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my quick answer as I dash to rounds is that i think there are differences. I can send a matlab function that realigns tags to tags and controls to controls later today. ciao. darren At 09:50 AM 7/13/2005, Geoffrey K Aguirre wrote: >On Jul 13, 2005, at 1:03 PM, Ze Wang wrote: > >> For ASL images, I always realign them before subtraction, and >>for realignment, I just follow the general procedure, although >>Geoffrey >>Aguirre recommends the separate realignment. > > Because the label and control images have systematically >different image intensities, it seems wise to realign them back to >separate label and control initial images. I have, however, never >tested to see if this makes any substantive difference. > >>According to >>JJ Wang's paper, spatial smoothing is very important for ASL images. > > The concept here is that whereas spatial smoothing of BOLD data >is associated with some deleterious effects upon the intrinsic >temporal noise structure, this unwanted effect is absent for ASL >data. In other words, spatial smoothing of ASL data provides the full >benefit that would be expected. There is no necessary incentive to >smooth ASL data, however, beyond what would be desired to maximize >detection of spatially extended signal changes. > > For those interested in the technical details: BOLD fMRI data >have enhanced noise at low temporal frequencies. These low-frequency >temporal fluctuations tend to share phase across space, so spatial >smoothing of BOLD data acts to enhance the noise present at low >temporal frequencies. > >Geoff > >-- > >Geoffrey Karl Aguirre, M.D., Ph.D. >[log in to unmask] >Assistant Professor of Neurology >Hospital of the University of Pennsylvania >Center for Cognitive Neuroscience >Philadelphia, PA ------------------------------------------------------------------------- Darren R. Gitelman, M.D. Cognitive Neurology and Alzheimer¹s Disease Center Northwestern Univ., 320 E. Superior St., Searle 11-470, Chicago, IL 60611 Voice: (312) 908-9023 Fax: (312) 908-8789 -------------------------------------------------------------------------