Enhancement of radio frequency magnetic field for a 1.5 T magnetic resonance system using a high impedance surface

Abstract

This study presents a novel design of a high impedance surface (HIS) to improve the magnetic field strength of a radio frequency (RF) coil at 63.8 MHz, suitable for 1.5 T magnetic resonance imaging systems. The HIS is miniaturised using distributed interdigital capacitors. A HIS with an electrically small unit cell (λ /94) and is positioned between a RF transmit‐receive coil and an RF shield. The magnitude of the magnetic field from an RF coil when coupled to a dielectric phantom load (mimicking a biological sample) was estimated using numerical simulation (finite integral methods) and corroborated with measurements. Simulated median improvements in magnetic field inside a dielectric phantom range from approximately 18 to 32% depending on the area of the HIS.

A Magnetic Resonance Imaging Surface Coil Transceiver Employing a Metasurface for 1.5T Applications

“…Numerical simulations were performed using the time domain solver within CST Microwave Studio 2015 where the coil was fed with a 1A current source as shown in Fig. 1b and it was assumed that the impedance surface, modelled as a tabulated surface impedance, was 150mmx150mm which has been shown to provide reasonable results in previous studies [15]. The simulation was surrounded by an absorbing boundary condition.…”

“…There are a range of options for the geometry of the metallic elements of the FSS, including square patches, Jerusalem crosses, spirals etc. Classic FSS designs tend to have periodic elements which can be a significant fraction of a wavelength which would not be appropriate for MRI at 63.8MHz, as such an interdigital approach was adopted that has been previously reported [15], and has a high capacitance density which provides electrically small unit cell designs. Fig.…”

“…The RF coil design is based on a previously reported method [15], with the coil matched to a 50Ω source. The FSS was manufactured on double sided FR4 using a PCB etching technique to the dimensions specified in Section II-B.…”

“…Lens techniques have been adopted through the use of capacitively loaded split ring resonators [8]- [10], that provide localised image and SNR enhancement. High impedance surfaces (HIS) have also been employed as in-phase reflective shields to improve field strength at 7T [11]- [13], and also at 1.5T [14], [15]. There has also been significant research into wire media based metamaterials and metasurfaces, initially [16] showed how a metallic endoscope can be used to guide fields to an external receive probe.…”