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Dear SPM Experts: I'm interested in the optional binarization step in VBM John mentioned in Human Brain Function 2nd ed. It says, "A further possible step after segmentation would be the binarization of the resulting tissue class images. Many tissue classification methods produces images where each voxel is the a posteriori probability that that voxel should be assigned to a particular tissue type according to the model. These probabilities are values between zero and one. Binarization would involve assigning each voxel to its most likely tissue class." I have been trying to implement this step of binarization, but I'm struggling with setting threshold for this binarization. This is because non gray matter regions have some values other than zero (such as 0.01, 0.03 etc), so simply performing Imcalc i1>0 results in covering other regions than gray matter. I change the threshold manually right now (eg i1>0.04, which seems to be pretty good to me), but I think there has to be some good formula to set the threshold to extract just gray matter. And also I'm wondering if there is a way to compare the segmented image with the binarization image and see whether right regions are binarized or not. Could you please let me indicate how to do that? I would be grateful for any suggestions. Regards, Kiyotaka -- Kiyotaka Nemoto Department of Psychiatry Ibaraki Prefectural Tomobe Hospital Ibaraki, Japan [log in to unmask]