I was hoping that you might be able to give me some assistance on a
technical problem I am currently having during the spatial pre-processing
of fMRI data using SPM2.
I have 2 data sets. The first of which (data set 1) appears to be being
processed as expected. The dimensions of the 3mm anatomical slice used for
co-registration and for determining parameters for normalisation are
64x64x64 (with a voxel size of 3x3x3). The dimensions of the fMRI data set
are 64x30x64 (with a voxel size of 3x6x3). This produces a final
realigned, normalised, smoothed image with a voxel size of -2x2x2.
Images from the second data set (data set 2) however, are being initially
displayed with dimensions of 64x64x64 (with a voxel size of -3x3x3) for the
3mm anatomical data and dimensions of 64x30x64 (with a voxel size of -
3x6x3) for the fMRI data set. The final realigned, normalised, smoothed
images have a voxel size of -2x2x2.
However as you can see from the images attached, the final images produced
from data set 2 are smaller in comparison to those produced from data set
1, even though the voxel size apparently has remained the same.
Initially, with data set 2, I changed the ‘resize {x}’ value to -1 to flip
all of the original images to cancel the negative voxel size. However,
this didn’t make any difference to the final realigned, normalised,
smoothed images, which still have a voxel size of -2x2x2 and were still
smaller on the ‘display’ than those from data set 1.
As I am new to using SPM2, I cannot think of what the problem might be and
why the final images are being produced with a negative voxel size, which
seems to be independent of whether the original images (those before pre-
processing) have a negative voxel value or not and why the images appear to
be smaller on display.
Any help on resolving this problem would be greatly appreciated.
Jenny Bell (PhD Student - University of Nottingham)
