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Dear SPM experts, I have two questions regarding the weighting images within the normalization of PET images in SPM5: 1) I have a PET scan which is somewhat hard to normalize accurately, ie. the result is really distorted (I already tried to use coregistration first or increase regularization, etc - without success). So I thought to use a weighting image containing 1s within the brain and 0s outside (similar to the "skull stripping" approach in MRI). Interestingly, this weighting image doesn't really improve the normalization process. However, it does work when I first multiply the weighting image with the actual scan and then normalize the scan (which is then basically the brain only). I thought the two approaches would be the same - could someone explain the difference to me, please? 2) Is it possible/feasible to use a weighting image to focus on specific parts of the brain, eg. brainstem? For instance, I have a problem with (another) PET scan where the cortex is nicely registered to the template but the brainstem is not. Radioligand uptake is high for both, but of course brainstem is smaller than cortex, thus, I guess the cost function will be mostly based on the cortex - so should a weighting image help or is there a better approach? Thank you for your help in advance, Andreas. -- Andreas Hahn, MSc. Functional Neuroimaging Group - PET & fMRI Department of Psychiatry and Psychotherapy Medical University of Vienna, Austria Phone: +43 1 40400 3825 Email: [log in to unmask] Web: http://www.meduniwien.ac.at/neuroimaging/