algorithm was tested on 4 dual-row TxArray coils with 8 to 32 channels. RESULTS By minimizing the modal reflected power, we designed an 8-element TxArray coil to have a low reflection for 7 out of 8 dimensions of the excitation space. Similarly, the minimization of the modal reflected power of a 16-element TxArray coil enabled us to enlarge the dimension of the excitation space by 50% compared with commonly employed design techniques. Moreover, we demonstrated that the low total reflected power for some critical excitation modes, such as the circularly polarized mode, can be achieved for all TxArray coils even with a high level of coupling. CONCLUSION Eigenmode analysis is an efficient method that intuitively provides a quantitative and compact representation of the coil's power transmission capabilities. This method also provides insight into the excitation modes with low reflection.
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Affiliation(s)
- Ehsan Kazemivalipour
- Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey.,National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
| | - Alireza Sadeghi-Tarakameh
- Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey.,National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
| | - Ergin Atalar
- Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey.,National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey
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Brizi D, Fontana N, Costa F, Tiberi G, Galante A, Alecci M, Monorchio A. Design of Distributed Spiral Resonators for the Decoupling of MRI Double-Tuned RF Coils. IEEE Trans Biomed Eng 2020; 67:2806-2816. [PMID: 32031927 DOI: 10.1109/tbme.2020.2971843] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
OBJECTIVE A systematic analytical approach to design Spiral Resonators (SRs), acting as distributed magnetic traps (DMTs), for the decoupling of concentric Double-Tuned (DT) RF coils suitable for Ultra-High Field (7 T) MRI is presented. METHODS The design is based on small planar SRs placed in between the two RF loops (used for signal detection of the two nuclei of interest). We developed a general framework based on a fully analytical approach to estimate the mutual coupling between the RF coils and to provide design guidelines for the geometry and number of SRs to be employed. Starting from the full-analytical estimations of the SRs geometry, electromagnetic simulations for improving and validating the performance can be carried out. RESULTS AND CONCLUSION We applied the method to a test case of a DT RF coil consisting of two concentric and coplanar loops used for 7 T MRI, tuned at the Larmor frequencies of the proton (1H, 298 MHz) and sodium (23Na, 79 MHz) nuclei, respectively. We performed numerical simulations and experimental measurements on fabricated prototypes, which both demonstrated the effectiveness of the proposed design procedure. SIGNIFICANCE The decoupling is achieved by printing the SRs on the same dielectric substrate of the RF coils thus allowing a drastic simplification of the fabrication procedure. It is worth noting that there are no physical connections between the decoupling SRs and the 1H/23Na RF coils, thus providing a mechanically robust experimental set-up, and improving the transceiver design with respect to other traditional decoupling techniques.
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Li Y, Lee J, Zhang L, Chen Q, Tie C, Luo C, Zhang X, Liang D, Liu X, Zheng H. Design and testing of a 24-channel head coil for MR imaging at 3 T. Magn Reson Imaging 2019; 58:162-173. [DOI: 10.1016/j.mri.2019.01.020] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2018] [Revised: 12/07/2018] [Accepted: 01/22/2019] [Indexed: 11/29/2022]
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10
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Cui J, Dimitrov IE, Cheshkov S, Gu M, Malloy CR, Wright SM. An Adjustable-Length Dipole Using Forced-Current Excitation for 7T MR. IEEE Trans Biomed Eng 2018; 65:2259-2266. [PMID: 29989961 DOI: 10.1109/tbme.2017.2788864] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Ultrahigh field imaging of the body and the spine is challenging due to the large field-of-view (FOV) required. It is especially difficult for RF transmission due to its requirement on both the length and the depth of the ${\rm{B}}_{1}^{{\rm + }}$ field. One solution is to use a long dipole to provide continuous current distribution. The drawback is the natural falloff of the ${\rm{B}}_{1}$ field toward the ends of the dipole, therefore the ${\rm{B}}_{1}^{{\rm + }}$ per unit square root of maximum specific absorption rate ${\rm{(B}}_{1}^{{\rm + }}{\rm{/ \surd SAR}}_{{\rm{max}}})$ performance is particularly poor toward the end of the dipole. In this study, a segmented element design using forced-current excitation and a switching circuit is presented. The design provides long FOV when desired and allows flexible FOV switching and power distribution without additional power amplifiers. Different element types and arrangements were explored and a segmented dipole design was chosen as the best design. The segmented dipole was implemented and tested on the bench and with a phantom on a 7T whole body scanner. The switchable mode dipole enabled a large FOV in the long mode and improved ${\rm{B}}_{1}^{{\rm + }}{\rm{/ \surd SAR}}_{{\rm{max}}}$ efficiency in a smaller FOV in the short mode.
