On Fri, Aug 14, 2009 at 12:21 AM, Kwaku Akrofi
<[log in to unmask]> wrote:
Hi All,
I have a few questions regarding the Normalisation step of SPM5/8:
1. Are the template images that come with SPM in MNI or Tailarach space? A
colleague has been asking me this and I don't know the answer.
MNI space
2. My colleague is also asking me about AC-PC alignment. I haven't seen
anything like that in SPM5/8 so I'm assuming it's either automatically done or
unnecessary. Am I right? Is so, why?
AC-PC alignment is not done automatically; but needs to be performed manually -- in some cases. The "unified segmentation" algorithm as well as normalization algorithm both work on the basis that the solution is the minimization of a cost function. As the translation or rotation of the brain necessary to match the template increases, the value of the cost function also increases. Thus, it is possible to reach a minimum of the cost function without normalizing the brain properly if each the translation or rotation is substantially different than the template. The solution is to make the origin of the image the AC (+/-10mm) and rotate the brain so that the AC-PC line matches that in the template (+/-20 degrees). This will ensure that the brain is properly normalized.
As a side note, AC-PC alignment is a term that AFNI uses as an intermediary in their talairach normalization, or at least used to use.
3. I've been following the Face Data example in Chapter 29 of the SPM5
manual. There, images are normalised (Normalise:Write option) using
parameters obtained from segmentation of the subject's structural image.
Whereas the options in Normalise have an "ICBM Space Template", those in
Segmentation have separate templates for Europeans and Asians.
3a. What is the best choice when a group comprises Europeans, Asians and
Africans? Would "Average sized template" be appropriate?
Thanks and regards,
Kwaku.
--
Best Regards, Donald McLaren
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D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
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