Pablo and Dave,
Thank you both for your replies.
I do have a few more questions for Dave though.
>The fast GRE sequences acquire a single line of k-space per echo, but you
>have less control over TR/TE than the standard GRE sequences. EPI gives
>distorted fieldmaps.
So you are saying it is OK to use Fast GRE, since you only acquire one line
per echo?
>3) Most likely, but will need a bit of explanation. You've acquired your
>field map images using a multi-channel receive coil, probably the 8-channel
>head coil. On GE scanners, when images from all the coils in the array are
>being combined (with ASSET off) it calculates the square root of the sum of
>squares for each coil. This even happens when you output real and imaginary
>parts. I tend to output real and imaginary so I'm not sure what happens to
>the phase, presumably it either calculates the square root of the sum of
>squares for each coil, or it calculates the phase from the already combined
>real & imaginary images. In either case this is invariably not what you
>want!
You are correct, I did use an 8-channel head coil, I should have mentioned
that. After writing the first message, I tried what the GE engineer
suggested and set CV opphasset_factor = 1 (engineer had said asset_factor,
but we are using software version 14.0_M4) after having chosen ASSET. This
gave me coil combined phase images that were correctly scaled to -1000pi ->
1000pi. I guess this is accomplishing the same result as saveinter = 1 w/out
the separate images from each coil.
>There are two ways to get around the problem. i) on the scan interface
>change to the body coil for your fieldmap. ii) after you've hit download on
>the scan interface, but before scanning, change the CV saveinter to 1. You
>will then get the separate images from each coil, plus the coil-combined
>images. For example, if you want magnitude + phase using the 8-channel coil,
>you'll get a total of 18 [2x(8+1)] images per echo-time per slice. You can
>then calculate the (wrapped) fieldmap using the method described in
>Bernstein et al, Magn Reson Med 32, 330-334, 1994.
I am unable to access the article, and would have to order it from the
library. However, does this article describe how to use the combined image
to get the fieldmap, or how to use the separate coil images?
>The SNR in the field maps
>will be much better than using the body coil, with the only drawback being
>increased storage space, but then disc space is cheap.
I have been told that a fieldmap does not require a high SNR, is this
correct? If it is, then it seems much simpler to just use the body coil. =]
>In both cases (i & ii) the phase images should be correctly scaled to
>-1000pi -> + 1000pi. Personally, I prefer to output the real & imaginary
>parts, then calculate the phase with the atan2 function.
>
Are you saying that when using the real and imaginary images you do not have
to use the method described in Bernstein, et al.? Just use the atan2
function (not familiar with it) in FSL?
Thanks again!!! We have been struggling with this for a while and now it
seems as if we've almost got it!
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