Sascha Brunheim1,2, Sören Johst3, Viktor Pfaffenrot3,4, Stefan Maderwald3, Harald H Quick3,4, Benedikt A Poser5. 1. Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Kokereiallee 7, Building C84, 45141, Essen, Germany. sascha.brunheim@uni-due.de. 2. High Field and Hybrid MR Imaging, University Hospital Essen, Hufelandstrasse 55, 45147, Essen, Germany. sascha.brunheim@uni-due.de. 3. Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Kokereiallee 7, Building C84, 45141, Essen, Germany. 4. High Field and Hybrid MR Imaging, University Hospital Essen, Hufelandstrasse 55, 45147, Essen, Germany. 5. Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht Brain Imaging Centre, Maastricht University, PO Box 616, 6200 MD, Maastricht, The Netherlands.
Abstract
OBJECTIVE: A new technique for 2D gradient-recalled echo echo-planar imaging (GE-EPI) termed 'variable slice thickness' (VAST) is proposed, which reduces signal losses caused by through-slice susceptibility artifacts, while keeping the volume repetition time (TR) manageable. The slice thickness is varied across the brain, with thinner slices being used in the inferior brain regions where signal voids are most severe. MATERIALS AND METHODS: Various axial slice thickness schemes with identical whole-brain coverage were compared to regular EPI, which may either suffer from unfeasibly long TR if appropriately thin slices are used throughout, or signal loss if no counter-measures are taken. Evaluation is based on time-course signal-to-noise (tSNR) maps from resting state data and a statistical group-level region of interest (ROI) analysis on breath-hold fMRI measurements. RESULTS: The inferior brain region signal voids with static B0 inhomogeneities could be markedly reduced with VAST GE-EPI in contrast to regular GE-EPI. ROI-averaged event-related signal changes showed 48% increase in VAST compared to GE-EPI with regular "thick" slices. tSNR measurements proved the comparable signal robustness of VAST in comparison to regular GE-EPI with thin slices. CONCLUSION: A novel acquisition strategy for functional 2D GE-EPI at ultrahigh magnetic field is presented to reduce susceptibility-induced signal voids and keep TR sufficiently short for whole-brain coverage.
OBJECTIVE: A new technique for 2D gradient-recalled echo echo-planar imaging (GE-EPI) termed 'variable slice thickness' (VAST) is proposed, which reduces signal losses caused by through-slice susceptibility artifacts, while keeping the volume repetition time (TR) manageable. The slice thickness is varied across the brain, with thinner slices being used in the inferior brain regions where signal voids are most severe. MATERIALS AND METHODS: Various axial slice thickness schemes with identical whole-brain coverage were compared to regular EPI, which may either suffer from unfeasibly long TR if appropriately thin slices are used throughout, or signal loss if no counter-measures are taken. Evaluation is based on time-course signal-to-noise (tSNR) maps from resting state data and a statistical group-level region of interest (ROI) analysis on breath-hold fMRI measurements. RESULTS: The inferior brain region signal voids with static B0 inhomogeneities could be markedly reduced with VAST GE-EPI in contrast to regular GE-EPI. ROI-averaged event-related signal changes showed 48% increase in VAST compared to GE-EPI with regular "thick" slices. tSNR measurements proved the comparable signal robustness of VAST in comparison to regular GE-EPI with thin slices. CONCLUSION: A novel acquisition strategy for functional 2D GE-EPI at ultrahigh magnetic field is presented to reduce susceptibility-induced signal voids and keep TR sufficiently short for whole-brain coverage.
Keywords:
7 Tesla ultrahigh field MRI; Echo-planar imaging; Repetition time; Slice thickness; Susceptibility artifact; fMRI