Hi Haakon,
I can only give you my uninformed and naive non-physicist's opinion
which is:
> Am I right in
> assuming that this is suboptimal, and that we SHOULD acquire field maps
> that are in exact correspondence to the functional images with regard to
> 1: voxel size 2: number of slices and 3: acquisition orientation.
I believe you are right, at least this is the reason why I collect my
fieldmap data by copying slices and voxel sizes from the functionals.
The difference this makes is that the fieldmap data does not have to be
resliced in order to match the fMRI data, which, I somewhat hope,
improves results. However,...
> Second: Suppose I have a rather large amount of data with fieldmaps for
> in which voxel size and number of slices is not in correspondence with
> the functional data. Is it still possible to perform fieldmap based
> unwarping?
I would say yes as the fieldmap toolbox is meant to do that and will
reformat the fieldmap data accordingly. The question on what kind of
improvement it carries depends on your dataset, your fieldmap, and your
field strength, and EPI sequence, and ...
> Third: Suppose I run multiple experiments during one scanning session,
> in which the acquisition parameters(no. of slices, voxel size, slice
> gap, TR, TE) vary between experiment. Is it necessary to acquire a
> fieldmap matched to each experiment?
I would not say necessary as this would significantly increase your
scanning time. There is an option in the fieldmap to provide one map for
each session, but if you do not do that, it will happily take one map to
unwarp all sessions. I would take this as an indication that it is not
blasphemy to proceed this way :)
> I have a sneaking suspicion that the reason why I can't find these
> questions answered anywhere is because the answers are rather obvious to
> those well-versed in these matters, but I hope you will find time to
> answer me in spite of this.
Well, one alternative explanation is that nobody has looked into these
effects in a systematic fashion which is why everybody proceeds as
he/she deems fit :)
Best,
Marko
--
=====================================================================
Marko Wilke (Dr.med./M.D.)
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Universitäts-Kinderklinik University Children's Hospital
Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
Hoppe-Seyler-Str. 1, D - 72076 Tübingen
Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473
=====================================================================