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Georget E, Luong M, Vignaud A, Giacomini E, Chazel E, Ferrand G, Amadon A, Mauconduit F, Enoch S, Tayeb G, Bonod N, Poupon C, Abdeddaim R. Stacked magnetic resonators for MRI RF coils decoupling. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2017; 275:11-18. [PMID: 27951426 DOI: 10.1016/j.jmr.2016.11.012] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/04/2016] [Revised: 11/01/2016] [Accepted: 11/20/2016] [Indexed: 06/06/2023]
Abstract
Parallel transmission is a very promising method to tackle B1+ field inhomogeneities at ultrahigh field in magnetic resonant imaging (MRI). This technique is however limited by the mutual coupling between the radiating elements. Here we propose to solve this problem by designing a passive magneto-electric resonator that we here refer to as stacked magnetic resonator (SMR). By combining numerical and experimental methodologies, we prove that this novelty passive solution allows an efficient decoupling of elements of a phased-array coil. We demonstrate the ability of this technique to significantly reduce by more than 10dB the coupling preserving the quality of images compared to ideally isolated linear resonators on a spherical salty agar gel phantom in a 7T MRI scanner.
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Affiliation(s)
- Elodie Georget
- Université Paris-Saclay, CEA-Saclay, DRF/I2BM/Neurospin/UNIRS, 91191 Gif-sur-Yvette Cedex, France.
| | - Michel Luong
- Université Paris-Saclay, CEA-Saclay, DRF/IRFU/SACM, 91191 Gif-sur-Yvette Cedex, France.
| | - Alexandre Vignaud
- Université Paris-Saclay, CEA-Saclay, DRF/I2BM/Neurospin/UNIRS, 91191 Gif-sur-Yvette Cedex, France.
| | - Eric Giacomini
- Université Paris-Saclay, CEA-Saclay, DRF/I2BM/Neurospin/UNIRS, 91191 Gif-sur-Yvette Cedex, France.
| | - Edouard Chazel
- Université Paris-Saclay, CEA-Saclay, DRF/I2BM/Neurospin/UNIRS, 91191 Gif-sur-Yvette Cedex, France.
| | - Guillaume Ferrand
- Université Paris-Saclay, CEA-Saclay, DRF/IRFU/SACM, 91191 Gif-sur-Yvette Cedex, France.
| | - Alexis Amadon
- Université Paris-Saclay, CEA-Saclay, DRF/I2BM/Neurospin/UNIRS, 91191 Gif-sur-Yvette Cedex, France.
| | | | - Stefan Enoch
- Aix Marseille Université, CNRS, Centrale Marseille, Institut Fresnel, UMR 7249, 13013 Marseille, France.
| | - Gérard Tayeb
- Aix Marseille Université, CNRS, Centrale Marseille, Institut Fresnel, UMR 7249, 13013 Marseille, France.
| | - Nicolas Bonod
- Aix Marseille Université, CNRS, Centrale Marseille, Institut Fresnel, UMR 7249, 13013 Marseille, France.
| | - Cyril Poupon
- Université Paris-Saclay, CEA-Saclay, DRF/I2BM/Neurospin/UNIRS, 91191 Gif-sur-Yvette Cedex, France.
| | - Redha Abdeddaim
- Aix Marseille Université, CNRS, Centrale Marseille, Institut Fresnel, UMR 7249, 13013 Marseille, France.
