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Dear FSL Experts, Let me premise this by saying that I am new to this but I have searched the FSL archives and have the following question. My goal is to calculate the total grey matter volume in an ROI and I have come across two methods: 1. The first (which is not suggested) is to simply sum values in the ROI in the GM_mod_merg_s3 file. 2. The second (which is the suggested method) is to multiply the mean GM value in the ROI by the total ROI volume. It was my understanding that the value in each voxel in the GM_mod_merg_s3 file is the GM volume of that voxel which suggests that simply summing these values would give you the GM volume of all voxels in an ROI. My question is why is the second method highly preferred? Is my understanding incorrect? If so what does the number in each voxel of a GM_mod_merg_s3 file represent? Many thanks in advance, Sante -- Sante Kotturi University of California, San Diego Cognitive Neuroscience