Dear Oskar,
> using SPM for VBM analysis of adult healthy/Parkinson/(dementia type AD/DLB) subjects, do you need to create a study-specific template, or can/should you use a generic one, e.g. the IXI template in MNI space that is included in Christian Gaser's VBM toolbox?
In principle, I am always more comfortable using a template that is
composed of images from subjects similar to those in the current
experiment. So if doing a study on healthy adults, the IXI template
included in the VBM toolbox would be perfect. For patients, the worry
would be that if you have a template based on healthy brains, patient
brains will be more difficult to deform in order to match the
template. Thus, you have a confound (one group is easier to spatially
normalize than another group) that may impact the results.
In practice, I don't know how much this affects results, and I am not
aware of anyone who has compared this. So, the actual impact on your
comparison is hard to know.
For myself, I try to always use a template composed of equal numbers
of people from each group in question (if the study is unbalanced,
this may involve creating a template from a subset of subjects).
(Another caveat is that the number of subjects may also play into the
accuracy: if you have 10 patients, a template based on these 10 scans
may be worse than a template based on 200 healthy brains, simply
because of the amount of data available.)
Would be interested to hear others' experiences on this however…
Hope this helps!
Best regards,
Jonathan
--
Jonathan Peelle, PhD
Assistant Professor
Department of Otolaryngology
Washington University in St. Louis
Office: (314) 362-9044
http://peellelab.org || http://jonathanpeelle.net
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