Thanks guys - this is interesting.
Chris, I was also thinking of going to higher b-values because of the
fixation, but I noticed that there _was_ a big difference between signal
in grey matter in the 2500 and 4500 images, and between the GM/WM contrast
in the two images, so I thought that I was getting a decent amount of
diffusion encoding. It just didn't seem to have the orientational
properties that I was expecting. If Sean is right, that the Myelin breaks
down, then this could definitely be an explanation; But, like Chris, I
also remember reading papers reporting normal FA in fixed brains.
In fact, I have a 30 direction dataset from this brain with b=2000, and
the FA appears around normal. Would this be possible in the case of
myelin break-down?
T
On Mon, 31 Oct 2005, Sean Deoni wrote:
> Hi Tim,
>
> With the risk of being moderated off this list for stupidity, I seem
> to remember previous T1 and T2 literature which saw a reversal of
> gray/white matter contrast in formalin fixed brain samples after a
> few days - which was attributed to a break-down of the myelin
> structure over time. Could this help explain why you're not seeing
> much difference between 2500 and 4500 - the structure you are hoping
> to see isn't there?
>
> SD
>
> ----------
> "They keep creating new ways to celebrate mediocrity." Mr. Incredible
>
> Sean Deoni
> Centre for Neuroimaging Sciences
> Institute of Psychiatry, King's College London
>
> +44 020 7919 3069
> [log in to unmask]
>
>
--
-------------------------------------------------------------------------------
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
-------------------------------------------------------------------------------
|