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Magn Reson Med. 2016 Jul;76(1):83-93. doi: 10.1002/mrm.25856. Epub 2015 Jul 30.

Dynamic multi-coil tailored excitation for transmit B1 correction at 7 Tesla.

Author information

1
Department of Diagnostic Radiology, Yale University School of Medicine, Magnetic Resonance Research Center, 300 Cedar Street, New Haven, Connecticut, USA.

Abstract

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 (B 1+) 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 B 1+ 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.

KEYWORDS:

B1 shimming; DYNAMITE; dynamic shimming; multi-coil; radio frequency; wavelength effects

PMID:
26223503
DOI:
10.1002/mrm.25856
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