Hello all,
I am trying to run ROI analyses by taking a large ROI mask (derived from a group analysis), applying it an individual subject's GLM analysis, and extracting the N (for example, 50) most active voxels within that mask. In theory, I would:
1) reshape the 3-dimensional image array into a 1-dimensional vector
2) sort the vector in descending order
3) find the value of the Nth item in the vector
4) use imcalc to find all the voxels that are greater than or equal to that value
However, I imagine there's some better/more clever way to do this, either in imcalc or elsewhere. I'm also concerned that I won't necessarily be able to get exactly N voxels, in case there are some duplicate values (However, I don't think this is a big problem because images are stored in floating point, right? So it would not likely have repeats...). Can anyone help? Thanks!
Dani
Daniel Simmonds
Developmental Cognitive Neurology
Kennedy Krieger Institute
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