Automated slice‐specific z‐shimming for functional magnetic resonance imaging of the human spinal cord

Abstract

Functional magnetic resonance imaging (fMRI) of the human spinal cord faces many challenges, such as signal loss due to local magnetic field inhomogeneities. This issue can be addressed with slice-specific z-shimming, which compensates for the dephasing effect of the inhomogeneities using a slice-specific gradient pulse. Here, we aim to address outstanding issues regarding this technique by evaluating its effects on several aspects that are directly relevant for spinal fMRI and by developing two automated procedures in order to improve upon the time-consuming and subjective nature of manual selection of z-shims: one procedure finds the z-shim that maximizes signal intensity in each slice of an EPI reference-scan and the other finds the through-slice field inhomogeneity for each EPI-slice in field map data and calculates the required compensation gradient moment. We demonstrate that the beneficial effects of z-shimming are apparent across different echo times, hold true for both the dorsal and ventral horn, and are also apparent in the temporal signal-to-noise ratio (tSNR) of EPI time-series data. Both of our automated approaches were faster than the manual approach, lead to significant improvements in gray matter tSNR compared to no z-shimming and resulted in beneficial effects that were stable across time. While the field-map-based approach performed slightly worse than the manual approach, the EPI-based approach performed as well as the manual one and was furthermore validated on an external corticospinal data-set (N > 100). Together, automated z-shimming may improve the data quality of future spinal fMRI studies and lead to increased reproducibility in longitudinal studies.

Thermal Stimulus Task fMRI in the Cervical Spinal Cord at 7 Tesla

“…Several groups have developed methods to address distortion and through-slice dephasing in single-shot GRE EPI at 3 T through slicewise dynamic shimming methods using either slice-specific gradient offsets (62) or z-gradient refocusing blips (63,64). These methods, applied to single-shot EPI, may be promising alternatives to multi-shot 2D or 3D EPI at 7 T but, at the time of this study, were not sufficiently refined to implement routinely at 7 T. Also 3D multi-shot EPI sequences suitable for implementation in fMRI experiments were not readily available at the time of this study, despite encouraging results demonstrating their robustness to B 0 inhomogeneity in resting-state fMRI studies at 7 T (12,13).…”

Simultaneous multislice imaging with slice‐specific z‐shim

“…However, in a recent study, it was observed that the reference acquisition approach sometimes outperforms a field map-based calculation. 12 In conclusion, the proposed RF envelope shift may help to combine SMS acceleration with slice-specific z-shim, which may be particularly helpful for BOLD-based functional neuroimaging of the spinal cord.…”

“…Using a field map acquisition to determine the through‐slice field inhomogeneity and calculate the corresponding z‐shim appears to be more elegant than the reference acquisition with a discrete number of z‐shims performed here, particularly because the field map can be acquired with a more robust sequence than EPI. However, in a recent study, it was observed that the reference acquisition approach sometimes outperforms a field map–based calculation 12 …”

“…A low‐threshold and easy‐to‐implement compromise is to apply so‐called “z‐shim” gradient pulses in the slice direction between the RF excitation and the data acquisition, which ameliorates the dephasing effect of the field inhomogeneities 5,6 . With single, slice‐specific z‐shim gradient pulses, the signal intensity in the human spinal cord could be enhanced significantly without affecting the temporal resolution 7 —an approach that has been applied in several spinal cord fMRI studies 8–12 …”

“…5,6 With single, slice-specific z-shim gradient pulses, the signal intensity in the human spinal cord could be enhanced significantly without affecting the temporal resolution 7 -an approach that has been applied in several spinal cord fMRI studies. [8][9][10][11][12] Simultaneous multislice (SMS) imaging 13,14 is an attractive tool to accelerate fMRI acquisitions and is used widely in human brain applications. It involves multiband RF excitations that cover several, separated frequency bands to excite multiple, distant slices simultaneously.…”