I am trying to unwarp some images and have some questions concerning the
instructions in the documentation. To begin with I am acquiring separate
phase and magnitude images in DICOM format from the scanner (GE 3T) and am
converting them to nii format. Using fslinfo for the phase images I receive
data_type INT16 and datatype 4. So then this should mean I should converting
to floating point format (not sure though what datatype 4 means, though)
using the fslmaths command stated in 2)b of the documentation. The value for
the -div operation is stated as 2048, but the manual states this needs to be
adjusted for each scanner. How can this value be determined?
When these files are then converted to radians how do I determine their ranges?
In acuqiring the two phase images, does one images just have to have a
slightly longer TE than the other, does it have to be an asymmetric echo (as
described in some posts), or both? What exactly is an asymmetric echo? One
post, #002856, describes the asym time for a gradient echo as "
the factor required to convert the phase difference in the scans to a
rad/s field value." I assume this factor is one of the values used in the
above fslmaths argument, but which one and how is this implemented in the
scan? This is really confusing to me.
Lastly, I have read that it is recommended to take spin-echo scans for these
phase maps, but this takes an extremely long amount of time. The
documentation states gradient echo images are OK (which are what I have been
acquiring), but do these require any extra processing?
I apologize for all the questions, but I have been trying to figure this out
for a while and am currently stuck!
Thanks!
-Carlos
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