title: Hadamard-encoded sub-slice fMRI for reduced signal dropout.
abstract: T2*-weighted blood oxygen level-dependent functional magnetic resonance imaging is adversely affected by susceptibility-induced field gradients in brain regions adjoining air interfaces that cause image distortion and signal dropout. Reducing slice thickness diminishes signal dropout but is accompanied by reduced signal-to-noise ratio (SNR). This study proposes simultaneous excitation of subslices with total width equal to the desired slice thickness, employing alternating Hadamard-encoded radiofrequency pulses coupled with incoherent addition of the subslices to achieve reduction of through-plane dephasing with minimal SNR loss but at the expense of a reduction in temporal resolution. Using a sensory task and hypercapnic challenge with breathholding (BH), results with two subslices per slice and a twofold reduction in temporal resolution show improved activation relative to a conventional acquisition. Average (eight subjects) T-scores in the BH task increased by 16% (P<.0003), and activation extent increased by 12% (not significant). In frontal brain regions, significant improvements in BH activation extent (11.4%, P<.05) and T-scores (18%, P<.0002) were demonstrated. Higher temporal resolution can be achieved by tradeoff of SNR.
authors.affiliation: Department of Radiology, Stanford University, Stanford, CA 94305-5488, USA. gary.glover@stanford.edu
authors.forename: Gary H
authors.initials: GH
authors.lastname: Glover
authors.forename: Catie
authors.initials: C
authors.lastname: Chang
mesh_terms: D000328:Adult; D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000502:physiology*; D001931:Brain Mapping / Q000379:methods*; D005071:Evoked Potentials / Q000502:physiology*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity