Enhancement of transmit and receive efficiencies with hybridized meta-atom in 7T head coil array

Abstract

B1 + heterogeneity remains an important challenge for ultra-high field MRI scanners (B0≥7T). There are two methods to mitigate this problem: parallel transmission pTx [1], and passive shimming using highdielectric pads [2], in this paper we will focus on passive shimming.High-dielectric constant pads have been proposed and optimized for passive shimming purposes [2][3][4]. Common formulations of dielectric pads for 7T applications are based on BaTiO3 mixed with water present some drawbacks such as performance decay over time. While previous studies tackled directly the formulation problem introducing new dielectric materials and solvent [5], we adopted a new approach based on metamaterial [6]. We demonstrated that the hybridization of four parallel metallic wires arranged on a square unit cell provides the ability to control radiofrequency field inside a 7T head birdcage.In the first part of this work, we show that these hybridized meta-atom (HMA) can be used like a dielectric pad placed on the side of the patient's head. In this configuration, we observe a strong enhancement of B1 amplitude near the metamaterial as well as a significant reduction of the inhomogeneities on the metamaterial side. Moreover, a significant gain in terms of signal to noise ratio is also observed as the reception of the MR signal is enhanced in a 1T/1R birdcage coil.In a second part, we address the HMA performances when used with a single channel transmit birdcage equipped with 32 channels receive array. In this configuration the HMA is deported between the emission coil and the receive array in order to maintain the patient's comfort. We demonstrated analytically using the impedance matrix [6], numerically and with measurement that the HMA structure is perfectly compatible with a birdcage emitter and does not perturb the receive array. Moreover, it has the ability to enhance both transmit and receive signal with the possibility to fill gaps usually observed in the brain temporal lobes.This metamaterial based passive shimming strategy would provide a cost effective and long-lasting solution without impacting the patient's comforts during the examination, especially in the presence of a head receive array routinely used for in vivo MRI protocol.

Hilbert Fractal Inspired Dipoles for B1+Field Control in Ultra-High Field MRI

“…The constructive and destructive interferences between wavelengths at ultra-high magnetic field (7 Teslas,300MHz) in clinical magnetic resonance imaging (MRI) lead to head coils' B 1 + magnetic field inhomogeneities in the temporal lobe regions. Approaches like high-permittivity dielectric pads [1]- [3] and coupled-wire metamaterials [4]- [5] have been employed to mitigate these effects. They present a powerful and controllable improvements of the B 1 + field inhomogeneities, but there remain some challenges to overcome.…”

“…This adds constraints in the set up and reduction of the patient comfort. On the other hand, the metamaterial approach is based on the mode hybridization principle, using long resonant dipoles coupled into a specific geometry [4]- [5]. Designs such as the Hybridized Meta-Atom (HMA) provide stronger enhancement than dielectric pads since it is possible to control the coil-resonator coupling strength by adapting the length of the wires and the position with respect to the patient's head, giving more degrees of freedom.…”