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Rowena Handley
Psychological Medicine
TEL: 0207 848 0599
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
On Behalf Of John Ashburner
Sent: 02 June 2008 15:09
To: [log in to unmask]
Subject: Re: [SPM] HDW question
> I am wondering, in the HDW toolbox, is it possible to specify a mask
> image over which the likelihood potentials are calculated?
No. Not without quite a lot of additional coding.
> Or does it even make
> sense to do that? I am wondering if it is possible to constrain the
> warping algorithm such that its cost function is calculated only in
> the masked regions. I have noticed that in some images where there is
> extremely bright dura near the brain, the presence of the dura in one
> image but not the other seems to be driving the warping, i.e. the
> warping field calculated over the brain voxels near this bit of dura
> seems to be heavily influenced by the presence or absence of the dura.
> In other words, we are finding large regions of "expansion" or
> "contraction" in the brain voxels near the bright dura, but it seems
> to be a result of the algorithm trying to match the dura between the
> two scans. I have tried simply extracting the brain from both images,
> which seems to help in some ways, but I believe it introduces
> artificial edges which have their own unintended effects on the
> result. Sorry for the long winded explanation, but is there a way to
> mask out certain voxels from the likelihood potential calculation
> without introducing an artificial edge? Thanks for any insight you can
> offer,
I don't have any bright ideas that would help here. Are you doing
inter-subject registration? If so, then you could try the DARTEL
toolbox, which warps GM to GM and WM to WM.
Best regards,
-John
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