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Magn Reson Med. 2016 Mar;75(3):1119-31. doi: 10.1002/mrm.25695. Epub 2015 May 7.

Fast iterative pre-emphasis calibration method enabling third-order dynamic shim updated fMRI.

Author information

1
Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland.
2
Clinic of Affective Disorders and General Psychiatry, University Hospital of Psychiatry Zurich, Zurich, Switzerland.
3
Max Planck Institute for Biological Cybernetics, Tuebingen, Germany.

Abstract

PURPOSE:

To calibrate a pre-emphasis to sufficiently compensate eddy currents for application of dynamic shim updating to fMRI without extension of scan times.

METHODS:

Eddy current effects induced into all shim terms up to third-order were characterized by spatiotemporal field monitoring, using a third-order field camera. Pre-emphasis settings were derived from the measurements and iteratively evaluated and refined. The calibrated pre-emphasis was applied to slice-wise dynamic shim updating in combination with a dynamic excitation frequency (F0) determination and a slice-wise B0 optimization routine for in vivo echo planar imaging and resting-state functional MRI.

RESULTS:

The described method for pre-emphasis calibration led to settling times of remaining eddy current effects below 2 ms, allowing for the application of dynamic shim updating to fMRI without extension of scan times or induction of eddy current related artifacts. A dynamic F0 determination compensates frequency shifts induced by the superposition of different shim fields, and therefore, prevents an image shift within the field of view. Hardware limitations necessitate the reduction of the maximum applicable B0 shim field amplitudes and restrict the shim performance.

CONCLUSION:

The proposed method enables accurate pre-emphasis calibration, and therefore, the application of dynamic shim updating to fMRI.

KEYWORDS:

B0 shimming; dynamic shim updating; eddy current compensation; pre-emphasis calibration; resting-state fMRI; spatio-temporal field monitoring

PMID:
25950147
DOI:
10.1002/mrm.25695
[Indexed for MEDLINE]

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