Multiple gradient- and spin-echo EPI acquisition technique with z-shimming to compensate for susceptibility-induced off- resonance effects

INTRODUCTION – Images obtained with gradient-echo echo planar imaging (GRE-EPI) suffer from magnetic field variations within the brain, such as in regions adjacent to air-tissue interfaces. In these regions, magnetic field inhomogeneities cause spins to dephase more rapidly, resulting in signal dropouts. Thus, conventional GRE-EPI pulse sequences used for dynamic susceptibility-contrast perfusion-weighted imaging (DSC-PWI) or blood oxygenation-dependent functional MRI (BOLD-fMRI) usually confine image analysis to homogeneous brain regions. Compensation mechanisms based on z-shimming (originally proposed in [1]) apply additional field gradients prior to repeated signal readouts of the same imaging plane, with the drawbacks of prolonging scan time ([2],[3]) or increasing image distortions ([4],[5]). Extending PERMEATE [6], a multi-echo EPI pulse sequence developed for DSC-perfusion quantification, z-shimming gradients were added prior to the first and second echo train, to restore T2*related signal dropouts in the first echo train, while maintaining regular contrast for the following echo trains. Using this technique, functional brain activity could be successfully detected in typical dropout regions close to the auditory canals in a breath-holding fMRI experiment [7], therefore facilitating whole-brain EPI-based BOLD-fMRI with unchanged acquisition time and low spatial image distortions due to parallel imaging accelerated acquisition. In this study, we propose adding a similar z-shimming approach to a combined gradient-echo/spin-echo (GRE-/SE-) EPI sequence [8], in which each EPI train is preceded by different z-shim gradients (see Fig. 1) in order to restore susceptibility-induced signal-dropouts and facilitate full-brain coverage in combined gradientand spin-echo measurements, for application in DSC-PWI and fMRI.