Simultaneous multislice imaging with slice-specific z-shim.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Neuroimaging
• Pulse Sequence Design

Background:

• Simultaneous multislice (SMS) imaging accelerates MRI acquisition by exciting multiple slices at once.
• Slice-accelerated T2*-weighted imaging, crucial for functional neuroimaging, suffers from signal loss due to magnetic field inhomogeneities.
• Conventional z-shimming methods are not optimal for SMS due to the need for slice-specific magnetic field corrections.

Purpose of the Study:

• To implement and evaluate slice-specific z-shimming for simultaneous multislice (SMS) imaging.
• To minimize signal losses in slice-accelerated T2*-weighted acquisitions, specifically for spinal cord functional neuroimaging.

Main Methods:

• Developed a novel method using RF envelope shifts to achieve slice-specific z-shimming in SMS imaging.
• Performed echo-planar imaging (EPI) with 2-fold SMS acceleration on a 3T MRI system.
• Compared image quality and signal-to-noise ratio (SNR) across three conditions: no z-shim, conventional average z-shim, and slice-specific z-shim.

Main Results:

• Phantom experiments confirmed the equivalence of envelope shift and conventional z-shimming for non-SMS excitations.
• The "mixed" z-shim approach, combining envelope shifts for slice differences and conventional methods for the mean, yielded the best image quality in SMS.
• This optimized approach improved signal amplitude and temporal SNR in phantoms and in vivo compared to non-slice-specific methods, with minimal TE differences.

Conclusions:

• RF envelope shifts enable slice-specific z-shimming in SMS imaging.
• This technique has the potential to significantly improve slice-accelerated functional neuroimaging, particularly for the spinal cord.