A new gradient shimming method based on undistorted field map of B-0 inhomogeneity

Most existing gradient shimming methods for NMR spectrometers estimate field maps that resolve B-0 inhomogeneity spatially from dual gradient-echo (GRE) images acquired at different echo times. However, the distortions induced by B-0 inhomogeneity that always exists in the GRE images can result in estimated field maps that are distorted in both geometry and intensity, leading to inaccurate shimming. This work proposes a new gradient shimming method based on undistorted field map of B-0 inhomogeneity obtained by a more accurate field map estimation technique. Compared to the traditional field map estimation method, this new method exploits both the positive and negative polarities of the frequency encoded gradients to eliminate the distortions caused by B-0 inhomogeneity in the field map. Next, the corresponding automatic post-data procedure is introduced to obtain undistorted B-0 field map based on knowledge of the invariant characteristics of the B-0 inhomogeneity and the variant polarity of the encoded gradient. The experimental results on both simulated and real gradient shimming tests demonstrate the high performance of this new method. (C) 2015 Elsevier Inc. All rights reserved.