I did 3 repetitions of the phasemap scans on the phantom, the difference in phasemap at the same echo time between scan 2 and scan1 is slightly different from the difference in phasemap at the same echo time between scan 3 and scan 2. Most voxels are more negative in scan2-1, compared to scan3-2.
Resultant fieldmaps from each scan are also slightly different.
Thanks for all your replies so far!
Binu
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of Mark Jenkinson
Sent: Friday, March 09, 2012 10:31 AM
To: [log in to unmask]
Subject: Re: [FSL] Phasemap for distortion correction
Hi,
It's hard to say whether the differences are important or not. You expect some differences
due to noise, and some scanners have some B0/shim drift. The question is whether the
phase difference is reliable and repeatable or not, as that is all you care about for getting a
fieldmap.
All the best,
Mark
On 9 Mar 2012, at 16:25, Binu Thomas wrote:
These scans were done on a phantom (water bottle), which did not move, so the B0 field should not have changed. The phase difference was measured on the phase unwrapped images, I also see differences on the phase images before unwrapping.
Thanks,
Binu
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf Of Stephen Smith
Sent: Friday, March 09, 2012 1:40 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [FSL] Phasemap for distortion correction
Hi - it's the *difference* between the two phasemaps that should be consistent (though there's also the issue of phase unwrapping).
This however also assumes that the B0 field hasn't changed - and this can change if the position of the head changes.
Cheers.
On 8 Mar 2012, at 23:28, Binu Thomas wrote:
Dear FSL'ers,
Recently I acquired a phasemap from a phantom using a gradient echo sequence with TE1=6.5ms and TE2=8.5ms. I repeated the same phasemap acquisition without changing anything and made sure that the center frequency (F0) and shim values were the same for both scans. I expected the phasemaps (units = radians) from both scans to be identical, but looks like there are some differences between phasemaps (phasemap from scan1 at echo 2 compared to phasemap from scan2 at echo 2). The average difference for all voxels between the phasemaps is around 0.073 radians and there is no particular trend in the difference, but phase values at each voxel are different between scans. Can someone explain why I see this difference in phase between identical acquisition of phasemaps?
Binu Thomas, M.S.
Ph.D. student, Biomedical Engineering.
Advanced Imaging Research Center
UT Southwestern Medical Center
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