Hi,
I'd like to ask a question regarding which way would be more accurate to
calculate and prepare the Fieldmap into Rad/secs, and to use to do
undistortion of the EPI DTI data. Using different softwares, I got a
Phase-difference map of intensity 0-4094 or of an intensity of -4096 to
4092. When i prepare and calculate the Fieldmaps into rad/secs from these
two Phase-difference maps, I get different ranges of the resulting
Fieldmaps and slightly different undistorted results, one a little more
distortion_corrected than another in the y- direction.
My raw "Fieldmap" is a phase-difference map as described in the fsl
practical page.
1. When I used the phase difference map of intensity 0 to 4095 (converted
from dicom to nifti by debabeler) and the following commands from the fsl
practical
fslmaths phasedif_map -sub 2048 -div 2048 -mul 3.14159 phase_rad -odt float
prelude -p phase_rad -a Magnitude -o uphase_rad
fslmaths uphase_rad -div 0.00246 FieldMap (0.00246 is the TE difference of
the fieldmap sequence)
The FieldMap.nii.gz intensity range is -3165.861328 2801.697021
2. When I used the phase difference map of intensity -4096 to 4092
(converted from dicom to nifti by MRIcron) and the following commands from
the fsl discussion board
fslmaths phasedif_map -div 4096 -mul 3.14159 -div 000246 phase_rad -odt float
prelude -p phase_rad -a Magnitude -o FieldMap
The FieldMap.nii.gz inensity range is -2801.073486 2790.472900
May I ask if it is ok to do unwarping of the Fieldmap when it is in
rad/secs unit?
Which order would make more sense to prepare the fieldmap to use to
undistort the dti scans?
Many thanks
Siewmin
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