Hi Derek - thanks for the quick response.
> Congrats on putting some lifeblood back into the mailing list!
:)
> I wondered whether you could also put up the two 'raw' images so that we
> could have a gander.
http://www.fmrib.ox.ac.uk/~behrens/b2500.tiff
http://www.fmrib.ox.ac.uk/~behrens/b4500.tiff
The AP diffusion sensitisiation is not obvious, but it is there.
(e.g. - callosal areas are slightly brighter than arcuate areas).
The reason for trying higher b was to get more sensitisation - hohum!
> My initial thought, before looking at the image, is that you might have
> been looking at the restricted component - (i.e. the hindered component
> has already been suppressed).
This always confuses me a bit - Are you saying that signal from the
restricted component should be insensitive to, or drop linearly with, the
b-value?
Do you (or anybody else) know anything from the literature about this
issue in fixed brains.
>
> Looking at the image, a few more bits of info might be helpful:
>
> - was the diffusion time unusually long?
>
I'm afraid I don't know. I'd have to ask the chaps in yankland who
actually acquired the image, and they're asleep at the mo.
> - what was the typical SNR of the two images?
>
just zooming around the images - something like 15-20 - so pretty good.
(obviously better in the b2500)
> - the dark patches at the boundaries of the lateral ventricles - is the
> 2500/4500 ratio a *positive* value here? Is this a slight
> misregistration, or some other effect?
Nice spot - Looks like an eddy current effect to me. (very slight phase
encode stretch and shift.
Thanks v. much for your help
T
>
> D
>
>
>
>
> _____________________________
> Centre for Neuroimaging Sciences
> Institute of Psychiatry, P089
> De Crespigny Park
> London, SE5 8AF
> United Kingdom
> Tel: +44-207-919-3057
> Fax: +44-207-919-2116
>
> -----DIFFUSION <[log in to unmask]> wrote: -----
>
> To: [log in to unmask]
> From: Tim Behrens <[log in to unmask]>
> Sent by: DIFFUSION <[log in to unmask]>
> Date: 10/31/2005 08:29AM
> 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
>
> --
> -------------------------------------------------------------------------------
> 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
> -------------------------------------------------------------------------------
>
>
>
>
--
-------------------------------------------------------------------------------
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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