Format

Send to

Choose Destination
Magn Reson Med. 2012 Oct;68(4):1109-16. doi: 10.1002/mrm.23319. Epub 2011 Dec 27.

Slice-by-slice B1+ shimming at 7 T.

Author information

1
Centre for Functional and Metabolic Mapping, Robarts Research Institute, Department of Medical Biophysics, The University of Western Ontario, London, Ontario, Canada.

Abstract

Parallel transmission has been used to reduce the inevitable inhomogeneous radiofrequency fields produced in human high-field MRI greater than 3 T. Further improvements in the transmit homogeneity and efficiency are possible by leveraging the additional degree of freedom permitted by multislice acquisitions. Compared to simple scaling of the flip angle to compensate for B1+ falloff along the radiofrequency coil, calculation of B1+ shim solutions on a slice-by-slice basis can markedly improve homogeneity and/or reduce transmitted power and global SAR. Performance measures were acquired at 7 T with a 15-channel head-only transceive array featuring elements distributed over all three logical axes, facilitating B1+ shimming over arbitrary orientations. Compared to a circularly polarized volume mode of the same coil, shimming to maximize excitation efficiency on a slice-by-slice basis yielded improvements in mean B1+ by 12.8±2.4% and a reduction in standard deviation of B1+ of 16.3±6.8%, while reducing relative SAR by 6.2±3.1%. When shimming for greater uniformity, the mean and standard deviation of B1+ were further improved by 15.9±2.6% and 26.2±10.4%, respectively, at the expense of a 135±8% increase in global SAR. Robust multislice-shim solutions are demonstrated that can be quickly calculated, applied in real time, and reliably improve on volume coil modes.

PMID:
22213531
DOI:
10.1002/mrm.23319
[Indexed for MEDLINE]
Free full text

Supplemental Content

Full text links

Icon for Wiley
Loading ...
Support Center