Evidence for a vascular contribution to diffusion FMRI at high b value.
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.
Miller
Karla L
KL
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.
Bulte
Daniel P
DP
Devlin
Hannah
H
Robson
Matthew D
MD
Wise
Richard G
RG
Woolrich
Mark W
MW
Jezzard
Peter
P
Behrens
Timothy E J
TE
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