Auto-Calibrating the Z-shim for Slice Specific Recovery of Susceptibility Losses

Background Z-shim is a method for recovering susceptibility related signal loss in T2*-weighted images. Several pulse sequences incorporate the Z-shim for fMRI studies investigating problematic areas of the brain such as amygdala and orbital frontal cortex humans. [1-5] Essentially, these brain regions are affected by local field present across the slice. This gradient field causes spin dephasing resulting in signal loss. The dephasing can be refocused using a small compensation gradient called the Z-shim. Calibrating the Z-shim manually for each slice is time consuming and operator dependent. Using a field map and a linear approximation for the field gradient across the slice, the Z-shim can be calibrated automatically for each slice. Methods Fig.1 Field Map to Gradient Map Field maps [Fig. 1] are obtained using spin echo EPI on a 3T Siemens Trio MR Scanner using two acquisitions, one with a 2 ms echo offset. The field map is masked and median filtered to remove spurious pixel values. At each pixel the distortion gradient is approximated by a linear fitting across three adjacent slices. The z-shim is estimated by the mean distortion gradient in the strongest region of susceptibility artifact and the requested echo time. The method is demonstrated in vivo using the Z-SAGA [1] pulse sequence, but is generally applicable to most z-shim methods.

The field map is masked and median filtered to remove spurious pixel values. At each pixel the distortion gradient is approximated by a linear fitting across three adjacent slices. The z-shim is estimated by the mean distortion gradient in the strongest region of susceptibility artifact and the requested echo time [fig. 2]. Fig. 2 In Vivo Image Profile of Field Gradient Distortion The method is demonstrated in vivo using the Z-SAGA [1] pulse sequence, but is generally applicable to most z-shim methods. Results Fig. 3 Z-shim result Discussion The auto-calibration procedure is effective in determining slice specific z-shim values. Extending the method to include more than a single z-shim is possible as in Ref. 5, but at the expense of temporal resolution and/or slice coverage. Acknowledgment This work was supported in part by the National Institutes of Health, Georgia Research Alliance and the Whitaker Foundation.

References

1. Heberlein KA, Hu X. Simultaneous acquisition of gradient-echo and asymmetric spin-echo for single-shot z-shim: Z-SAGA. Magn Reson Med 2004;51(1):212-216
2. Guo H, Song AW. Single-shot spiral image acquisition with embedded z-shimming for susceptibility signal recovery. J Magn Reson Imaging 2003;18(3):389-395
3. Li Z, Wu G, Zhao X, Luo F, Li SJ. Multiecho segmented EPI with z-shimmed background gradient compensation (MESBAC) pulse sequence for fMRI. Magn Reson Med 2002;48(2):312-321
4. Gu H, Feng H, Zhan W, Xu S, Silbersweig DA, Stern E, Yang Y. Single-shot interleaved z-shim EPI with optimized compensation for signal losses due to susceptibility-induced field inhomogeneity at 3 T. Neuroimage 2002;17(3):135-136
5. Deichmann R, Josephs O, Hutton C, Corfield DR, Turner R. Compensation of susceptibility-induced BOLD sensitivity losses in echo-planar fMRI imaging. Neuroimage 2002;15(1):120-135