This bias might be caused by interaction between your diffusion gradients
and your imaging gradients. At higher tesla (and bvalue, you didn't specify
the bvalue) this interaction becomes stronger and needs to be corrected for.
Many scanners will output the effective bvalues for each image, which are a
bit different from the bvalue you specified in the scanner. If you use the
effective bvalues in your tensor calculation, the bias will disappear. One
thing that would argue against this theory is that the biases were not
consistent across subjects (was the exact same acquisition used?).
Peace,
Matt.
-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of roberto toro
Sent: Thursday, June 11, 2009 7:03 AM
To: [log in to unmask]
Subject: Re: [FSL] tensor bias
Dear FSLers,
I have DTI data (in-vivo, 6 directions, 7T) from 6 mice, which I
processed for eddy current correction, and DT fitting. It worked
nicely for 4 out of the 6 volumes, but the 2 others had a global bias
in V1 orientation. One volume appeared all red (V1 had an overall
left/right direction), and the other volume appeared all blue (V1 were
biased in the dorsal/ventral direction).
Would there be a way of correcting (subtracting) these global biases?
Would it be possible that the bias is produced by a global difference
of intensity in some of the volumes encoding the 6 gradient
directions?
There's a jpg at http://dl.free.fr/q3VgvTUtr showing a single slice
from B0-B7. Top half is a mouse that worked OK, bottom half a mouse
with a left/right bias.
thank you very much in advance!
roberto
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