Simone A Winkler1, Paul A Warr2, Jason P Stockmann3, Azma Mareyam3, Boris Keil4, Ronald D Watkins1, Lawrence L Wald3, Brian K Rutt1. 1. Department of Radiology, Stanford University, Stanford, California, USA., Grant sponsor: NIH K99 EB24341., Grant sponsor: NIH P41 EB015891., Grant sponsor: NSERC., Grant sponsor: BWF. 2. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, Massachusetts, USA. 3. Department of Electrical and Electronic Engineering, University of Bristol, UK. 4. Technische Hochschule Mittelhessen, Germany.
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.
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.
Authors: Christoph Juchem; Peter B Brown; Terence W Nixon; Scott McIntyre; Douglas L Rothman; Robin A de Graaf Journal: Magn Reson Med Date: 2011-03-25 Impact factor: 4.668
Authors: Christoph Juchem; Terence W Nixon; Scott McIntyre; Vincent O Boer; Douglas L Rothman; Robin A de Graaf Journal: J Magn Reson Date: 2011-07-23 Impact factor: 2.229
Authors: Graham C Wiggins; Jonathan R Polimeni; Andreas Potthast; Melanie Schmitt; Vijay Alagappan; Lawrence L Wald Journal: Magn Reson Med Date: 2009-09 Impact factor: 4.668