Dynamic multi‐coil tailored excitation for transmit B1 correction at 7 Tesla. Issue 1 (30th July 2015)

• Record Type: Journal Article
• Authors: Umesh Rudrapatna, S.; Juchem, Christoph; Nixon, Terence W.; de Graaf, Robin A.
• Is Part Of: Magnetic resonance in medicine. Volume 76:Issue 1(2016)
• Page Start: 83
• Page End: 93
• Publication Date: 2015-07-30
• Subjects: B1 shimming -- wavelength effects -- radio frequency -- DYNAMITE -- multi‐coil -- dynamic shimming; Nuclear magnetic resonance -- Periodicals; Electron paramagnetic resonance -- Periodicals; 616.07548
• DOI: 10.1002/mrm.25856
• Languages: English
• ISSNs: 0740-3194

Abstract

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.