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 19 F/ 1 H 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 1 H images of a mouse under anesthesia with 19 F images of a tiny tube filled with a fluorine-containing liquid and attached to the body of the mouse. Magnetic resonance provides unique instrumentation for modern biomedical studies. In clinical and preclinical applications, radiofrequency (RF) coils play an important role of antennas exciting spins in a subject under an applied strong static magnetic field with a desired flip angle and receiving weak echo signals at the Larmor frequency. Thus, a signal-to-noise (SNR) ratio of images, strongly dependent on electromagnetic properties of RF coils, becomes particularly important if a density of an investigated nucleus in the sample is low. In multi-nuclei studies, it is possible to retrieve additional imaging and/or spectroscopy information using the magnetic resonance of several nuclei of interest typically taking place at different Larmor frequencies for the same magnet field B 0. The Larmor frequency of a nucleus is determined by its gyromagnetic ratio γ as f L = γ·B 0. The corresponding RF-coils, suitable for multi-nuclei MRI, must operate at all desirable Larmor frequencies simultaneously given that the input impedance is matched to the transceiver channel(s) and the RF-field distribution in a subject ensures maximum + B 1 per unit power efficiency in transmission over the region of interest. The afore-mentioned properties also ensure a high SNR of the same coil in reception improving imaging quality and resolution of measured molecular spectra. Preclinical studies impose additional limitations on RF-coil designs. The dedicated coils must be compatible with other equipment required for the biomedical experiment, such as excitation systems and sample beds. Another challenge comes from the electromagnetic environment of preclinical scanners, where RF-coils must operate inside an electrically narrow, shielded tunnel. Finally, RF-coils are typically very sensitive to variation of a subject's electrical properties. In contrast to large coils (for instance, those used for clinical applications) where the noise from the subject dominates, in the so-called mid-range preclinical coils, the subject noise becomes comparable to the intrinsic noise of the coil 1. This last fact results from power losses in the coil's components and strongly depends on the coil's design and quality of its materials. RF-coils for small-animal imaging and spectroscopy at the fields 3-17 T are typically volumetric coils which have linear polarization of the RF magnetic field such as solenoids providing high SNR if the sample losses are not
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Anna Hurshkainen, Anton Nikulin, Elodie Georget, Benoît Larrat, Djamel Berrahou, et al.. A Novel Metamaterial-Inspired RF-coil for Preclinical Dual-Nuclei MRI. Scientific Reports, Nature Publishing Group, 2018, 8 (1), ⟨10.1038/s41598-018-27327-y⟩. ⟨hal-01951316⟩

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