Dear SPMers,
We intend to analyze a dataset for cerebral asymmetry using the VBM approach
in SPM5. I have not come across any paper discussing the use of SPM5 for
such a study (though there are studies with SPM99 and SPM2). After pursuing
some threads in this mail list, I have formed a tentative plan for the
processing and analysis. Any comments / suggestions would be highly appreciated
Objective: To compare brain asymmetry in two groups (one disorder and one
normal control)using VBM type analysis
1. Segment images in native space using the default asymmetric tissue priors
in SPM5
2. Create symmetrical tissue priors by flipping and averaging the default
asymmetrical priors of SPM5
2. Normalize the native space GM images to the MIN space using the
symmetrical tissue priors* (created in previous step) as the reference
images with "preserve amount" option.
3. Flip the normalized (symmetrically normalized) images to obtain a
Original Image (OI) series and a Flipped Image (FI) series.
4. Obtain a Asymmetry Image (AI) as AI=(OI-FI)/(OI+FI)*0.5
5. Smooth the AI images
6. Specify a full factorial 2x2 model with two factors - Diagnosis (levels:
disorder, normal); Gender (levels: Male, Female) and total gray as covariate.
The reasoning behind Steps 1 to Steps 3 is as follows...
Directly 'symmetrical-ising' the priors would introduce a unintended
smoothing of priors. Therefore segment in native space and normalize in a
separate step (This was gleaned form a post by Marko Wilke).The assumption
is that the default tissue priors in SPM5 are NOT smoothed. Is that true?
*Should we be using the 'symmetrical-ised' GM prior for normalization or a
'symmetrical-ised' T1 (whole brain) template instead?
**How much "template image smoothing" to apply to take into account
smoothing implied in 'symmetrical-ising'
Would like to hear from some of you before we venture into what could turn
out to be a misadventure borne out of my ignorance!
Best Regards
Harsha
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Harsha Halahalli
Research Associate
MBIAL, Department of Psychiatry
National Institute of Mental Health and Neuro Sciences(NIMHANS)
Bangalore - 560029. INDIA.
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