Dual-polarity reconstruction for 3D multi-shot EPI improves image quality in high-resolution fMRI

High-resolution functional MRI (fMRI) at 7 T enables sub-millimeter mapping of brain activity, but 3D multi-shot echo-planar imaging (EPI) is often affected by Nyquist ghost and fuzzy ripple artifacts that degrade image quality and may bias functional interpretation. Dual-polarity GRAPPA (DPG) is a robust strategy for correcting polarity-related artifacts, yet its practical implementation and systematic evaluation in 3D multi-shot EPI for high-resolution fMRI remain limited. Here, a practical dual-polarity reconstruction pipeline was implemented for 3D multi-shot EPI and evaluated in phantom and in vivo experiments at 7 T. The method uses a two-step strategy in which acquired RO+/RO- lines are first replaced with phase-corrected data and the remaining missing samples are subsequently reconstructed with conventional GRAPPA. In the 3D EPI setting, kz-extended kernels were used to account for slice-/partition-dependent phase behavior. Relative to linear phase correction (LPC), the proposed implementation consistently reduced Nyquist ghost and fuzzy ripple artifacts across imaging protocols. In sub-millimeter resting-state fMRI, it improved temporal SNR in artifact-prone temporal-lobe regions. In sub-millimeter auditory task fMRI, it significantly reduced false-positive activations relative to LPC while showing good repeatability across repeated runs. These results provide a systematic evaluation of dual-polarity reconstruction in high-resolution 3D multi-shot fMRI and demonstrate its practical value for improving image fidelity and activation specificity in this setting.