[Neuroimaging] [Nipy-devel] [dipy] PIESNO
stjeansam at gmail.com
Fri Jul 24 17:31:42 CEST 2015
Well, it actually depends on your own scanner. Actually, the Siemens
scanner in Sherbrooke is using a 12 channel head coils array, but in the
way is it operated, it is the same as a 4 channel head coils array,
hence we use N=4.
This is because Siemens reconstruction algorithm (GRAPPA) yields a
noncentral chi distribution of the noise due to the algorithm working in
fourier space, as opposed to Philips and GE which are using a
reconstruction in the image space (SENSE), which yields Rician
distributed noise (and hence N=1 in this case).
The parameter N is really related to the algorithm used by the
reconstruction (and it depends on the number of coils of the receptor
antenna), less than the grappa/sense factor itself. Problems is, you
kind of need to know your scanner to use it. Some people [1,2] are using
maximum likelihood estimation form the background to get N, but I have
not tried it myself. See  if you want to know more about the various
reconstruction algorithm and the noise distribution they yield.
 Varadarajan, D., Haldar, J., 2015. A Majorize-Minimize Framework for
Rician and Non-Central Chi MR Images. IEEE Trans. Med. Imaging 0062,
 Aja-Fernández, S., Vegas-Sánchez-Ferrero, G., Tristán-Vega, A.,
2014. Noise estimation in parallel MRI: GRAPPA and SENSE. Magn. Reson.
Imaging 32, 281–90. doi:10.1016/j.mri.2013.12.001
 Dietrich, O., Raya, J.G., Reeder, S.B., Ingrisch, M., Reiser, M.F.,
Schoenberg, S.O., 2008. Influence of multichannel combination, parallel
imaging and other reconstruction techniques on MRI noise
characteristics. Magn. Reson. Imaging 26, 754–62.
Le 2015-07-24 11:18, Arnaud Boré a écrit :
> Dear dipy experts,
> I try to figure out how to use PIESNO to estimate the noise. In the
> http://nipy.org/dipy/examples_built/piesno.html#example-piesno we
> don't know about the grappa factor. Is it 3 so we have N = 12 (number
> of coils) / 3 (grappa factor) ?
> Last question if we don't use grappa then N = 12 ?
> Thank you
> Arnaud BORE
> Research assistant
> Cellulaire : (001) 514-647-8649
> Neuroimaging mailing list
> Neuroimaging at python.org
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