Dear all,
A few months ago, I started looking at the problem of fieldmap-based unwarping of EPI when the usual fieldmap sequences (phase-difference volumes) were not available. With lots of help from Mark I was able to eventually come up with what seemed sensible-looking fieldmaps. Essentially the steps I used was
- Extract from T1 a mask to differentiate between air and "everything else", i.e. skull+brain+CSF
- Use b0calc to estiamte the magnetic field variations
- Scale by 2*pi* gyromanetic ratio
- Remove spherical harmonics from the resulting volume
Unfortunate after comparing fieldmap determined from this approach with fieldmap computed using conventional methods (we were able to acquire newer data), there are noticeable differences between the fieldmaps as seen in the two files linked below.
Fieldmap computed from phasemap https://db.tt/57Lb8HzM
Fieldmap computed from T1 volume https://db.tt/Ygte0Zh2
Since we still have substantial data that we would like to process as much as reasonably possible, hopefully we can still get something using the T1-b0calc approach? I'd really appreciate it if someone can suggest where I might be able to improve or where problems might have occurred in the above description?
Thanks very much!
Regards,
Louis.
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