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Connell IRO, Menon RS. General Coupling Matrix Synthesis for Decoupling MRI RF Arrays. IEEE TRANSACTIONS ON MEDICAL IMAGING 2016; 35:2229-2242. [PMID: 27093549 DOI: 10.1109/tmi.2016.2553844] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Multi-channel radio-frequency (RF) arrays, composed of multiple resonant coils, provide significant benefits for MRI during both signal reception (receive) and excitation (transmit). Demonstration of increased signal-to-noise ratio (SNR) and acceleration factors during parallel acquisitions has lead to the development of receive arrays. Conversely, transmit arrays have demonstrated considerable potential for mitigating excitation inhomogeneity arising at ultra-high magnetic field strengths ( ≥ 7 T) , present due to wave-like interactions inside the sample. Due to geometric constraints, the design of both receive and transmit arrays requires the resonating coils to be closely spaced. Significant overlap in the near-field distributions from each coil results in coupling. Without an adequate decoupling strategy applied between individual elements in an RF array, the MRI performance of the array can be significantly degraded. This work presents a method to design decoupling networks for arbitrarily large RF arrays based on direct synthesis of a coupling matrix. Reflection coefficients are fitted to transfer polynomials with transmission coefficients simultaneously minimized through a nonlinear optimization. The method demonstrates the design of nth-order distributed filters and lumped element networks that compensate for all first-order and cross-coupling terms arising in an RF array suitable for MRI. The synthesis results are computed for 4-, 8-, and 32-channel RF arrays. Monte Carlo analyses and experimental results for two RF array constructions demonstrate the robustness of this approach.
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13
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Hurshkainen AA, Derzhavskaya TA, Glybovski SB, Voogt IJ, Melchakova IV, van den Berg CAT, Raaijmakers AJE. Element decoupling of 7T dipole body arrays by EBG metasurface structures: Experimental verification. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2016; 269:87-96. [PMID: 27262656 DOI: 10.1016/j.jmr.2016.05.017] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/12/2015] [Revised: 05/07/2016] [Accepted: 05/25/2016] [Indexed: 06/05/2023]
Abstract
Metasurfaces are artificial electromagnetic boundaries or interfaces usually implemented as two-dimensional periodic structures with subwavelength periodicity and engineered properties of constituent unit cells. The electromagnetic bandgap (EBG) effect in metasurfaces prevents all surface modes from propagating in a certain frequency band. While metasurfaces provide a number of important applications in microwave antennas and antenna arrays, their features are also highly suitable for MRI applications. In this work we perform a proof-of-principle experiment to study finite structures based on mushroom-type EBG metasurfaces and employ them for suppression of inter-element coupling in dipole transceive array coils for body imaging at 7T. We firstly show experimentally that employment of mushroom structures leads to reduction of coupling between adjacent closely-spaced dipole antenna elements of a 7T transceive body array, which reduces scattering losses in neighboring channels. The studied setup consists of two active fractionated dipole antennas previously designed by the authors for body imaging at 7T. These are placed on top of a body-mimicking phantom and equipped with the manufactured finite-size periodic structure tuned to have EBG properties at the Larmor frequency of 298MHz. To improve the detection range of the B1+ field distribution of the top elements, four additional elements were positioned along the bottom side of the phantom. Bench measurements of a scattering matrix showed that coupling between the two top elements can be considerably reduced depending on the distance to the EBG structure. On the other hand, the measurements performed on a 7T MRI machine indicated redistribution of the B1+ field due to interaction between the dipoles with the structure. When the structure is located just over two closely spaced dipoles, one can reach a very high isolation improvement of -14dB accompanied by a strong field redistribution. In contrast, when put at a certain height over the antennas the structure provides a moderate isolation improvement together with a slight increase of B1+ level. This study provides a tool for the decoupling of dipole antennas in ultrahigh field transceive arrays, possibly resulting in denser element placement and/or larger subject-element spacing.
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Affiliation(s)
- Anna A Hurshkainen
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
| | - Tatyana A Derzhavskaya
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
| | - Stanislav B Glybovski
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
| | - Ingmar J Voogt
- University Medical Center Utrecht, 3584 CX, Netherlands.
