Hi Mark!
Thank you so much for your quick response!
Here are the two original phase images:
If what you are saying is the problem, could you please specify what you mean by "more sophisticated methods of reconstructing phase from multiple coils"
Thank you so much!!
Best,
Anna
----- Original Message -----
Sent: Tuesday, May 24, 2016 8:04:05 AM
Subject: Re: [FSL] Prelude Unwrapping Problem
Hi,
It's hard to say exactly what is happening without seeing the original, wrapped phase images, but I suspect that the problem lies in phase reconstruction errors. We have seen this before when phase is reconstructed from multiple coils that are not combined
correctly. It can be identified as a phase wrapping line (discontinuity) that just stops at some point. This does not represent a true phase signal that can be unwrapped, and is a consequence of switching from one coil's phase measurement to another, normally
where the signal is very low. Unfortunately, PRELUDE is only capable of dealing with phase images that derive from a single, continuous, phase image that is wrapped. A piecewise phase image, with other discontinuities at the borders, cannot be unwrapped
like this and needs attention in the acquisition/reconstruction. We have found that acquiring the phase using a single coil produces correct phase, as can more sophisticated methods of reconstructing phase from multiple coils. Either way, it needs to be
dealt with prior to running the FSL tools.
If you want to check if this is really the problem then send us an image of the original, wrapped phase image and we can confirm if this is the case.
Sorry I don't have better news.
All the best,
Mark
Date: Monday, 23 May 2016 at 10:15
Subject: [FSL] Prelude Unwrapping Problem
Dear FSL Experts,
I am trying to prepare fieldmaps for FUGUE and have a problem with the prelude command where the fieldmaps dont fully unwrap.
I am using Siemens 7T data with two pahse and two magnitude images which
I can not feed into fsl_prepare_fieldmap (due to having seperate phase images), so I followed the
steps of non-siemens data.
The phase images are in radians as prescribed.
After using the proposed prelude commands:
prelude -a fieldmap_mag -p phase0_rad -o phase0_unwrapped_rad
prelude -a fieldmap_mag -p phase1_rad -o phase1_unwrapped_rad
I found, that the images are not fully unwrapped with sharp edges.
So I changed it to the following:
prelude -a fieldmap_mag -p phase0_rad -o phase0_unwrapped_rad -n 30 -m
prelude -a fieldmap_mag -p phase1_rad -o phase1_unwrapped_rad -n 30 -m
(with m being the mask from the magnitude image)
The unwrapping was still not good so I changed the n value several times,
but everytime one area that was unwrapped previously got fixed, another
showed up in a different area:
Does anyone know how to fix this?
Thank you so much for your help in advance!
Best,
Anna