 |
 |
I have read the archives on Grey Matter thresholds, but I am still confused. First, is the Global Mean the average of _all_ the voxels in the matrix or just the brain voxels ? If the Global Mean is just the brain voxels, then how does SPM know which voxels to include in the calculation of the Global Mean. Let me give an explicit example. Take a PET image with a 128 X 128 matrix and 63 slices. Let the Global Mean be brain voxels only, the Global Mean be 50, and the grey matter threshold be the default - 80% (0.8). Is it then correct that 1) A voxel would have to have a value greater than 40 to be included in the search volume. AND a voxel with a value of 25 (50%) of global mean would be excluded from the search volume OR is it correct that 2) A voxel with a value of 11 would be included in the search volume because it is _within_ 80% of the Global Mean. In other words, A) does a voxel need to be greater than 0.8 * Global Mean to be included in the search volume OR B) does a voxel need to be greater than 0.2 * Global Mean to be included in the search volume. It appears to me that alternatives 1 and A are what SPM uses, since changing the Grey Matter threshold to 0.6 leads to more voxels in the search volume. If 2 and B are what SPM uses, then setting the grey matter threshold to 0.6 would exclude more voxels, since a voxel would have to be greater than 0.4* Global Mean to be included. If 1 and A are SPM uses, then 0.8 seems to be a overly stringent threshold since some sub-cortical areas may only be 0.6 - 0.7 * Global Mean. If 2 and B are what SPM uses, then 0.8 may be overly liberal and include substantial amounts of white matter. sg %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%