Gain of Imaging Fidelity by Employing a Higher Number of Independent Transmit Channels Together with Slice-Selective Radio-Frequency (RF) Shimming at 7T.

Niravkumar Darji,Oliver Speck,Gopesh Patel

doi:10.3390/ma7010030

Abstract

Dielectric resonance effects and radio-frequency (RF) power deposition have become challenging issues for magnetic resonance imaging at ultrahigh-field (UHF) strengths. The use of transmit (Tx) coil arrays with independently-driven RF sources using a parallel transmission system is a promising method for alleviating the resulting RF inhomogeneities. In this study, the effect on homogeneity and RF-power when employing a higher number of transmit channels with multi-slice acquisition in vivo at high field strength (7T) is scrutinized. An 8-channel head coil array was driven to emulate circular polarized (CP) and 2-, 4-, and 8-channel independent transmit configurations at 7T. Static RF shimming was employed on human subjects in order to homogenize the B1+ field in the excited volume. Slice-selective and global RF shimming methods were applied with CP and 2-, 4-, and 8-channel transmit channel configurations. RF shimming was performed from CP to 2-, 4-, and 8-channel Tx configurations globally and slice-selectively. Systematic improvement in B1+ homogeneity and/or reduction in RF-power were observed. RF shimming in the human brain with 8-channel transmit and slice-selective shimming yields an increase in B1+ homogeneity of 43% and/or reduces RF-power by 68% when compared with CP global RF shimming at 7T.

Highlights

Magnetic resonance imaging of human subjects at high field strengths (≥3 T) suffers from radio frequency (RF) field inhomogeneity artifacts caused by dielectric resonance effects
RF shimming in the human brain with 8-channel transmit and slice-selective shimming yields an increase in B1+ homogeneity of 43% and/or reduces RF-power by 68% when compared with circularly polarized (CP) global RF shimming at 7T
In the case of 1-channel Tx, an identical RF-amplitude was applied for the whole slice stack in global RF shimming or for individual slices in slice-selective RF shimming, with CP phases, scaled to reach the desired mean flip angle (FA)

Summary

Introduction

Magnetic resonance imaging of human subjects at high field strengths (≥3 T) suffers from radio frequency (RF) field inhomogeneity artifacts caused by dielectric resonance effects. The circularly polarized (CP) transmit B1 (B1+) field that creates the tipping of the magnetization produces variable tissue contrast depending on the geometry and the magnetic field strengths, while the inhomogeneity of the signal reception field (B1−) gives rise to variable signal reception sensitivity. While this latter effect is correctable given the coil sensitivities, the former effect is not readily correctable [1,2]. The parallel transmit hardware allows channel specific control over RF amplitudes and excitation phases that can mitigate high field B1+

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Conclusion