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Dynamic multi‐coil tailored excitation for transmit B 1 correction at 7 Tesla
Author(s) -
Umesh Rudrapatna S.,
Juchem Christoph,
Nixon Terence W.,
de Graaf Robin A.
Publication year - 2016
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.25856
Subject(s) - shim (computing) , excitation , flip angle , imaging phantom , electromagnetic coil , multislice , mathematics , acoustics , nuclear magnetic resonance , physics , optics , magnetic resonance imaging , medicine , quantum mechanics , erectile dysfunction , radiology
Purpose Tailored excitation (TEx) based on interspersing multiple radio frequency pulses with linear gradient and higher‐order shim pulses can be used to obtain uniform flip angle in the presence of large radio frequency transmission (B1 + ) inhomogeneity. Here, an implementation of dynamic, multislice tailored excitation using the recently developed multi‐coil nonlinear shim hardware (MC‐DTEx) is reported. Methods MC‐DTEx was developed and tested both in a phantom and in vivo at 7 T, and its efficacy was quantitatively assessed. Predicted outcomes of MC‐DTEx and DTEx based on spherical harmonic shims (SH‐DTEx) were also compared. Results For a planned 30 ° flip angle, in a phantom, the standard deviation in excitation improved from 28% (regular excitation) to 12% with MC‐DTEx. The SD in in vivo excitation improved from 22 to 12%. The improvements achieved with experimental MC‐DTEx closely matched the theoretical predictions. Simulations further showed that MC‐DTEx outperforms SH‐DTEx for both scenarios. Conclusion Successful implementation of multislice MC‐DTEx is presented and is shown to be capable of homogenizing excitation over more than twofold B1 +variations. Its benefits over SH‐DTEx are also demonstrated. A distinct advantage of MC hardware over SH shim hardware is the absence of significant eddy current effects, which allows for a straightforward, multislice implementation of MC‐DTEx. Magn Reson Med 76:83–93, 2016. © 2015 Wiley Periodicals, Inc.