Dear Romain,
You must have a magnitude image and an image that encodes the
field differences. We opt for using the units rad/s, which is easy to
convert from Hz (just multiply by 6.28) or Tesla (multiply by 42 MHz/T
to get Hz and then multiply by 6.28 to get rad/s).
The latter information is derived from phase differences.
Depending on your scanner (and I have *no* experience with Bruker
I'm afraid), you may get one image (the difference) or two. These
may be phase values directly (though often scaled to span a large
integer range, like 0 to 4096 for 0 to pi radians) or may be in
real and imaginary parts. The tool fslcomplex has been written
to allow you to convert between the different forms. However,
once you have a phase difference you still need to divide by the
time difference between the images (normally the echo time
difference of the fieldmapping sequence if it is gradient-echo
based, but there are other alternatives too). Only after you've
divided by this time difference will you get units of rad/s rather
than just radians.
I hope this helps.
All the best,
Mark
On 16 May 2012, at 01:39, romain valabregue wrote:
> Dear all
>
> I am trying to unwarp dti data from a mouse brain (with a Bruker 11.7 T)
>
> I get a little bit confused with the documentation
> http://www.fmrib.ox.ac.uk/fsl/fugue/feat_fieldmap.html
>
> from what I understand I have a magnitude volume (that is easy) and a phase volume that is in Hz
> does anyone use Buker data and can confirm this ?
>
>
> So the only scaling I need is to multiply this phase volume by 6.28 and I do not need to enter the TE difference
> is that right ?
>
> I get value between -9000 and +4000 (or -3000 + 3000 if I restrict to the brain)
> does it seems reasonable ?
>
> thank you for your help
>
> Romain
>
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