Adaptive slice-specific z-shimming for 2D spoiled gradient-echo sequences

Magn Reson Med. 2021 Feb;85(2):818-830. doi: 10.1002/mrm.28468. Epub 2020 Sep 10.

Abstract

Purpose: To reduce the misbalance between compensation gradients and macroscopic field gradients, we introduce an adaptive slice-specific z-shimming approach for 2D spoiled multi-echo gradient-echoe sequences in combination with modeling of the signal decay.

Methods: Macroscopic field gradients were estimated for each slice from a fast prescan (15 seconds) and then used to calculate slice-specific compensation moments along the echo train. The coverage of the compensated field gradients was increased by applying three positive and three negative moments. With a forward model, which considered the effect of the slice profile, the z-shim moment, and the field gradient, R 2 maps were estimated. The method was evaluated in phantom and in vivo measurements at 3 T and compared with a spoiled multi-echo gradient-echo and a global z-shimming approach without slice-specific compensation.

Results: The proposed method yielded higher SNR in R 2 maps due to a broader range of compensated macroscopic field gradients compared with global z-shimming. In global white matter, the mean interquartile range, proxy for SNR, could be decreased to 3.06 s-1 with the proposed approach, compared with 3.37 s-1 for global z-shimming and 3.52 s-1 for uncompensated multi-echo gradient-echo.

Conclusion: Adaptive slice-specific compensation gradients between echoes substantially improved the SNR of R 2 maps, and the signal could also be rephased in anatomical areas, where it has already been completely dephased.

Keywords: R 2 relaxometry; T 2 relaxometry; field inhomogeneities; gradient-echo; z-shim.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Brain / diagnostic imaging
  • Echo-Planar Imaging*
  • Magnetic Resonance Imaging
  • Phantoms, Imaging
  • White Matter*