TY - JOUR
T1 - Fast iterative pre-emphasis calibration method enabling third-order dynamic shim updated fMRI
AU - Fillmer, Ariane
AU - Vannesjo, Signe Johanna
AU - Pavan, Matteo
AU - Scheidegger, Milan
AU - Pruessmann, Klaas Paul
AU - Henning, Anke
N1 - Publisher Copyright:
© 2015 Wiley Periodicals, Inc.
PY - 2016/3/1
Y1 - 2016/3/1
N2 - 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.
AB - 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.
KW - B shimming
KW - dynamic shim updating
KW - eddy current compensation
KW - pre-emphasis calibration
KW - resting-state fMRI
KW - spatio-temporal field monitoring
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U2 - 10.1002/mrm.25695
DO - 10.1002/mrm.25695
M3 - Article
C2 - 25950147
AN - SCOPUS:84928964138
SN - 0740-3194
VL - 75
SP - 1119
EP - 1131
JO - Magnetic resonance in medicine
JF - Magnetic resonance in medicine
IS - 3
ER -