Simultaneous multislice imaging with slice-specific z-shim

Abstract

Purpose: To implement slice-specific z-shim in simultaneous multislice (SMS) imaging in order to minimize signal losses in slice-accelerated T₂ *-weighted acquisitions, such as for spinal cord functional neuroimaging.

Methods: The RF envelopes of the individual slice bands are temporally shifted on the plateau of the slice-selection gradient pulse before being combined to the multiband RF envelope. Thus, optimum z-shims can be realized for each slice of an SMS excitation, which is in contrast to conventional z-shimming. EPI with 2-fold SMS acceleration was performed on a 3T whole-body MR system in phantoms and the cervical spinal cord of healthy volunteers (i) without z-shim, (ii) with conventional z-shim using the average value of the slices of the SMS excitation, and (iii) with optimal, slice-specific z-shims for each slice using envelope shifts.

Results: Phantom experiments demonstrate the equivalence of the envelope shift and conventional z-shimming for non-SMS excitations. With SMS, the best image quality is obtained with "mixed" z-shim, where only the z-shim differences of the slices of an SMS excitation are implemented by an envelope shift while their mean z-shim is applied conventionally with a gradient pulse after the echoes acquired for N/2 ghost correction. In phantoms and in vivo, this setup outperforms the approaches without slice-specific z-shim with respect to signal amplitude and temporal SNR at the expense of slight TE differences (<1 ms) between the slices.

Conclusion: With RF envelope shifts, slice-specific z-shims can be combined with SMS imaging, which could improve slice-accelerated functional neuroimaging in the spinal cord.

Keywords: RF envelope shift; SMS; multiband RF pulses; simultaneous multislice imaging; z-shim.