Dear SPM community,
in an arterial spin labelling (ASL) perfusion MRI study, a sequence of
40 interleaved labelled and non-labelled images (label, non-label,
label, ...) is acquired and pairwise subtracted to get the perfusion
images. Averaging over the 40 images is done in order to increase SNR.
My question concerns the order of (pre-) processing steps, since there
doesn't seem to be consent in the ASL community.
Would it make more sense...
1) to first realign all of the 40 label and 40 non-label images
separately and then substract the two mean realigned images? Or...
2) to first subtract the label and non-label images and then realign the
40 difference images (incl. averaging)?
Furthermore, if you are doing a ROI analysis (extracting the mean
perfusion in a large ROI like the vascular region of the posterior
cerebral artery) instead of a voxel-based analysis (no random field
theory!), is it necessary to smooth the image? If so, what would be a
criterion for the size of the smoothing kernel, e.g. for a acquired
voxel size of [2 2 8], normalized to a voxel size of [2 2 2]?
Thanks in advance,
Michael Hermes
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