Variable slice thickness (VAST) EPI for the reduction of susceptibility artifacts in whole-brain GE-EPI at 7 Tesla
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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.
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.
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.
KeywordsfMRI Echo-planar imaging Susceptibility artifact Repetition time Slice thickness 7 Tesla ultrahigh field MRI
S Brunheim: project development, programming, data collection, data analysis, manuscript writing. S Johst: project development, programming, data collection, manuscript editing. V Pfaffenrot: data collection, data analysis, manuscript editing. S Maderwald: data collection, manuscript editing. HH Quick: project supervision, manuscript editing. BA Poser: project supervision, project development, programming, data analysis, manuscript editing.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflicts of interest.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
Informed consent was obtained from all individual participants included in the study.
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