| | - Irina V Melchakova
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
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Yan X, Zhang X, Xue R, Gore JC, Grissom WA. Optimizing the ICE decoupling element distance to improve monopole antenna arrays for 7 Tesla MRI. Magn Reson Imaging 2016; 34:1264-1268. [PMID: 27469314 DOI: 10.1016/j.mri.2016.07.008] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2016] [Revised: 07/19/2016] [Accepted: 07/21/2016] [Indexed: 11/15/2022]
Abstract
The induced current elimination (ICE) method has been previously applied to decouple monopole coil arrays in ultrahigh field MRI. However, the method creates low B1+ spots near the decoupling elements. In this study, we aim to improve the performance of ICE-decoupled monopole array in human head imaging at 7 Tesla. Eight-channel ICE-decoupled monopole arrays were optimized by varying the position of the decoupling elements. A series of numerical studies were performed using the co-simulation method. In simulation, decoupling performance, quality (Q-) values and transmit field (B1+) were comparatively investigated. In addition, we constructed an optimized ICE-decoupled monopole array and compared its performance with the unoptimized array. The simulation results showed that a good trade-off between decoupling and B1+ loss can be obtained when decoupling elements were moved 2.5-cm away from coil elements. This was validated by in-vivo MR imaging using the constructed array. Compared with the unoptimized ICE decoupled monopole array, the optimized array had a more homogeneous transmit field and no dark spots or signal cancellations in the MR images.
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Affiliation(s)
- Xinqiang Yan
- Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA; Department of Radiology, Vanderbilt University, Nashville, TN, USA.
| | - Xiaoliang Zhang
- Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA; UCSF/UC Berkeley Joint Graduate Group in Bioengineering, San Francisco, CA, USA
| | - Rong Xue
- State Key Laboratory of Brain and Cognitive Science, Beijing MRI Center for Brain Research, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China
| | - John C Gore
- Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA; Department of Radiology, Vanderbilt University, Nashville, TN, USA
| | - William A Grissom
- Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA; Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, USA
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Yan X, Cao Z, Zhang X. Simulation verification of SNR and parallel imaging improvements by ICE-decoupled loop array in MRI. APPLIED MAGNETIC RESONANCE 2016; 47:395-403. [PMID: 27034578 PMCID: PMC4808813 DOI: 10.1007/s00723-016-0764-x] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/21/2015] [Revised: 12/08/2015] [Indexed: 06/03/2023]
Abstract
Transmit/receive L/C loop arrays with the induced current elimination (ICE) or magnetic wall decoupling method has shown high signal-to-noise (SNR) and excellent parallel imaging ability for MR imaging at ultrahigh fields, e.g., 7 T. In this study, we aim to numerically analyze the performance of an eight-channel ICE-decoupled loop array at 7 T. Three dimensional (3-D) electromagnetic (EM) and radiofrequency (RF) circuit co-simulation approach was employed. The values of all capacitors were obtained by optimizing the S-parameters of all coil elements. The EM simulation accurately modeled the coil structure, the phantom and the excitation. All coil elements were well matched to 50 ohm and the isolation between any two coil elements was better -15 dB. The simulated S parameters were exactly similar with the experimental results, indicating the simulation results were reliable. Compared with the conventional capacitively decoupled array, the ICE-decoupled array had higher sensitivity at the peripheral areas of the image subjects due to the shielding effect of the decoupling loops. The increased receive sensitivity resulted in an improvement of signal intensity and SNR for the ICE-decoupled array.
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16
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Connell IRO, Gilbert KM, Abou-Khousa MA, Menon RS. Design of a parallel transmit head coil at 7T with magnetic wall distributed filters. IEEE TRANSACTIONS ON MEDICAL IMAGING 2015; 34:836-845. [PMID: 25415982 DOI: 10.1109/tmi.2014.2370533] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Ultra-high field magnetic resonance imaging (MRI) scanners ( ≥ 7T) require radio-frequency (RF) coils to operate in the range of the electromagnetic spectrum where the effective wavelength in the tissue approaches the patient dimensions. Multi-channel transmit arrays, driven in parallel, have been developed to increase the transmit field (B1(+)) uniformity in this wavelength regime. However, the closely packed array elements interact through mutual coupling. This paper expands on the ability of a distributed planar filter (the "magnetic wall") to decouple individual elements in an entire array. A transmit RF coil suitable for neuroimaging at 7T was constructed. The transmit coil, composed of 10 individual surface coil elements, was decoupled with magnetic walls. A separate receive coil array was used for signal reception. The hardware and imaging performance of the transmit coil was validated with electromagnetic simulation, bench-top measurements, and in vivo MRI experiments. Analysis and measurements confirmed that the magnetic wall decoupling method provides high isolation between transmit channels, while minimally affecting the B1(+) field profiles. Electromagnetic simulations confirmed that the decoupling method did not correlate to local specific absorption rate (SAR) "hot spots" or increase local-to-global SAR fractions in comparison to previously reported 7T multi-channel transmit arrays employing different decoupling methods.
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