Variable slice-thickness (VAST) EPI for the reduction of signal voids in GE-EPI at 7 Tesla

Abstract

2D gradient-echo EPI of the human brain suffers from signal loss artifacts caused by susceptibility-induced gradients perpendicular to the slice plane. The problem is most pronounced near the skull's air cavities and compromises BOLD fMRI studies in regions including orbito-frontal and temporal cortex. The spins across the excited rectangular target slice profile are effectively dephased by through-plane gradient GS,z as can be seen for two slices in Fig 1 at 7 T. According to the expression on the right (see Fig 1) many methods exist to address the issue, including z-shimming[1], tailored RF excitations[2], short or slice-dependent TE[3] and the use of thin slices for a defined region[4]. The drawback is that optimizing parameters for a certain region will compromise sensitivity elsewhere and result in unfeasibly long TR for whole brain coverage. Hence, we propose a new technique dubbed 'Variable SliceThickness EPI' (VAST EPI) to reduce signal loss and keeping TR manageable in 2D GE-EPI. The slice thickness is varied across the brain, with thinner slices being used in the inferior regions where signal voids are most severe. Various scheme of slice thickness variation are evaluated and compared to regular EPI.