And the Abstract of this paper indicates that the noise distribution is
nearly Gaussian for SNR larger than two.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2254141/
cheers,
-MH
On Mon, 2010-11-01 at 08:09 -0500, Michael Harms wrote:
> Just to add briefly to Gael's point:
> Once your SNR gets above a certain threshold (mentioned as 10 in
> Triantafyllou et al, NI, 26:243-250, 2005), the Rician distribution is
> well approximated as a Gaussian distribution.
>
> cheers,
> -MH
>
> On Mon, 2010-11-01 at 13:28 +0100, Anders Eklund wrote:
> > Good point, I mean that the MRI observations have Rician-distributed
> > noise. My question however remains, is this considered in SPM or is it
> > assumed that the noise is Gaussian?
> >
> > /Anders
> >
> > 2010-10-31 10:19, Gael Varoquaux skrev:
> > > On Sat, Oct 30, 2010 at 07:45:07PM +0200, Anders Eklund wrote:
> > >
> > >> An interesting discussion, do you know if SPM uses the fact that the
> > >> noise in MRI is Rician distributed and not Gaussian distributed?
> > >>
> > > Forgive me for asking a naive question, but is the noise in fMRI really
> > > Rician-distributed? The MRI-observation noise is Rician-distributed. I
> > > believe that this comes directly from the measurement process. However,
> > > with EPI, there are much more processes contributing to 'noise' than
> > > imaging noise, such as residual movement or vascular and respiratory
> > > noise.
> > >
> > > I am not even sure that the EPI-specific noise (such as
> > > field-inhomogeneity fluctuations that can clearly be seen in the
> > > ventricles) are Rician-distributed. If someone on the mailing-list who
> > > understands the physics behind the EPI noise could enlight me, I'd be
> > > much obliged.
> > >
> > > Gael
> > >
> >
|