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      A 32-channel parallel transmit system add-on for 7T MRI

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          Abstract

          Purpose

          A 32-channel parallel transmit (pTx) add-on for 7 Tesla whole-body imaging is presented. First results are shown for phantom and in-vivo imaging.

          Methods

          The add-on system consists of a large number of hardware components, including modulators, amplifiers, SAR supervision, peripheral devices, a control computer, and an integrated 32-channel transmit/receive body array. B 1 + maps in a phantom as well as B 1 + maps and structural images in large volunteers are acquired to demonstrate the functionality of the system. EM simulations are used to ensure safe operation.

          Results

          Good agreement between simulation and experiment is shown. Phantom and in-vivo acquisitions show a field of view of up to 50 cm in z-direction. Selective excitation with 100 kHz sampling rate is possible. The add-on system does not affect the quality of the original single-channel system.

          Conclusion

          The presented 32-channel parallel transmit system shows promising performance for ultra-high field whole-body imaging.

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          Most cited references44

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          Transmit SENSE.

          The idea of using parallel imaging to shorten the acquisition time by the simultaneous use of multiple receive coils can be adapted for the parallel transmission of a spatially-selective multidimensional RF pulse. As in data acquisition, a multidimensional RF pulse follows a certain k-space trajectory. Shortening this trajectory shortens the pulse duration. The use of multiple transmit coils, each with its own time-dependent waveform and spatial sensitivity, can compensate for the missing parts of the excitation k-space. This results in a maintained spatial definition of the pulse profile, while its duration is reduced. This work introduces the concept of parallel transmission with arbitrarily shaped transmit coils (termed "Transmit SENSE"). Results of numerical studies demonstrate the theoretical feasibility of the approach. The experimental proof of principle is provided on a commercial MR scanner. The lack of multiple independent transmit channels was addressed by combining the excitation patterns from two separate subexperiments with different transmit setups. Shortening multidimensional RF pulses could be an interesting means of making 3D RF pulses feasible even for fast T(2)(*) relaxing species or strong main field inhomogeneities. Other applications might benefit from the ability of Transmit SENSE to improve the spatial resolution of the pulse profile while maintaining the transmit duration. Copyright 2003 Wiley-Liss, Inc.
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            Signal-to-noise ratio and MR tissue parameters in human brain imaging at 3, 7, and 9.4 tesla using current receive coil arrays.

            Relaxation times, transmit homogeneity, signal-to-noise ratio (SNR) and parallel imaging g-factor were determined in the human brain at 3T, 7T, and 9.4T, using standard, tight-fitting coil arrays.
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              B(1) destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil.

              RF behavior in the human head becomes complex at ultrahigh magnetic fields. A bright center and a weak periphery are observed in images obtained with volume coils, while surface coils provide strong signal in the periphery. Intensity patterns reported with volume coils are often loosely referred to as "dielectric resonances," while modeling studies ascribe them to superposition of traveling waves greatly dampened in lossy brain tissues, raising questions regarding the usage of this term. Here we address this question experimentally, taking full advantage of a transceiver coil array that was used in volume transmit mode, multiple receiver mode, or single transmit surface coil mode. We demonstrate with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B(1) in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B(1) proved remarkably similar for the different coil elements, although with opposite rotational direction. Additional simulation data closely matched our phantom results. In the human brain the phase patterns were more complex but still exhibited similarities between coil elements. Our results suggest that measuring spatial B(1) phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B(1) in user-defined areas of the brain.
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                Author and article information

                Contributors
                Role: ConceptualizationRole: Data curationRole: Formal analysisRole: InvestigationRole: MethodologyRole: Project administrationRole: SoftwareRole: ValidationRole: Writing – original draftRole: Writing – review & editing
                Role: ConceptualizationRole: Funding acquisitionRole: InvestigationRole: MethodologyRole: SupervisionRole: Validation
                Role: MethodologyRole: SoftwareRole: Validation
                Role: MethodologyRole: Software
                Role: InvestigationRole: Validation
                Role: MethodologyRole: Software
                Role: ConceptualizationRole: Formal analysisRole: InvestigationRole: MethodologyRole: SupervisionRole: Validation
                Role: ConceptualizationRole: Methodology
                Role: Formal analysisRole: MethodologyRole: Validation
                Role: Validation
                Role: ConceptualizationRole: Validation
                Role: Methodology
                Role: Data curationRole: Validation
                Role: Software
                Role: Conceptualization
                Role: Supervision
                Role: ConceptualizationRole: Funding acquisitionRole: MethodologyRole: Project administrationRole: SupervisionRole: Writing – review & editing
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                1932-6203
                12 September 2019
                2019
                : 14
                : 9
                : e0222452
                Affiliations
                [1 ] Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Essen, Germany
                [2 ] RF & Microwave Technology, University of Duisburg-Essen, Duisburg, Germany
                [3 ] High-Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany
                [4 ] Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
                [5 ] Electromagnetic Theory and Applied Mathematics, Faculty of Electrical Engineering and Information Technology, FH Aachen – University of Applied Sciences, Aachen, Germany
                [6 ] Faculty of Physics and Astronomy and Faculty of Medicine, University of Heidelberg, Heidelberg, Germany
                New York University School of Medicine, UNITED STATES
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Author information
                http://orcid.org/0000-0001-9784-4354
                http://orcid.org/0000-0001-5023-5105
                Article
                PONE-D-19-09705
                10.1371/journal.pone.0222452
                6742215
                31513637
                b24f470d-b112-485c-911f-2b9a8d687ac4
                © 2019 Orzada et al

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 5 April 2019
                : 29 August 2019
                Page count
                Figures: 10, Tables: 0, Pages: 20
                Funding
                Funded by: funder-id http://dx.doi.org/10.13039/100011199, FP7 Ideas: European Research Council;
                Award ID: 291903
                Award Recipient :
                The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 291903 MRexcite. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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                Diagnostic Radiology
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                Magnetic Resonance Imaging
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