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SPM  December 2007

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Subject:

Re: Segmentation - VBM

From:

John Ashburner <[log in to unmask]>

Reply-To:

John Ashburner <[log in to unmask]>

Date:

Thu, 20 Dec 2007 14:27:16 +0000

Content-Type:

text/plain

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> I have a 'stupid' question: for how many millimeters could we say the
> SPM5 segmentation can introduce errors (any ref?)

This is entirely data dependent.  Statistics are presented for some simulated 
data in the "Unified Segmentation" paper, but the behaviour with real data is 
likely to be different depending on signal/noise, artifacts etc.

The current "gold standard" for assessing image segmentation algorithms 
involves comparing their results against manually segmented images. This 
should imply that human judgement is perfectly acceptable for assessing how 
well an image is segmented.

It also depends on what is meant by the term "grey matter".  The segmentation 
algorithm defines it in terms of a mathematical model of the data.  The only 
information it has about grey matter is in terms of the tissue probability 
maps in the spm5/tpm directory.

> I have a reviewer saying that effects observed in the visual cortex and
> cerebellum in a VBM study of mine can be attributed to segmentation
> error (well obviously it would also mean that the error was always in
> the same direction ... )

The reviewer has a perfectly plausible explanation.  The differences could 
also be attributed to registration errors (see Bookstein's criticism of VBM), 
cortical thickness differences, folding differences, differences in the MR 
properties of the tissue, partial volume effects etc.  A significant 
difference among the pre-processed data has many possible explanations, but 
it would essentially predict where the null hypothesis (of no difference) 
would be deemed unlikely to hold if the same pre-processing was applied to 
similar images of a different set of subjects from the same populations.

For VBM studies, such differences are usually explained in terms of tissue 
volume, but other explanations are always possible.   Similar issues apply in 
all branches of science.

Best regards,
-John

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