Hi Michael,
It is certainly true that once signal from multiple voxels has been
compressed into a single voxel, it is hard to put it back with a field map
alone. For diffusion data, phase up / phase down corrections have a better
chance of dealing with this issue, as you have the data both compressed and
rarified and can figure out what the undistorted image would have looked
like. It is not possible to do this with BOLD data, as you can't acquire a
run and then a second run with the phase encode direction flipped and unwarp
and average them (which is what happens with the diffusion data). That
being said, perhaps if you acquired the slices of phase up and phase down
interleaved with one of these faster TR sequences you could pull off phase
up / phase down correction on BOLD data (though you would want to run slice
timing correction first).
If I recall correctly from a conversation Jesper and I had, R/L (or L/R)
phase encode direction actually have the least severe distortions, but
people don't like them because the distortions are not symmetrical. If you
were most interested in stuff in the orbitofrontal region, it might make
sense to use a phase encode that stretches rather than compresses this
region (though of course other regions would be compressed instead).
The field map is good at geometrical distortion correction, but less good at
correction of intensities from signal stretching or compression (or outright
loss).
Peace,
Matt.
-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Michael Harms
Sent: Wednesday, October 13, 2010 11:25 AM
To: [log in to unmask]
Subject: [FSL] effectiveness of field correction as a function of phase
encoding direction
Hello,
Is the effectiveness of field correction for compensating for B0
distortions at least partially dependent on the phase encoding direction
used to acquire the BOLD or DTI data that is to be corrected?
To use Siemen's parameter lingo, a "P>>A" phase encoding direction
stretches signal from frontal cortex outward (anteriorly, into empty
space), whereas "A>>P" (which is unfortunately Siemen's default)
compresses signal from frontal cortex (i.e., moves frontal signal
posteriorly, onto existing brain). It seems that once signal from
multiple voxels is compressed into 1 voxel that not much correction
would be possible... Is that indeed the case?
thanks,
-MH
--
Michael Harms, Ph.D.
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