Dear Leia,
> In my experiment, The Siemens GRE field map sequence only acquire
> one volume images(It looks like phase
> images, but i am not sure),but the tool need two images( phase and
> magnitude images) or four images( two pair real and imagnary images).
> Can someone give some cue how to acquire the raw data for FieldMap
> toolbox.In seimens 3T Trio MRI, which sequence should I use?
I have no personal experience of acquiring fieldmaps on a Siemens
scanner (maybe Chloe or Nicholaus could comment?) but the principle is
as follows. When measuring a fieldmap we want to know the change in
phase between two different echo times. To have knowledge of the phase
we need either real-imaginary data or to get phase-images straight off
the scanner. One can either evaluate/get the phase at both echo-times
and then calculate the change, or I believe that some vendors phase-map
sequences will give you the phase-change map directly. Note that the
scaling of these maps is sometimes a little inscrutiable, and that Chloe
has posted a script that will rescale it to -pi -- pi for you.
The reason that FieldMap asks also for a magnitude image is that it
needs some means of assessing the SNR at ecah voxel (the magnitude being
the S).
As I said, I am afraid I will have to defer the details of how to
actually acquire these to someone with experience of Siemens fieldmaps.
> By the way,when I compiled the code of FieldMap under winXP, VC++
> 6.0, some error have been reported.Can someone tell me where the
> DBL_MAX are defined ?
>
> > cd C:\MATLAB6p5\toolbox\spm2\toolbox\FieldMap
> >> make
> pm_invert_phasemap_dtj.c
> pm_invert_phasemap_dtj.c(44) : error C2065: 'DBL_MAX' : undeclared
> identifier
DBL_MAX is simply the largest value that can be represented by a double
on your architecture. On my system it is defined in float.h in the gcc
specific include directory. When looking at pm_invert_phasemap_dtj.c I
realise that float.h is not explicitly included. I can only assume that
on my system it is somehow included through <math.h>. In your case I
would start by explicitly including float.h by adding a line
#include <float.h>
after the line #include <math.h>
Good luck Jesper
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