 |
 |
Dear Khalil and Thomas, The two volumes are actually not identical, the first one is continuous while the second is a discretised volumetric approximation of a sphere. For simple continuous shapes such as a sphere, there is a known formulae to compute the Resel counts, see section 3.2 and table 1 p.18 of this paper from Keith Worsley: http://www.math.mcgill.ca/keith/unified/unified.pdf otherwise, for voxel data, the Resel counts is computed using equations from section 3.3 of the same paper. Have a look at spm_resels.m for the implementation. This is also described in section 3.5 of this chapter from the SPM book: http://www.fil.ion.ucl.ac.uk/spm/doc/books/hbf2/pdfs/Ch14.pdf Best regards, Guillaume. On 25/04/17 17:12, Khalil I Thompson wrote: > Hey Thomas! > > > I just ran across this issue with my data as well a couple of weeks ago. > I believe the problem was that the ROI sphere will always cover the > volume you specified when originally designing the mask so centering > during VOI time-series extraction doesn't make any difference, the > volume location is always consistent. The SVC correction, however, is > completely dependent on the centered location so it won't necessarily > overlap with the ROI sphere you originally designed. Does that make sense? > > > Khalil Thompson > > Graduate Student > > Cognitive & Affective Neuroscience > > Georgia State University > > [log in to unmask] > > ------------------------------------------------------------------------ > *From:* SPM (Statistical Parametric Mapping) <[log in to unmask]> on > behalf of Arnold, Thomas <[log in to unmask]> > *Sent:* Tuesday, April 25, 2017 10:01:24 AM > *To:* [log in to unmask] > *Subject:* [SPM] SVC Resel discrepancy > > > Greetings, > > > I've performed small-volume correction using the a 6mm sphere as well as > using an ROI image of a 6mm sphere. I noted that the P-values differed > between the methods. Both spheres are centered at the same location and > have the same FWHM values, voxel number, and voxel size. Despite this > the resels are different (ROI is half that of sphere). Why would this be > the case if the resels are a function of volume and FWHM, which are the > same in both cases? > > > Thank you for any help, > Thomas Campbell Arnold > -- Guillaume Flandin, PhD Wellcome Trust Centre for Neuroimaging University College London 12 Queen Square London WC1N 3BG