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Materials 2014, 7(1), 30-43; doi:10.3390/ma7010030
Article

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

1,* , 1
 and
1,2,3
1 Department of Biomedical Magnetic Resonance, Otto-von-Guericke University Magdeburg, Leipziger Straße 44, Magdeburg 39120, Germany 2 German Centre for Neurodegenerative Diseases (DZNE), site Magdeburg, Magdeburg 39120, Germany 3 Leibniz Institute for Neurobiology, Magdeburg 39118, Germany
* Author to whom correspondence should be addressed.
Received: 24 September 2013 / Revised: 29 November 2013 / Accepted: 16 December 2013 / Published: 20 December 2013
(This article belongs to the Special Issue High Field Magnetic Resonance Methods and Materials 2013)
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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.
Keywords: image homogeneity; radio frequency shimming; RF shimming; RF-power control; transmit array; multi-slice; 7T image homogeneity; radio frequency shimming; RF shimming; RF-power control; transmit array; multi-slice; 7T
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Darji, N.; Patel, G.; Speck, O. Gain of Imaging Fidelity by Employing a Higher Number of Independent Transmit Channels Together with Slice-Selective Radio-Frequency (RF) Shimming at 7T. Materials 2014, 7, 30-43.

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