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Chaitee, SPM creates a mask of the brain during the analysis. Your ROI may have voxels in it that are not included in the SPM mask. If you send us your ROI.img/hdr files and the SPM mask.img/hdr files (located in the directory with your SPM results), we can see if that's what is going on. ------------------------------------------------------------------------ ---------- Joseph A. Maldjian, MD Associate Professor, Dept. of Radiology and Biomedical Engineering Director Advanced Neuroscience Imaging Research Core (ANSIR) Wake Forest University School of Medicine Office: 336 716-2815 fax: 336 716-2870 email: [log in to unmask] website: www.ansir.wfubmc.edu/maldjian.htm <http://www.ansir.wfubmc.edu/maldjian.htm> ------------------------------------------------------------------------ ---------- -----Original Message----- From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Chaitee Sengupta Sent: Tuesday, May 03, 2005 4:27 PM To: [log in to unmask] Subject: [SPM] small volume ROI analysis and search volume We are doing an fMRI ROI analysis using manually created (in MRIcro) ROI image masks and the WFU pickatlas interface with SPM2 utilizing SVC. We are finding that the volume our ROIs as drawn and the search volume reported on the SPM results do not match. Here are the steps we have followed: 1. Draw ROI on normalized structural image (2x2x2mm voxels) in MRIcro 2. Export ROI as Analyze image 3. In SPM Results, use ROI analysis (WFU Pickatlas interface), specify mask image file. In one case the ROI we drew had 119 voxels, but the SPM2 analysis search volume was reported as 113 voxels with resels reported as 0.3 . In another case the ROI was 86 voxels, but the SPM2 search volume was 23 voxels with 0.1 resels. Why are we seeing the difference between the original ROI and the search volume? How does SPM calculate the search volume when doing an ROI analysis? Thanks, Chaitee Sengupta Research Assistant Cognitive Neuropsychology Lab University of Washington, Seattle