[Insight-users] Problem with pixel representation type

[Luis Ibanez]

Hi Soumend,


1) ITK image readers perform casting when reading
    the images. If you declare the reader as:

    typedef signed char                PixelType;
    typedef itk::Image< PixelType, 3 > ImageType;
    typedef itk::ImageFileReader< ImageType > ReaderType;

    ReaderType::Pointer reader = ReaderType::New();

    reader->SetFileName("inputfile.dcm");

    The "reader" will take the pixel type of the file
    and cast it to "PixelType". If image file has an
    image of pixel type = unsigned char and it gets
    displayed correctly using "unsigned char", it is
    likely that the image is not using the entire
    intensity range [-128:127].

    The simple way to know is for you to instantiate
    and ImageFileReader of pixel type = "unsigned char"
    read the image and use the MaximumMinimumCalculator
    in order to compute the maximum and minium values of
    the image.

    Once you get this two (min/max) numbers,
    please let us know.


2) This relates to (1).
    You may want to read carefully the IO chapter of
    the SoftwareGuide

         http://www.itk.org/ItkSoftwareGuide.pdf

                 Chapter 7, pdf-page 219.


    Note that if what you are trying to do is to
    create a "Generic" image viewer, you should
    prepare multiple instantiations of the ITK
    pipeline. You may find examples on how to do
    this on the VolView Plugins code in

        InsightApplications/VolviewPlugins

    Otherwise, your reader/viewer has been working
    fine just because you have been lucky so far
    with your data. Murphy's laws clearly state that
    you will run out of luck the day you demo this
    application to your supervisor or your most
    important client.


3) You can easily renormalize the image intensity
    values before display on your VTK visualization
    pipeline. You will have to post the code of your
    visualization pipeline if you want to get advice
    on this regard.


4) Resolution (image pixel spacing) is irrelevant
    in this context.



Regards,


    Luis



-----------------------------------
soumend at magnum.barc.ernet.in wrote:
> HI !!!!!
> 
> I have developed a dicom image viewer which will read the dicom image using 
> ITK and displayed through VTK. Still I have some dought in this regard.will 
> anybody help me out? 
> 
> i) Though the pixel representation field of CT image shows 0 (i.e unsigned 
> char),the image is properly displayed when pixel type is taken as signed char. 
> Whereas PET image has pixel representation 1 behaves good with signed char 
> pixel type as it should be. Why the discrepency is there for CT image?
> 
> ii) In the DICOMApphelper the pixel is read as unsigned char for CT image and 
> as float in the case of PET image (due to rescale slope and offset values). 
> But in my program i have defined  the pixel type of input image as signed 
> char. My question ....though the pixel values are read differently still 
> images get displayed doesnt matter what is the input pixel type. Why is it so?
> 
> iii) the range of pixel value is different for CT and Pet images ...now to 
> read from a single interface how can i set the pixel range dynamically in the 
> look up table(it is VTK related qs ..still)?
> (resaolution of CT images are much better than PET images) 
>     
> 
> 
> -------------------------------------------------
> 
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