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Dear SPM experts,



         During the model estimation, the SPM program produces a pile of
beta*** files as well as a mask.img file.

First of all, I wonder how does SPM create this mask.img.

         Second, this mask.img contains gray values ranging from 0 to 1,
instead of logical values (0 or 1) as I thought. Why? And how can I use this
mask.img to extract the inside-brain voxels?

         The last one, there are two sets of data which were collected using
different MRI machines. We processed them respectively and got different
masks. As attached please find mask1.img and mask2.img. The former comes
from one subject of the first data set, and the latter comes from one
subject of the second data set. The latter has a sharp edge, whereas the
form is something blurred. I have checked other subjects, the mask images
for the first data set are always blurred, whereas the mask images for the
second set are always sharp. What reason causes the problem? The head
motion? Or the difference of the resolution?



         Thanks in advance for any help!



Best wishes!



     Lu FENG

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