title: Evidence for a vascular contribution to diffusion FMRI at high b value.
abstract: Recent work has suggested that diffusion-weighted functional magnetic resonance imaging (FMRI) with strong diffusion weighting (high b value) detects neuronal swelling that is directly related to neuronal firing. This would constitute a much more direct measure of brain activity than current methods and represent a major advance in neuroimaging. However, it has not been firmly established that the observed signal changes do not reflect residual vascular effects, which are known to exist at low b value. This study measures the vascular component of diffusion FMRI directly by using hypercapnia, which induces blood flow changes in the absence of a change in neuronal firing. Hypercapnia elicits a similar diffusion FMRI response to a visual stimulus including a rise in percent signal change with increasing b value, which was reported for visual activation. Analysis of the response timing found no evidence for an early response at high b value, which has been reported as evidence for a nonhemodynamic response. These results suggest that a large component of the diffusion FMRI signal at high b value is vascular rather than neuronal.
authors: Centre for Functional MRI of the Brain (FMRIB), Oxford Centre for Clinical Magnetic Resonance (OCMR), and Department of Experimental Psychology, University of Oxford, Oxford, Oxon OX3 9DU, United Kingdom.
authors: Karla L
authors: KL
authors: Miller
authors: Daniel P
authors: DP
authors: Bulte
authors: Hannah
authors: H
authors: Devlin
authors: Matthew D
authors: MD
authors: Robson
authors: Richard G
authors: RG
authors: Wise
authors: Mark W
authors: MW
authors: Woolrich
authors: Peter
authors: P
authors: Jezzard
authors: Timothy E J
authors: TE
authors: Behrens
mesh_terms: D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology* / Q000378:metabolism / Q000473:pathology / Q000502:physiology; D001931:Brain Mapping / Q000379:methods*; D002560:Cerebrovascular Circulation; D004058:Diffusion; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008959:Models, Neurological; D015233:Models, Statistical; D009474:Neurons / Q000378:metabolism; D013997:Time Factors