Dear spm experts,
I obtained nearly three times the size of an activated region after
normalization and couldn't understand why. A similar question was posted
in Dec. 1999 but I thought I would post another e-mail to ask for help
because different way of analyzing data may make a difference.
I did statistics on two sets of image files: rsfmri*.img (before
normalization) and nrsfmri*.img (after normalization). Both were
processed from the same fMRI data. The original spatial resolution was
3.75x3.75x5mm and the smoothing kernal was twice the voxel size. When
doing normalization, I followed pretty much all the suggested option
except for the following:
neurological convention
don't mask brain
don't mask object
bounding box: template
voxel sizes: 4X4X4
I started with transposing the first fMRI volume to the template EPI.img
and then applied the transformation parameters to all volumes in the
time series.
No global scaling was done. The same statistical threshold was applied
on both analyses: P=0.001 (T=3.43) with extent threshold k=20 on
rsfmri*.img and k=22 on nrsfmri*.img series. The statistics table listed
smoothness FWHM = [11.9, 11.4, 15.1] mm for rsfmri*.img and FWHM =
[12.5, 13.2, 16.6] mm for nrsfmri*.img. The voxel sizes only differed by
10%: [3.75, 3.75, 5] for rsfmri*.img and [4, 4, 4] for nrsfmri*.img.
In the results, the largest cluster from rsfmri*.img had T=11.65, Z=6.88
and k=307. The largest cluster from nrsfmri*.img had T=12.18, Z=7.03,
and k=1167. Both the largest clusters were located on similar anatomic
structure. It looked like the overall Z score increased in nrsfmri*.img
and therefore, more voxels were above the same threshold.
My questions are:
1) Why was the activated region so much larger in the nrsfmri*.img than
that in the rsfmri*.img?
2) Which one is correct?
Your reply and suggestion will be very much appreciated.
Yi-Fen Yen, Ph.D.
Medical Engineering Department
Wake Forest University School of Medicine
Winston-Salem, NC 27157, USA
|