[Insight-users] Re: Help with Dicom PET images

[Ziv Yaniv]

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>Today's Topics:
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>   1. Help with Dicom PET images (Markus Weigert)
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>----------------------------------------------------------------------
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>Message: 1
>Date: Mon, 25 Sep 2006 13:41:59 +0200
>From: "Markus Weigert" <m.weigert at fz-juelich.de>
>Subject: [Insight-users] Help with Dicom PET images
>To: <insight-users at itk.org>
>Message-ID: <008101c6e097$9c81beb0$2f085e86 at ime.kfajuelich.de>
>Content-Type: text/plain; charset="iso-8859-1"
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>Hi all,
>
>I have a problem regarding dicom PET image series.
>To load and use those images, I use itk::image<short,3> as container.
>The problem is that those PET images show very high values, which
>are over the range of short datatype (also above unsigned short).
>I tried to switch to "int" and was able to read and display the series, 
>but unfortunately, I also need to write the data in dicom format.
>However, when I try to write a dicom series using an itk::image<int,3>,
>I get an error message, that dicom does not support this datatype!!!!
>So what can I do?
>I do not want to rescale the image though this would violate the physical
>meaning of the values.
>I just wonder that dicom is someway coding these high values (because the
>original values can be loaded properly from a serie), but doesn't let me write it.
>
>
>Kind regards,
>Markus
>
>
> 
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Hi Markus,

 From my experience with PET, the voxel values are float and not 
int/short. Before you continue with your work I suggest that you look at 
the DICOM header. The type of the gray level value is defined by 
multiple fields, Rescale Slope [0028-1053] (for PET usually some 
floating value), Rescale Intercept [0028-1054] (for PET usually zero), 
and Pixel Representation [0028-0103] (for PET 1==signed short). The 
final value is computed as
finalValue = RescaleSlope*pixelValue + RescaleIntercept. The way you are 
currently reading the images in ITK simply C-casts the float data to int.

To save your ITK image back to DICOM reverse the computation so that 
your "pixelValue" is a signed short and combined with the rescale slope 
and intercept yields the value found in your ITK image.
I am not aware of a nice way to do this in ITK, but perhaps the GDCM 
people have an elegant solution?

                      regards
                          Ziv