In addition to what Joe said, the locations of rCBF and BOLD changes
differ. rCBF effects are believed to be located in the arteries mostly,
while BOLD effects occur more strongly in the veins. Thus, BOLD
activation patterns are usually biased towards the sulci and a
voxel-by-voxel comparison of fMRI and rCBF images will probably result
in a slight mismatch of activation foci.
Just my tuppence
Johannes
----- Original Message -----
From: Joseph Devlin <[log in to unmask]>
Date: Friday, February 13, 2004 3:44 pm
Subject: Re: [FSL] question - interactions
> Dear Arun,
>
> Ambitious idea. I wonder about your basic assumption though:
>
> >The assumption from this point on is that there is a linear
> >relationship between the PET data and my fmri results on a voxel by
> >voxel level. In the preprocessing, I also took care that the final
> >smoothing of the data is the same in both modalities (i.e. the
> fmri data
> >was smoothed more than the pet data).
>
> I think this may be a large assumption to make. The most obvious
> fact is
> that BOLD and rCBF are not the same thing, even though they are
> clearlyrelated, and thus their response characteristics probably do
> not meet this
> assumption. In fact, there is evidence from some parametric
> studies done
> in London showing that rCBF has a much more linear response profile
> thanBOLD (see Rees et al. Neuroimage. 1997 Nov;6(4):270-8). In
> addition,voxel-by-voxel there are differences due to the
> distortions in the B0 field
> so that in the BOLD case, you get a shift towards draining veins
> and also
> macroscopic susceptibility effects such as distortion and signal
> loss, none
> of which are present in PET images. I would expect these effects
> to be
> rather significant in any cross-modality voxel-by-voxel comparison.
>
> I'm sorry this doesn't answer your question!
>
> Joe
>
> --------------------
> Joseph T. Devlin, Ph. D.
> FMRIB Centre, Dept. of Clinical Neurology
> University of Oxford
> John Radcliffe Hospital
> Headley Way, Headington
> Oxford OX3 9DU
> Phone: 01865 222 738
> Email: [log in to unmask]
>
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