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

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.

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, $R_2^{\ast }$ 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.

The proposed method yielded higher SNR in $R_2^{\ast }$ 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.

Adaptive slice-specific compensation gradients between echoes substantially improved the SNR of $R_2^{\ast }$ maps, and the signal could also be rephased in anatomical areas, where it has already been completely dephased.