Hi Mahinda,
to add a few words to Stam's answer: very generally, you can see an increase of mode when there is a transition from "disc" shape to more "cigar" shape anisotropy.
This can mean a selective degeneration in two crossing fibres in a neurodegenerative disorder for instance (as in my poster that you're mentioning), this can also mean a more linear anisotropy, which you would maybe see in healthy subjects with, say, motor learning for instance, just like an increase of FA would then also be seen.
Hope this helps,
Gwenaelle
--- En date de : Mer 23.6.10, Stamatios Sotiropoulos <[log in to unmask]> a écrit :
> De: Stamatios Sotiropoulos <[log in to unmask]>
> Objet: Re: [FSL] Anisotropy mode
> À: [log in to unmask]
> Date: Mercredi 23 juin 2010, 22h03
> Hi Mahinda,
>
> The mode characterises directly the tensor shape. A
> negative mode indicates an oblate profile (i.e. l1 roughly
> equal to l2, both much larger than l3) and a positive mode a
> prolate profile (l1 much larger than both l2 and l3), where
> l's are the tensor eigenvalues. Therefore, you can interpret
> directly changes in the mode to changes in the relative
> relationship between the tensor eigenvalues.
>
> It has been empirically observed that voxels with two
> crossing fibres tend to have an oblate tensor shape (and
> this approximation improves as the crossing becomes more
> perpendicular, the fibres more similar and the volume
> fractions more identical). That's why a negative mode value
> may indirectly suggest a fibre crossing.
>
> Cheers,
> Stam
>
>
>
> On 23 Jun 2010, at 17:33, Mahinda Y wrote:
>
> > Dear All,
> >
> > I have seen some recent and papers using the mode of
> anisotropy to distinguish between where changes in FA are
> due to changes in voxels containing 2 fibre bundles to 1
> fibre bundle (causing an increase in the mode to more
> positive values) - is this interpretation only valid
> in areas where one knows there are crossing fibres - how
> does one interpret such changes in the mode in areas where
> crossing fibres are less likely ?
> >
> > Thanks.
> >
> > Mahinda
> >
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Gwenaëlle Douaud, PhD
FMRIB Centre, University of Oxford
John Radcliffe Hospital, Headington OX3 9DU Oxford UK
Tel: +44 (0) 1865 222 523 Fax: +44 (0) 1865 222 717
www.fmrib.ox.ac.uk/~douaud
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