A Novel Metamaterial-Inspired RF-coil for Preclinical Dual-Nuclei MRI

Abstract

In this paper, we propose, design and test a new dual-nuclei RF-coil inspired by wire metamaterial structures. The coil operates as a result of resonant excitation of hybridized eigenmodes in multimode flat periodic structures comprising several coupled thin metal strips. It was shown that the field distribution of the coil (i.e. penetration depth) can be controlled independently at two different Larmor frequencies by selecting a proper eigenmode in each of two mutually orthogonal periodic structures. The proposed coil requires no lumped capacitors to be tuned and matched. In order to demonstrate the performance of the new design, an experimental preclinical coil for 19F/1H imaging of small animals at 7.05T was engineered and tested on a homogeneous liquid phantom and in-vivo. The results demonstrate that the coil was both well tuned and matched at two Larmor frequencies and allowed image acquisition at both nuclei. In an in-vivo experiment, it was shown that without retuning the setup it was subsequently possible to obtain anatomical 1H images of a mouse under anesthesia with 19F images of a tiny tube filled with a fluorine-containing liquid and attached to the body of the mouse.

Small‐animal, whole‐body imaging with metamaterial‐inspired RF coil

“…The coil design was inspired by so‐called ‘mushroom’ metamaterial structures, which are periodic arrays of wires loaded by square capacitive metal patches forming an artificial magnetic conductor . The operational principle of the coil is based on a hybridization of eigenmodes of coil structural elements in an array of parallel non‐magnetic wires . The coil comprises a wire resonator inductively coupled with a small non‐resonant magnetic loop.…”

“…It has been shown that, in such a resonator type, multiple surface eigenmodes can be excited, all having different B 1 + patterns. The fundamental eigenmode has the most homogeneous B 1 + distribution with the highest penetration depth into the subject . Inductive coupling of the resonator with the feeding loop provides excitation of the first eigenmode of the resonator in both transmission and reception regimes.…”

“…20 The operational principle of the coil is based on a hybridization of eigenmodes of coil structural elements in an array of parallel non-magnetic wires. 21 The coil comprises a wire resonator inductively coupled with a small non-resonant magnetic loop.…”

“…The fundamental eigenmode has the most homogeneous B 1 + distribution with the highest penetration depth into the subject. 21 Inductive coupling of the resonator with the feeding loop provides excitation of the first eigenmode of the resonator in both transmission and reception regimes. Tuning the first eigenmode of the resonator to the Larmor frequency of 1 H at 7 T (300.8 MHz) is carried out by changing the length of the tubes, in the range 57-80 mm, whilst the value of the capacitance is kept constant as a result of the fixed size of the capacitive patches (9 × 9.5 mm 2 ).…”

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

“…To do so, the wires used need to be either of a resonant length 10 or miniaturized by high-permittivity loading, 11 or by capacitive interconnections. 12 These studies demonstrate the possibility of using such metasurface structures to achieve a dual-frequency coil for proton and fluorine imaging, 13 and a coil for whole-body imaging with higher SNR than a volume coil. 14 It has been shown that the optimal impedance matching of such metasurface-based coils is obtained with a nonresonant feed loop when one of the metasurface resonances coincides with the targeted Larmor frequency.…”