Analysis of High Impedance Coils Both in Transmission and Reception Regimes

Abstract

Theory of a high impedance coil (HIC)-a cable loop antenna with a modified shield-is comprehensively discussed for MRI application in both transmitting and receiving regimes. Understanding a weakness of the previously reported HIC in transmitting regime, we suggest another HIC which is advantageous in both transmitting and receiving regimes compared to a conventional loop antenna. In contrast with the claim of previous works that the reported HICs are advantageous in transmission regime, we show only this HIC is a practical transceiver HIC. Using the perturbation approach and adding gaps to both shield and inner wire of the cable, we tune the resonance frequency to be suitable for ultra-high field (UHF) magnetic resonance imaging (MRI). These gaps reduce the quality factor of the enhanced HIC which makes its resonant frequency more stable with respect to different loadings. Our theoretical model and applicability of our HIC for MRI applications are verified by simulations. Using the theoretical model, we have designed and fabricated an array of three HICs operating at 298 MHz. The operation of the array has been experimentally studied in the presence of different phantoms used in ultrahigh field MRI and the results compared with those obtained for a conventional array.

Flexible Multi-Turn Multi-Gap Coaxial RF Coils: Design Concept and Implementation for Magnetic Resonance Imaging at 3 and 7 Tesla

“…7 (see also [38]), operating far from its self-resonance can cause a similar "loopole"-type current distribution and might yield improved performance compared to a simple loop coil, depending on the coil orientation relative to B 0 [54]. Furthermore, the asymmetric current pattern might be an explanation [55] for improved inter-element decoupling observed in references [29], [38].…”

“…dipoles alone or combined with loop coils) might be more advantageous for large penetration depths at UHF [52], [53]. Although a homogeneous current distribution along the oCo was targeted in this work (as with most segmented standard coils), it is conceivable that a non-uniform placement of the gaps leading to unbalanced current distribution on the oCo [29] could also be exploited at UHF, in analogy to "loopole" designs [54], [55] which combine loop and dipole antenna properties. The 1T1G coil shown in Fig.…”

“…Further, lightweight wearable array design and imaging of moving body parts becomes possible. CCs are especially interesting as some of the challenges arising in flexible coil development seem to be alleviated: they demonstrate robustness against coil deformation, load variation and inter-element coupling (overlap change) in an array configuration [29], [34], [38]. The re-occurring research interest in coaxial coil design has clearly been inspired by previous work on "shielded loop" (surface) [42], [43] and "coaxial cavity" (volume) resonators [44].…”

“…The purpose of this work is to explore the size limits of 1T1G-CCs and introduce and examine the extended design concept of coaxial coils with more than one gap and/or cable turns -multi-turn multi-gap CCs (MTMG-CCs) -as proposed by Laistler and Moser [45]. Some publications have suggested [46] or proven [29] the feasibility of introducing multiple gaps (at arbitrary positions) in the coaxial coil conductor to tune the surface coil to a desired resonance frequency. However, to the knowledge of the authors, this approach has never been investigated more systematically.…”

Interelement Decoupling Strategies at UHF MRI

“…It can vary from around 8 to 12 cm for operation at 7T, and there is a need for using techniques for increasing/decreasing the coil diameter. In the work in reference [22], it is proposed to introduce multiple gaps on the shield and/or inner conductor to obtain the desired coil size.…”