My best guess is that the modulated images are stored as some form of integer
datatype, with a scalefactor. To help me narrow down the cause of the
problem, I need a few more details before I try to figure out the cause.
Which version of SPM are you using?
Which options did you use to generate the modulated images?
Why are they called m0wc1*.nii instead of mwc1*.nii?
Best regards,
-John
On Tuesday 05 May 2009 21:05, Neil Chatterjee wrote:
> Dear SPMers,
>
> I came across several oddities in my m0wc1*.nii (modulated, normalized,
> gray matter) images yesterday, and I am hoping someone here can shed some
> light on the situation. Apologies for the length of this correspondence,
> but I wanted to be precise in explaining the problem observed.
>
> Anyways, looking at a typical m0wc1*.nii image, the voxel values have the
> following strange properties:
>
> 1) There are no voxels with a value greater than 1
> 2) There are ~580,000 voxels with a value of exactly 1. Actually, they all
> have a value of exactly 1.000000059138983, which in itself is kind of
> strange. 3) Of the non-zero valued voxels in the m0wc1*.nii image, 42.4% of
> them are exactly (to double precision) the same as in the wc1*.nii images.
>
> In a previous thread, Dr. Ashburner said that
>
> > The contents of a modulated image are a voxel compression map multiplied
>
> by tissue belonging
>
> > probabilities (which range between zero and one)
> > ...
> > The total volume of grey matter in the original image can be
> > determined by summing the voxels in the modulated, spatially
> > normalised image and multiplying by the voxel volume (product of voxel
> > size).
>
> That the total volume of gray matter in the original image can be
> determined by integration implies conservation of probability of gray
> matter. It follows that the voxel compression map would have values >1 in
> areas where there has been positive compression (shrinking) and values <1
> in areas where there has been negative compression (expansion). With this
> in mind, the properties described above lead me to the following
> conclusions:
>
> A) There are no voxels with high probability (p~1.0) of being gray matter
> that were positively compressed (shrunk) in normalizing, else there would
> exist modulated voxels with value > 1.
> B) There exist several voxels that either i) had a gray matter probability
> of exactly 1 and were not compressed even one iota or ii) were compressed
> in exact (to double precision!) proportion to their uncertainty of being
> gray matter. Else there would not exist modulated voxels with value = 1
> exactly C) 42.4% of probable gray matter voxels neither shrunk nor expanded
> in the process of morphing to standard space.
>
> I just can't wrap my head around any of those conclusions. I feel like
> either I'm totally misunderstanding what happens with modulation or
> something is very very wrong with my images. I understand that the
> non-linear only modulation (m0 instead of m) changes things, but
> substituting "non-linear compression" for "compression" above does not make
> the observations any less strange. If any guru out there can make sense of
> all this, it would be much appreciated.
>
> Regards,
> Neil
>
> Neil Chatterjee
> Research Assistant
> Stanford Systems Neuroscience and Pain Lab
> 650-724-0522
> [log in to unmask]
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