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.; Dubois, M.; Jomin, P.; Berrahou, D.; Raaijmakers, A.; Avdievich, N.; Melchakova, I.; Enoch, S.; Belov, P.; Simovski, C.; Simovski, C.; Glybovski, S.
(2020) Physical Review Applied, volume 13, issue 6, pp. 1 - 14
(Article)
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
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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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Keywords: General Physics and Astronomy
ISSN: 2331-7019
Publisher: American Physical Society
Note: Funding Information: The theoretical work and the manufacture were supported by the Russian Science Foundation (Grant No. 18-19-00482). The experimental work received funding from the European Union's Horizon 2020 Research and Innovation Program under Grant No. 736937. The MRI test received financial support from the French "Investissement d'Avenir" program run by the Agence Nationale pour la Recherche, Grant No. ANR-11-INBS-0006 "Infrastructure d'avenir en Biologie Santé," and from the ERPT equipment program of the Leducq Foundation. The authors are thankful to Christopher Collins, Christophe Craeye, Dmitry Dmitriev, and Maxim Konstantinov for useful discussions and to Elizaveta Nenasheva for manufacture of the ceramic rings. Funding Information: The theoretical work and the manufacture were supported by the Russian Science Foundation (Grant No. 18-19-00482). The experimental work received funding from the European Union’s Horizon 2020 Research and Innovation Program under Grant No. 736937. The MRI test received financial support from the French “Investissement d’Avenir” program run by the Agence Nationale pour la Recherche, Grant No. ANR-11-INBS-0006 “Infrastructure d’avenir en Biologie Santé,” and from the ERPT equipment program of the Leducq Foundation. The authors are thankful to Christopher Collins, Christophe Craeye, Dmitry Dmitriev, and Maxim Konstantinov for useful discussions and to Elizaveta Nenasheva for manufacture of the ceramic rings. Publisher Copyright: © 2020 American Physical Society.
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