Impact of through-slice gradient optimization for dynamic slice-wise shimming in the cervico-thoracic spinal cord

This study investigates the effectiveness of through-slice gradient optimization in dynamic slice-wise B₀ shimming of the cervico-thoracic spinal cord to enhance signal recovery in gradient-echo (GRE) EPI sequences commonly used in functional MRI studies.

Six volunteers underwent MRI acquisitions with dynamic shim updating (DSU) using a custom-built 15-channel AC/DC coil at 3 T. A magnetization-prepared rapid gradient echo was acquired to segment the spine and to provide a clear image of the anatomical region of interest in the figures. GRE B₀ field maps were used to measure field homogeneity before and after shimming; the pre-shimming field map was used for optimization. Shimmed fields were dynamically applied to GRE-echo planar imaging acquisitions simulating functional MRI acquisitions under two shimming conditions: DSU with and without through-slice gradient consideration.

DSU with through-slice gradient optimization increased the temporal signal-to-noise ratio at the T2 vertebral level by 201% compared with volume-wise shim and by 28% compared with DSU without through-slice. The residual geometric distortions were similar between DSU with and without through-slice gradient optimization. A high signal loss penalty parameter was effective in simulations for reducing through-slice gradient-induced signal loss but led to instability and reduced image quality in actual acquisitions due to excessive in-plane B₀ inhomogeneities.

Introducing a carefully balanced through-slice gradient parameter in slice-wise shimming substantially improves signal recovery in axial GRE images of the spinal cord, without compromising in-plane homogeneity. This effective approach can advance spinal cord functional MRI applications at high field strengths.