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

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Journal Title
Journal ISSN
Volume Title
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Date
2020-06-02
Major/Subject
Mcode
Degree programme
Language
en
Pages
14
Series
Physical Review Applied, Volume 13, issue 6
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 F-19 MRI system with a magnetic shield implemented as a periodic cylindrical metal structure with corrugations filled with a ceramic.
Description
| openaire: EC/H2020/736937/EU//M-CUBE
Keywords
impedance, surfaces, antennas
Other note
Citation
Lezhennikova , K , Abdeddaim , R , Hurshkainen , A , Vignaud , A , Dubois , M-C , Jomin , P , Berrahou , D , Raaijmakers , A , Avdievich , N , Melchakova , I V , Enoch , S , Belov , P , Simovski , K & Glybovski , S 2020 , ' Constructive Near-Field Interference Effect in a Birdcage MRI Coil with an Artificial Magnetic Shield ' , Physical Review Applied , vol. 13 , no. 6 , 064004 . https://doi.org/10.1103/PhysRevApplied.13.064004