I re-discovered this old post about a S/I gradient in field maps from
Philips scanners, as I'm currently looking at some Philips data which
displays exactly the same issue. Did you ever manage to resolve it?
On 09/07/12 16:32, Michael Harms wrote:
> My apologies for re-posting this one more time, but I'm really stumped.
> I would REALLY appreciate it (and would be happy to treat you to a
> beverage of your choice at the next OHBM) if someone that has
> successfully acquired and applied field map correction from a Philips
> Achieva could contact me with what they had to do.
> I'd be happy to then summarize whatever I learn to the broader FSL-list.
> On Fri, 2012-06-29 at 11:38 -0500, Michael Harms wrote:
>> Hello FEAT users with Philips scanners,
>> I would be highly appreciative if someone that has successfully applied
>> fieldmapping correction (B0 unwarping) using Philips scanner data would
>> contact me off the list so that I could pick your brain.
>> We have data from a multi-site study, of which one site was a Philips
>> scanner. I *thought* that I knew what needed to be done to apply
>> fieldmapping correction for the data from this site, but the comparison
>> in the Prestats report of "undistorted example_func to undistorted
>> fieldmap" clearly shows that we don't have something right.
>> The odd thing is that the "Unwarping shift map" from the Philips site
>> data shows a spatial pattern that is completely different from what we
>> observe from 4 different Siemens sites. Namely, it has a rather strong
>> S/I gradient, with negative voxel shifts inferiorly, and positive voxel
>> shifts superiorly. This makes me wonder if what we collected wasn't a
>> proper fieldmap (the site collected a FFE with "B0 field map = yes" and
>> "delta TE = 2.5 ms"), but if that's the case, I'd like to know what the
>> site needs to do differently at the time of acquisition.
>> Thanks in advance!