Browsing by Author "Hurshkainen, Anna"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Constructive Near-Field Interference Effect in a Birdcage MRI Coil with an Artificial Magnetic Shield
    (American Physical Society, 2020-06-02) Lezhennikova, Kseniia; Abdeddaim, Redha; Hurshkainen, Anna; Vignaud, A.; Dubois, Marie-Claude; Jomin, P.; Berrahou, D.; Raaijmakers, A.; Avdievich, N.; Melchakova, I.V.; Enoch, S.; Belov, P.; Simovski, Konstantin; Glybovski, S.; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Aix-Marseille Université; Université Paris-Saclay; Multiwave Imaging; University Medical Center Utrecht; Max Planck Institute for Biological Cybernetics; Department of Electronics and Nanoengineering
    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.
  • Thumbnail Image
    Item
    Decoupling of Closely Spaced Dipole Antennas for Ultrahigh Field MRI with Metasurfaces
    (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2021-02) Hurshkainen, Anna; Mollaei, Masoud Sharifian Mazraeh; Dubois, Marc; Kurdjumov, Sergei; Abdeddaim, Redha; Enoch, Stefan; Glybovski, Stanislav; Simovski, Constantin; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Kostantin Simovski Group; Aix-Marseille Université; CNRS; Department of Electronics and Nanoengineering
    Phased antenna arrays of dipoles are widely used in ultrahigh field (UHF) magnetic resonance imaging for creating the controllable radio frequency (RF) magnetic field distributions in a human body. Due to safety and imaging quality reasons each individual channel of the array should be decoupled - electromagnetically isolated from the others. The required number of channels is large and in some techniques the dipole antennas should be located in the close proximity of the human body. Their ultimately dense arrangement leads to a strong mutual coupling and makes the conventional decoupling structures inefficient. This coupling needs to be suppressed without a significant distortion of RF fields in the imaged area. In this work, we propose and study a novel decoupling technique for two UHF transceiver on-body dipole antennas. The decoupling is performed by a periodic structure of five parallel resonant wires referred to as a metasurface (MS). In contrast to conventional decoupling with a single resonant wire, the MS decoupled by means of excitation of a higher order coupled mode of the wires, which field is highly confined. The main advantage is a low distortion of the RF-field in the central region of the body.
  • Thumbnail Image
    Item
    Decoupling of dipole antennas by a split loop
    (2018-01-01) Simovski, Constantin; Mollaei, Masoud Shariflan Mazraeh; Glybovski, Stanislav; Hurshkainen, Anna; Department of Electronics and Nanoengineering; Kostantin Simovski Group; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO)
    The decoupling of two very closely located resonant dipoles by a similar dipole located in the middle between them has been recently demonstrated. An approximate analytical model of this decoupling was built, validated by exact numerical simulations and confirmed experimentally. In this work we show that the similar decoupling can be achieved using another scatterer, namely a split-loop resonator may nicely replace the dipole. This replacement allows us to broaden the operation band of the antenna system nearly twice.
  • No Thumbnail Available
    Item
    Double-resonant decoupling method in very dense dipole arrays
    (Elsevier, 2020-05-01) Sharifian Mazraeh Mollaei, Masoud; Hurshkainen, Anna; Kurdjumov, S.; Simovski, C.; Kostantin Simovski Group; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Department of Electronics and Nanoengineering
    In this paper an approach for broadening of operational band in a dense array of dipole antennas by implementing passive split-loop resonators (SLRs) as decouplers is presented. Compared to the previous method, where three closely located active dipoles were decoupled by two passive dipole, the operational band is significantly improved from 0.5% to 1.6% at the same level of decoupling −8 dB for the cross-talk and inter-channel transmittance. To delineate, the presence of two SLRs results in birefringence of the resonant interaction of SLRs which creates two different eigenmodes for decoupling. As a result, a dual-resonant decoupled band is obtained. Alongside with analytical investigation, numerical and experimental investigations verify the veracity of our approach. Moreover, the possibility of decoupling by SLRs for arrays with more active dipoles is investigated numerically.
  • Thumbnail Image
    Item
    Passive decoupling techniques in ultra-high field MRI
    (2018-01-01) Hurshkainen, Anna; Simovski, Constantin; Glybovski, Stanislav; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Department of Electronics and Nanoengineering
    In this work a comparison of two passive decoupling techniques for ultra-high field magnetic resonance imaging (MRI) is presented. An electromagnetic band-gap (EBG) structure and a single passive dipole are compared in terms of their capabilities to decouple two active dipole body array elements at 7 Tesla at an extremely short inter-element distance. Decoupling band as well as distribution of circularly polarized radiofrequency magnetic field inside a homogeneous phantom provided with these two techniques are compared.