Wireless coils based on resonant and nonresonant coupled‐wire structure for small animal multinuclear imaging

Abstract

Earlier work on RF metasurfaces for preclinical MRI has targeted applications such as whole‐body imaging and dual‐frequency coils. In these studies, a nonresonant loop was used to induce currents into a metasurface that was operated as a passive inductively powered resonator. However, as we show in this study, the strategy of using a resonant metasurface reduces the impact of the loop on the global performance of the assembled coil. To mitigate this deficiency, we developed a new approach that relies on the combination of a commercial surface coil and a coupled‐wire structure operated away from its resonance. This strategy enables the extension of the sensitive volume of the surface coil while maintaining its local high sensitivity without any hardware modification. A wireless coil based on a two parallel coupled‐wire structure was designed and electromagnetic field simulations were carried out with different levels of matching and coupling between both components of the coil. For experimental characterization, a prototype was built and tested at two frequencies, 300 MHz for 1H and 282.6 MHz for 19F at 7 T. Phantom and in vivo MRI experiments were conducted in different configurations to study signal and noise figures of the structure. The results showed that the proposed strategy improves the overall sensitive volume while simultaneously maintaining a high signal‐to‐noise ratio (SNR). Metasurfaces based on coupled wires are therefore shown here as promising and versatile elements in the MRI RF chain, as they allow customized adjustment of the sensitive volume as a function of SNR yield. In addition, they can be easily adapted to different Larmor frequencies without loss of performance.

RF Coils for Preclinical Multinuclear Imaging Based on Coupled-wire Structures Working in Resonant and Non-resonant Regime

“…It has been shown recently that the distribution of currents within the components of the coil plays an important role in the resulting B1 + field distribution [18]. A matched loop presents higher current amplitude compared to a feed loop, therefore the B1 + magnitude in case A is high but localized, compared to cases B and B with capacitors, where the B1 + field is more homogeneous as shown in the simulations (Figures 2 and 3).…”

“…However, their sensitivity extends along the whole phantom ( Figure 3B). This can be linked to the currents flowing inside the loop and the wires in each case [18]. Case C seems to maintain a high B1 + magnitude (25 µT), similar to case A (26 µT), in addition to a horizontal extension of the field.…”

Sensitivity Analysis of the Simply Noise-matched Receiving Coil for NMR Experiments

“…For our example the value ܸ = 10 ି V has been adopted. Other quantities: ݂ = 4.45 ‫ݖܪܯ‬ ܳ = 250 is quality factor of the unloaded coil ܺ = ߱ ‫ܮ‬ = 75 Ω is reactance of the coil ܴ ௨௧ is output resistance of the tuning and matching circuit, corresponding to ܴ ܴ = ఠ బ ொ is resistance of the unloaded coil Values of the capacitances ‫ܥ‬ ଵ and ‫ܥ‬ ଶ are given by (10) and (11): Auxiliary voltage ܸ ଷ is given by (12):…”

Decoupling of Closely Spaced Dipole Antennas for Ultrahigh Field MRI With Metasurfaces

“…The values of the quadrupole and magnetic dipole moments in this mode are balanced that makes the electric quadrupole (off-diagonal) and magnetic dipole not distinguishable in what concerns their electromagnetic fields the central transverse plane (H-plane of the array) [30]. Therefore, the currents of this mode create in the transverse plane the near magnetic field identical to that of a horizontal loop of the same length [31], [32]. This field is presented in the bottom panel of As we expected, when the mode order increases the corresponding electromagnetic field better concentrates around the MS.…”