Hi,
First thing's first - I have had a little experience with GE scanners over
the
past year and they put some large scaling values into their phase outputs.
If you can, get the raw complex data in real and imaginary format, rather
than magnitude and phase, as then there is no scaling issue.
Secondly, the input to prelude can either be separate phase and
absolute (magnitude) images using the -p and -a options *or* it can
be a single analyze file in _complex_ format, which contains both.
See the two example usage lines in the help for prelude (just run
prelude with no arguments). If you want to generate or manipulate
analyze format complex images use the tool: avwcomplex.
Thirdly, there are two programs - prelude and fugue. Prelude does the
unwrapping and can be run separately on the symmetric and asymmetric
results or you can make a 4D analyze file that contains both and run it all
at once. I tend to do the latter because I'm lazy and don't want to type
the command twice and have more files.
For fugue, however, the situation is not so flexible. It requires the two
unwrapped phase maps to be presented as a single 4D analyze file.
This is easily created from the individual files using avwmerge with the
-t option.
Hope that answers all of your questions.
All the best,
Mark
On Thursday, December 19, 2002, at 11:22 PM, Antti Tarkiainen wrote:
> Hi!
>
> This may be a stupid question, but I am very new to this field and need
> some
> help with prelude.
> I have read from the material on the web that the field map scan requires
> the collection of two spin-echo sequences, one symmetric echo and one
> asymmetric echo. On the other hand, for prelude you need one complex or
> two
> real analyse files giving the absolute and phase values separately.
> Basically, the question is, how do I get one absolute value file and one
> phase value file from one symmetric echo and one asymmetric echo scan?
> To be more specific, we have now a pulse sequence for our 3T GE scanner
> that
> enables us to measure the asymmetric scan. This will also give us the
> phase
> and magnitude images separately. So, if we e.g. take 10 slices using the
> asymmetric sequence, we get 10 magnitude images and 10 phase images. And
> for
> the symmetric scan, the same thing. Now we have actually 4 image files,
> one
> asymmetric phase value file, one asymmetric magnitude value file, one
> symmetric phase value file and one symmetric magnitude value file.
> For prelude I need one magnitude file and one phase file. How do I
> construct
> those two files from the four files available? I tried to read the earlier
> postings, but was unable to find an answer.
> Also, when I tried the prelude with just some phase and magnitude files
> (or
> with some combinations of them) I got this error message: "input phase
> image
> exceeds allowable phase range. Allowable range is 6.283 radians. Image
> range is: 18603 radians.". It sounds like I would need some scaling. How
> should that be done?
> I hope that somebody can show me the light...
>
> Thanks and Merry Christmas to everybody.
>
> Antti Tarkiainen
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