Dear Feng,
It's a related issue. See http://blogs.warwick.ac.uk/nichols/entry/fwhm_resel_details/ :
> Note, if you check SPM’s arithmetic, you may find that R is not exactly V / ( FWHM X × FWHM Y × FWHM Z ). The reason is that a more
> sophisticated method for measuring the search volume is used that involves counting voxel edges and faces, etc
Also see section 3.3 in Worsley et al. (1996, "A unified statistical approach for determining significant signals in images of cerebral activation").
Best
Helmut
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