Dear Eric


Thanks for this important remark. I am not an engineer, but I have checked 
this point when we first begun to perform fast imaging for fMRI. More 
technical comments will be welcome.


I have a comment about a small part of one of Geraint's recent replies 
which regarded low frequency fMRI noise:
> change is very low. Unfortunately many confounding signals of little
> interest (physiologically aliased signals, drifts in scanner gain and so
> on) also change at low frequencies.


Though I certainly agree that there is more noise at lower frequencies,
I have not seen any data to support their explanation in terms of either
a)	drifts in scanner gain

or
b)	physiologically aliased signals.

Though both of these seem reasonable a priori as explanations for low 
frequency noise, I have not found any evidence to support either in my own 
data or in the literature. In fact there is even some evidence to refute 
them.
In terms of evidence against model a), a drift in scanner gain would 
predict a correlation between the whole brain signal and variability in 
signal across space. I looked at this and saw no such correlation (has 
anyone else looked at this?).
In fact, for scanner noise (defined here as seen on phantoms), I would 
rather talk of 2 types of noise :
1- The first one concern the temporal evolution of the whole phantom (== 
global intensity) : In our scan (Brucker 2T upgraded for EPI with an 
adapted gradient coils fitted inside the scan) we haven't found a clearly 
structured noise (no peek frequency), but rather a white noise (sure this 
is true only after the steady state has been reached). Is this what you 
meant by 'correlation between the whole brain signal' ?
2- The second concern only certain regions : most of it is on the 
interfaces of the object (+++ in regions with susceptibly artifacts) or its 
projections by ghosts. Its frequency is ~ 0.0038 Hz and is intensity may 
reach ~ 0.7 % (min to max). However those results are probably highly re  
lated to the imaging parameters (we haven't check that, has anybody done it 
?). Due to this distribution, we interpreted it as gradient instability.

Model b) would predict no (or at least a decrease in) low frequency noise 
in non-physiological scanned objects. But, low frequency noise has been 
observed in water phantoms that looks very similar to that seen in human 
subjects. Furthermore, cardiac and respiratory rhythms have a strong 
periodic component that would therefore not alias into a 1/f noise 
spectrum.

I don't know if the authors of 'localization of cardiac-induced signal 
change in fMRI' are SPM listers, but it would be interesting that they give 
us the spectra of there observed cardiac noise.

I raise these issues because it would be wonderful if an engineer or MRI 
physicist could do some research into understanding the low frequency 
components of fMRI noise better. Or perhaps, some has already been done, 
and it just has not filtered its way into the mainstream neuroimaging 
literature?

Hope that there will be comments on this issue ...
Sincerely



Jack


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