Ajna,
I also use Chloe's great code, but modified that particular line
to be more absolute in its assignment of phase, based on the knowledge
that the phase difference maps on a Siemens Trio range in value from
-4095 to 4096, and represent -pi to +pi.
mn=min(vol(:));
mx=max(vol(:));
%svol=-pi+(vol-mn)*2*pi/(mx-mn);
% JNL. More accurate statement
svol = vol * pi / 4096;
Jim Lee
University of Utah
Dear SPMers,
I am wondering if anyone knows what is the significance of scaling the
phase difference image to +pi to -pi range...
The matlab code provided by Chloe Hutton among the updates finds the
entire image max and min and uses this to scale the phase difference
map
to this range. First problem I have with this code is that it does not
discriminate between brain and non-brain voxels in the image to create
the scale, and so the scaling ends up being based on noise outside the
brain and therefore in a way arbitrary...
Second, I have conceptually hard time understanding why the image would
be scaled to have the max and min set to +pi and -pi, rather than
simply
be somewhere within this range. I.e. if max phase difference was for
example lower than pi radians, why are we setting this voxel to pi?
All input would be greatly appreciated!
-Ajna Borogovac, Columbia University
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