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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 > > >