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Subject:

Re: comparing single patients to a group of normals

From:

[log in to unmask] (John Ashburner)

Reply-To:

[log in to unmask] (John Ashburner)

Date:

Thu, 17 Jun 1999 11:25:40 +0100

Content-Type:

text/plain

Parts/Attachments:

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text/plain (39 lines)


> From [log in to unmask] Tue Jun 15 16:18:23 1999
> Date: Tue, 15 Jun 1999 11:19:47 -0400 (EDT)
> MIME-version: 1.0
> Subject: Re: comparing single patients to a group of normals
> Thank you for your help; I have one more question regarding this project.
> In the spatial normalization step, would it be useful to create a mean of
> the control subjects (once I've obtained 12!), normalize that mean to the
> standard PET template, and then normalize the controls and patients using
> the normalized control mean as a user-defined template?  Would this be
> better (or worse) than normalizing everybody to the standard PET template?

The original mean of the control subjects is likely to be a bit meaningless
as all the different subjects will be in different positions.  You could
normalize the mean of each of the subjects' images to the template, create a
mean of these images, and smooth with an 8mm FWHM kernel.  This image could
then serve as a template for subsequent spatial normalizations.  The mean
would need to be what some people call a "soft mean", which you can do
using the "Image Calc" button:
	select the images
	enter an output filename
	enter an expression something like:
(i1+i2+i3+i4+i5+i6)./(eps+(i1~=0)+(i2~=0)+(i3~=0)+(i4~=0)+(i5~=0)+(i6~=0))

This should give you a slightly more appropriate template for your data,
as the image contrast will be more like that in your images.  The disadvantage
of doing it this way is that it is a two pass procedure, so registration
errors will be introduced twice.  The bottom line is that your images should
end up being more consistently matched to each other, but may not be quite
as well matched to the template provided with SPM.

Regards,
-John




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