Kerker Effect in Ultrahigh-Field Magnetic Resonance Imaging

Abstract

Ultrahigh-field (UHF) magnetic resonance imaging (MRI) systems are getting a lot of attention as they ensure high intrinsic signal-to-noise ratio resulting in higher spatial and temporal resolutions as well as better contrast. This promises improved clinical results with regard to morphological as well as functional and metabolic capabilities. Traditionally, MRI relies on volume coils (birdcage) able to deliver a homogeneous radio frequency field exciting the nuclei magnetic spin. However, this strategy is hindered at UHF because of the rf field inhomogeneities yielded by the increased Larmor frequency. A standard approach consists of inserting passive dielectric elements within the volume coil in order to locally enhance the rf field and mitigate these inhomogeneities. However, the lack of control over their electromagnetic properties prevents the development of optimal solutions. Here, a single meta-atom is used to achieve efficient and tunable rf field control in UHF MRI. We demonstrate theoretically and experimentally a full overlap between the electric dipolar and magnetic dipolar resonances of the meta-atom. This interaction is precisely tuned to reach the so-called Kerker scattering conditions when illuminated in the near field by a birdcage coil. At these conditions, a strong enhancement or suppression of the rf field is achieved in the vicinity of the meta-atom within the MRI volume coil.

Intelligent Metamaterials Based on Nonlinearity for Magnetic Resonance Imaging

“…The ability of metamaterials to manipulate the magnetic field has enabled their applications to inductive wireless power transfer, [18] enhancement of the magneto-optic effect, [19] high-quality sensing, [20,21] plasmonic perfect absorption, [22] and magnetic field confinement, [23,24] among others. [27] Recently, judiciously designed metamaterials, consisting of wire [28] or helical resonator arrays, [29] have been utilized to enhance the signal-to-noise ratio (SNR) of the MRI by amplifying the radio-frequency (RF) magnetic field strength due to their capacity for magnetic field enhancement. For example, negative permeability metamaterials have been employed as waveguides [25] and lenses [26] to image deep tissues using 1.5 Tesla (T) MRI systems and a cylindrical meta-atom has been developed to mitigate the field inhomogeneity in 7 T MRI systems based on the Kerker effect.…”

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

“…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.…”

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

“…Earlier works based on metamaterials aimed to bring more flexibility on this trade-off by attempting to control the sensitive volume and the SNR independently [6][7][8]. These configurations were based on coupled-wire arrays and relied on the hybridization mechanism [9]. The wires were either of a resonant length [9] or miniaturized using high permittivity materials [10] or capacitive interconnections [11].…”