On 23/10/07 22:12, "Carlos Faraco" <[log in to unmask]> wrote:
> 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?
Phase images from GE scanners are in radians multiplied by 1000, so should
be in the range around -3142 -> 3142. If the numbers are all positive, then
it's possible they've been acquired using a multi-channel receive coil, and
you should discuss it with whoever's responsible your data acquisition. It
us possible to use such data, but it requires a few changes on the
acquisition side, and the processing is slightly more complicated.
>
> 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.
You can use either gradient echoes with different Tes, or a spin-echo/and an
asymmetric spin echo. An asymmetric spin echo is just a spin-echo with the
180 degree refocusing pulse shifted earlier or later.
>
> 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?
They don't really require more processing, but I prefer them because they
don't suffer from signal dropout as much as gradient echoes, especially in
the regions where the correction is needed the most! You just need to know
the echo shift, which is twice the shift of the 180 degree pulse, and is
equivalent to delta TE for gradient echoes.
Also, apart from EPI, I don't recall any of the GE pulse sequences
supporting asymmetric spin-echoes without modification.
Hope that helps,
Dave
>
> 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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