Comparison of new element designs for combined RF‐shim arrays at 7 T

Abstract

Purpose: To identify novel concepts for RF-shim loop architectures suitable for 7T made of two concentric conducting loops fulfilling RF and DC functions, respectively, and to determine their relative SNR performance. The goal is to minimize interference between the two systems while making efficient use of the space closest to the body. Theory: We show by means of theoretical derivation of the frequency spectrum that the proposed two-loop structure exhibits an anti-resonant null and a resonant peak in the frequency domain. Methods: The proposed structure is comprised of two concentric wire loops either arranged as nested loops or in the form of a coaxial cable, in which the two conductors carry the RF and shim signals, respectively. We use theory, simulation, and phantom measurements to obtain frequency spectra and SNR maps for the proposed structures. Results: Retained SNR is found to be 75% in the coaxial loop and ranges from 57% to 67% in three different coaxial configurations. We have found both implementations to be a viable concept for the use in RF-shim devices if remaining SNR limitations can be overcome. Conclusions: We have investigated two new design modalities in 7T RF-shim coil design that separate the RF and shim conductors such that the previously proposed toroidal chokes are eliminated – thereby removing undesired additional loss, bulk, and design complexity.

Gradient and shim technologies for ultra high field MRI

“…One novel insight confirmed by the new modeling capabilities described above that proves particularly noteworthy for UHF applications, is the existence and analysis of a newly observed phenomenon called Lorentz damping (Winkler et al, 2015). By this theory, the intrinsically stronger vibrations of GC conductors exposed to higher field strengths lead to greater mechanical damping compared to lower fields.…”

“…The great advantage of such a combined device is the prospect for extremely efficient packaging of this hardware into a single closely coupled (tight fitting) coil array. In principle, this maximizes efficiency and capability of both RF and B 0 shimming systems, with minimal reduction in the performance of either system (Stockmann et al, 2013; Winkler et al, 2015). …”

“…Multiphysics modeling has gained significant importance in MRI over the last few years and is now commonly applied to vibrational analysis, opening up significant new insights (Zorumski, 1973; Edelstein et al, 2002; Mechefske et al, 2002; Mechefske et al, 2004; Shao and Mechefske, 2005; Taracila et al, 2005; Mechefske and Wang, 2006; Koch et al, 2009; Winkler et al, 2014; Winkler et al, 2015). State-of-the-art multiphysics overmodeling methods demonstrate a high level of agreement with experimental results (Lerch et al, 2000; Mechefske et al, 2002; Mechefske et al, 2004; Yao et al, 2004; Rausch et al, 2005; Mechefske and Wang, 2006; Liu et al, 2011; Jiang and Havens, 2012; Goora et al, 2013; Liu et al, 2013; Winkler et al, 2014; Wang et al, 2015; Winkler et al, 2015). Experimentally validated modeling can then be used to gain new understanding into the causes and types of vibrations observed in GCs.…”

“…In practice, however, it has been found that noise levels on whole-body UHF systems are not significantly higher than for clinical systems (Schmitt, F., Personal Communication). The aforementioned modeling efforts have shown that Lorentz damping, which results from the additional motional damping caused by the static magnetic field B 0 , is likely responsible for this observation, and suggests that acoustic levels do not in fact increase linearly with main field strength (Winkler et al, 2015). …”

In vivo B 0 field shimming methods for MRI at 7 T

“…Further thought has suggested other methods to integrate the shim current with the RF coil circuit (Winkler et al, 2015) and geometries that split the shim currents to form two or more shim loops per RF loop (Darnell et al, 2017). …”