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Hi Tony,

You can use SPM's Imcalc Function to mask for gray matter only by applying each subject's gray matter segmentation to the ROIs. 

Example: 
i1:  gray matter segment (use normalized)
i2: ROI

FORMULA: (i1>.5).*i2       

The formula within the parenthesis converts the segment to a binary image based on the threshold .5 (for example) and applies that image to the ROI.

Hope that helps,

Vy T.U. Dinh
Research Assistant, Neurological Sciences
Rush University Medical Center
Phone: (312) 563-3853
Fax: (312) 563-4660
Email: [log in to unmask]
From: SPM (Statistical Parametric Mapping) [[log in to unmask]] on behalf of tony han [[log in to unmask]]
Sent: Wednesday, August 10, 2011 3:24 AM
To: [log in to unmask]
Subject: [SPM] ROI across individuals

 
Dear all,
 
  I have several ROIs (3mm-diameter spheres) and want to use these ROIs to extract data from functional images. I create these ROIs using avg152T1 as reference. These ROIs should be in gray matter. However, due to anatomical variability between individuals, some ROIs are in white matter of several subjects. How could I solve this problem?
 
all the best,
Tony