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  Constructive Near-Field Interference Effect in a Birdcage MRI Coil with an Artificial Magnetic Shield

Lezhennikova, K., Abdeddaim, R., Hurshkainen, A., Vignaud, A., Dubois, M., Jomin, P., et al. (2020). Constructive Near-Field Interference Effect in a Birdcage MRI Coil with an Artificial Magnetic Shield. Physical Review Applied, 13(6): 064004, pp. 1-14. doi:10.1103/PhysRevApplied.13.064004.

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Item Permalink: http://hdl.handle.net/21.11116/0000-0006-966D-7 Version Permalink: http://hdl.handle.net/21.11116/0000-0006-966E-6
Genre: Journal Article

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 Creators:
Lezhennikova, K, Author
Abdeddaim, R, Author
Hurshkainen, A, Author
Vignaud, A, Author
Dubois, M, Author
Jomin, P, Author
Berrahou, D, Author
Raaijmakers, A, Author
Avdievich, N1, 2, Author              
Melchakova, I, Author
Enoch, S, Author
Belov, P, Author
Simovski, C, Author
Glybovski, S, Author
Affiliations:
1Department High-Field Magnetic Resonance, Max Planck Institute for Biological Cybernetics, Max Planck Society, ou_1497796              
2Max Planck Institute for Biological Cybernetics, Max Planck Society, Spemannstrasse 38, 72076 Tübingen, DE, ou_1497794              

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 Abstract: Radio-frequency (rf) coils are used in all clinical and research magnetic-resonance-imaging (MRI) systems to excite nuclear spins and to receive signals from them. The quality of imaging depends strongly on the signal-to-noise ratio (SNR) and the transmit efficiency of the coils. The birdcage volume coils used in most MRI scanners for homogeneous imaging of a sample are typically shielded from the external systems of the MRI scanner, i.e., the gradient coils, to confine the rf field within the region of interest. However, the near magnetic field of a conventional copper rf shield surrounding a birdcage coil interferes destructively with the primary field of the coil in the sample, which significantly limits the SNR and the transmit efficiency. In the work presented here, we theoretically study and experimentally demonstrate the possibility of creating an artificial magnetic rf shield for a birdcage coil with constructive interference in a sample. This effect is similar to the in-phase reflection of antenna far fields from a magnetic shield but affects the near field in MRI. We build an analytical model of a birdcage coil shielded with a cylindrical impedance boundary and analyze the conditions for increasing the efficiency of the coil by means of the shield. We conclude that by replacing a copper shield with an artificial magnetic one, it is possible to reduce the dissipative intrinsic losses of the coil and increase the power absorbed by the sample, which improves the efficiency. To demonstrate the effect, we perform a detailed numerical simulation and an experiment with a small birdcage in a 7-T 19F MRI system with a magnetic shield implemented as a periodic cylindrical metal structure with corrugations filled with a ceramic.

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 Dates: 2020-06
 Publication Status: Published in print
 Pages: -
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 Table of Contents: -
 Rev. Type: -
 Identifiers: DOI: 10.1103/PhysRevApplied.13.064004
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Title: Physical Review Applied
  Abbreviation : Phys. Rev. Appl.
Source Genre: Journal
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Publ. Info: College Park, Md. [u.a.] : American Physical Society
Pages: - Volume / Issue: 13 (6) Sequence Number: 064004 Start / End Page: 1 - 14 Identifier: ISSN: 2331-7019
CoNE: https://pure.mpg.de/cone/journals/resource/2331-7019