Hi,
I've not heard of anyone doing this before; so this is equivalent for
example to
L1 - MD = L1 - (L1+L2+L3)/3
= (2L1 - L2 -L3) / 3
I'm not sure that this does really make sense - if you want
'directionality' you could just use FA, MO etc?
Wrt the 'blank' areas, not sure what's going on here - you'll need to
check out each stage of the preprocessing carefully to track down at
what stage this is occurring.
Cheers.
On 10 Nov 2008, at 07:32, India Bohanna wrote:
> Hi. I am interested in doing a TBSS analysis of lambda parallel and
> perpendicular with MD
> subtracted (to discern the magnitude of directional rather than
> general changes in water
> movement). Has anyone done this before and is this a legitimate
> approach? Any comments
> would be appreciated.
>
> Also, I have found that when I subtract MD from L1 (fslmaths L1 -sub
> MD) I get a range of
> around 0 to 0.001500 and a mean of around 0.00025. When I do
> tbss_1_preproc (TBSS v
> 1.1 and using a scale factor of 6,000,000 (gives a range of 0 to
> 9000)), this works for most
> images, but there are a couple of subjects where subtracting MD from
> L1 gives large holes
> in my images when I look at the slicesdir output. The data range and
> mean seems to be
> similar for the images with and without holes. I am puzzled as to
> what this means. Do holes
> mean negative values?
>
> I have run dtifit again and can't see any obvious problems with the
> original images.
>
> Any help would be greatly appreciated!!
>
>
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