Dynamic multi-coil tailored excitation for transmit B1 correction at 7 Tesla.
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.
['Umesh Rudrapatna|S|S|', 'Juchem|Christoph|C|', 'Nixon|Terence W|TW|', 'de Graaf|Robin A|RA|']
D000465:Algorithms D016477:Artifacts D001921:Brain D004867:Equipment Design D019544:Equipment Failure Analysis D006801:Humans D007089:Image Enhancement D008279:Magnetic Resonance Imaging D019047:Phantoms, Imaging D015203:Reproducibility of Results D012680:Sensitivity and Specificity
multi-coil radio frequency B1 shimming DYNAMITE dynamic shimming wavelength effects