Hai Luo1, Bin Wang1, Gaojie Zhu1, Wenzhou Wang1, Xiang Zhou1, Ziyue Wu1, and Leping Zha1,2
1AllTech Medical Systems, Chengdu, China, People's Republic of, 2AllTech Medical Systems, Cleveland, OH, United States
3D dual echo gradient echo sequence is commonly used
to obtain the field map for B0 shimming. The maps contain the true B0 fields mixed
with eddy currents induced magnetic field changes averaged over the echo time
difference, which compromise the shimming accuracy. A calibration sequence with
alternating gradient polarities is proposed to measure the eddy currents term. Quadratic surface fitting is then applied to produce smooth eddy currents
calibration maps over the full imaging volume containing only the first and
second order components. The actual rapid in-vivo shimming sequence runs later,
using the calibration maps to remove the eddy currents influences during
the post-processing, with partial Fourier acquisition
on phase encoding and slice encoding directions to reduce the scan time. The fast
method provides means of eddy currents insensitive shimming, as well
as reduced sensitivity to motion.
