title: Optimal EPI parameters for reduction of susceptibility-induced BOLD sensitivity losses: a whole-brain analysis at 3 T and 1.5 T.
abstract: Most functional magnetic resonance imaging (fMRI) studies record the blood oxygen level-dependent (BOLD) signal using fast gradient-echo echo-planar imaging (GE EPI). However, GE EPI can suffer from substantial signal dropout caused by inhomogeneities in the static magnetic field. These field inhomogeneities occur near air/tissue interfaces, because they are generated by variations in magnetic susceptibilities. Thus, fMRI studies are often limited by a reduced BOLD sensitivity (BS) in inferior brain regions. Recently, a method has been developed which allows for optimizing the BS in dropout regions by specifically adjusting the slice tilt, the direction of the phase-encoding (PE), and the z-shim moment. However, optimal imaging parameters were only reported for the orbitofrontal cortex (OFC) and inferior temporal lobes. The present study determines the optimal slice tilt, PE direction, and z-shim moment at 3 T and 1.5 T, otherwise using standard fMRI acquisition parameters. Results are reported for all brain regions, yielding a whole-brain atlas of optimal parameters. At both field strengths, optimal parameters increase the BS by more than 60% in many voxels in the OFC and by at least 30% in the other dropout regions. BS gains are shown to be more widespread at 3 T, suggesting an increased benefit from the dropout compensation at higher fields. Even the mean BS of a large brain region, e.g., encompassing the medial OFC, can be increased by more than 15%. The maps of optimal parameters allow for assessing the feasibility and improving fMRI of brain regions affected by susceptibility-induced BS losses.
authors.affiliation: Wellcome Department of Imaging Neuroscience, Institute of Neurology, University College London, London, WC1N 3BG, UK. n.weiskopf@fil.ion.ucl.ac.uk
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weiskopf
authors.forename: Chloe
authors.initials: C
authors.lastname: Hutton
authors.forename: Oliver
authors.initials: O
authors.lastname: Josephs
authors.forename: Ralf
authors.initials: R
authors.lastname: Deichmann
mesh_terms: D000328:Adult; D001921:Brain / Q000033:anatomy & histology*; D002560:Cerebrovascular Circulation; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010100:Oxygen / Q000097:blood; D012016:Reference Values; D012680:Sensitivity and Specificity; D013702:Temporal Lobe / Q000033:anatomy & histology
chemical_list: D010100:Oxygen