Benno,
I did a comparison using phantom data, years ago, comparing the
situation when the fieldmap exactly matches the slices of the fmri
scans, and when it does not. The differences are very minor, nothing
like the effects you are seeing.
Are you sure you have the right units for your phase map? It sounds
like your corrections are stronger than they should be.
Jim Lee
On 4/2/14, Benno Gesierich <[log in to unmask]> wrote:
> Dear SPM fellows,
>
> I found some information on the selection of field map parameters (on the
> mailing list and the FieldMap manual), which appears to be in conflict with
> each other. Could anybody help me to resolve this conflict?
>
> In the Manual_FieldMap_v2.0.pdf, I found a sentence stating: "For the
> geometric distortion correction to work, it is extremely important that the
> number of slices, voxel size and slice orientation of the EPI and field map
> are the same."
>
> However, in a mailing list contribution from Dr. Chloe Hutton 2009-06-08
> (Email subject: Re: On the optimum settings for B0 field maps; item#
> 036809), I found following statement: "Ideally, as described above, the
> field map should be acquired with the
> same shim values as is acquired for the EPI data. The actual number of
> slices acquired and voxel size is not important as the FieldMap toolbox can
> take care of this using the positioning information encoded in the image
> headers."
>
> Maybe, I misunderstand this, but to me these statements seems to be in
> conflict, right?
>
> I am asking this, because I got problems with field map corrections, and I
> am trying to figure out whether the selection of my parameters could be the
> reason. Please see also my Email from yesterday, describing this problem
> (Email subject: FieldMap: unexpected jump and stretch of unwarped scan).
>
> Thanks a lot,
> Benno
>
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