I recently requested a feature to be added to SPM: the option to perform a logarithmic transform on series of fMRI images during preprocessing, thus transforming them from arbitrary scanner units to meaningful units of percentage signal change. With some helpful direction from Guillaume Flandin I wrote a small utility that acts as a toolbox and that adds precisely this functionality to the batch processing functions. I've made it available here in case others are interested to give it a try:
http://audio-fmri.langers.nl/downloads.html
(1st link under matlab routines; readme-file included)
It works for me; if it does for you too, or if for some buggy reason it doesn't, let me know!
Best,
Dave
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