Paul:
There is a bias correction step in the
segmentation "module" but it is different than
the bias correction that is used when you click
bias correction. Actually it was different in
spm2, I"m not 100% sure for spm5. In some
previous posting john said that the bias
correction algorithm in the segmentation program
is more robust or somehow better. I have also
noticed what you have, which is that sometimes
the other bias correction program performs better
than the one included with segmentation. Not sure
why, perhaps John can elaborate further.
darren
At 01:55 PM 1/11/2006, Paul Macey wrote:
>Hi
>
>I have been playing with the unified
>segmentation/bias correction/normalization in
>SPM5 and I have a question: should segmentation
>and normalization be performed on bias corrected
>images? In other words, run the "segment"
>procedure once to produce a bias corrected
>image, and run it again using the bias corrected image to produce the segments.
>
>Here is an example that illustrates why this
>seems like a good idea (compare images 2 and 4):
>1 - raw image (mean of motion-corrected series)
>[]
>
>2 - segmented gray matter (output from applying Segment to image 1)
>
>"bad"
>[]
>
>3 - bias-corrected mean (output from applying Segment to image 1)
>[]
>
>4 - segmented gray matter from bias-corrected
>(output from applying Segment to image 3)
>
>"good"
>[]
>
>
>The bias correction seems to do a great job, and
>the second-pass segmented gray matter image (#4 in table) seems good.
>
>However, I had assumed that the unified
>procedure avoids the need for iterating through
>these preprocessing steps, but that assumption
>seems incorrect. I thought that the unified
>procedure conceptually applies the bias
>correction and then segments according to
>bias-corrected image intensities (and priors).
>However, in this albeit extreme case this does
>not seem to be what is going on.
>
>Regards,
>Paul Macey
>Assistant Researcher
>Department of Neurobiology, UCLA
>10833 Le Conte Avenue
>Los Angeles, CA 90095-1763
>Email <mailto:[log in to unmask]>[log in to unmask]
>
>
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Darren R. Gitelman, M.D.
Cognitive Neurology and Alzheimer¹s Disease Center
Northwestern Univ., 320 E. Superior St., Searle 11-470, Chicago, IL 60611
Voice: (312) 908-9023 Fax: (312) 908-8789
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