Hi Tim,
Have you done this at multiple diffusion encoding orientations? Possibly
the variance across orientation in the 2500/4500 ratio would be a more
sensitive measure?
Geoff.
> -----Original Message-----
> From: DIFFUSION [mailto:[log in to unmask]] On Behalf
> Of Tim Behrens
> Sent: 31 October 2005 08:29
> To: [log in to unmask]
> Subject: Baffled
>
> Hello all -
> Hope you;re all well.
>
> I'm a bit baffled about some diffusion images we're getting
> from a fixed brain, and I was hoping someone might have an idea.
>
> The diffusion properties in this fixed brain are a bit
> different from a living one, so we were playing around seeing
> what the best b-values were. For example, we tried b=2500 and
> b=4500. We get some orientational sensitivity with both
> bvalues, but I was expecting to get _more_ in the b=4500. In
> fact, the drop in signal in the b=4500 image seems to be a
> constant factor throughout white matter - when I display the
> ratio of the two images (2500/4500), there is no
> orientational selectivity.
> http://www.fmrib.ox.ac.uk/~behrens/ratio.tiff
> (diffusion coding along A/P)
>
>
> Can anybody think of a reason for this?
>
> I must admit, I am a bit confused.
>
> T
>
> --
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> Tim Behrens
> Centre for Functional MRI of the Brain
> The John Radcliffe Hospital
> Headley Way Oxford OX3 9DU
> Oxford University
> Work 01865 222782
> Mobile 07980 884537
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