Evidence for a vascular contribution to diffusion FMRI at high b value
- Karla L. Miller†,‡,
- Daniel P. Bulte†,
- Hannah Devlin†,
- Matthew D. Robson§,
- Richard G. Wise¶,
- Mark W. Woolrich†,
- Peter Jezzard†, and
- Timothy E. J. Behrens†,‖
- †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; and
- ¶Cardiff University Brain Research Imaging Centre (CUBRIC), Cardiff University, Cardiff CF10 3AT, United Kingdom
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Edited by Marcus E. Raichle, Washington University School of Medicine, St. Louis, MO, and approved November 8, 2007 (received for review August 2, 2007)
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.
Footnotes
- ‡To whom correspondence should be addressed. E-mail: karla{at}fmrib.ox.ac.uk
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Author contributions: K.L.M., D.P.B., R.G.W., M.W.W., P.J., and T.E.J.B. designed research; K.L.M., D.P.B., H.D., and M.D.R. performed research; K.L.M. and T.E.J.B. analyzed data; and K.L.M. and T.E.J.B. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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This article contains supporting information online at www.pnas.org/cgi/content/full/0707257105/DC1.
- © 2007 by The National Academy of Sciences of the USA
