Hi,
On 15 Oct 2007, at 16:22, Najmeh Khalili M. wrote:
> Hi,
>
> Is there a way to get the residuals of eddy_correction or is
> there any way to estimate a metric of suseptibility distortions
> present in DTI?
Eddy_correction is not a model-fitting (in the way that dtifit is),
so you don't get "residuals" - however, you can look at the temporal
variance of the uncorrected and corrected datasets, though remember
that this includes variation due to the real valid diffusion effects.
To do it on either 4D dataset:
fslmaths <4Ddata> -Tstd dataSTD
However the majority of the variance you see here will not be related
to susceptibility distortions. The geometric distortions would be
modelled, for example, by using the FUGUE tool and a fieldmap (see
the manual for more details).
> More generally, I need to model the imaging noise in the
> between-subject DTI analysis and I wonder if you have any
> tools < or philosophy > that address this problem.
Again - I'm not sure this I follow - this is another separate issue -
the majority of between-subject variation will probably be neither
due to susceptibility distortions nor within-subject acquisition
noise, but due to real subject-subject variability in tract geometry
and FA values.
Cheers.
>
> Thank you,
> Naj
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
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