Hi,
Just to answer this thread as well.
Prelude should *not* be used with a map in rad/s - it requires a
map of *phase* in *radians* only, not radians per second.
Hence the first way is correct and the second way is incorrect.
The division by TE (which turns rad -> rad/s) should always be
done after running prelude.
All the best,
Mark
On 3 May 2009, at 00:16, Siew-Min Gan wrote:
> 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
>
|