Index parameter in CT data - DICOM

This is a discussion on Index parameter in CT data - DICOM ; Hi all, I am trying to extract the pixel data from a DICOM CT image slice in MATLAB. The Data Cursor option in MATLAB shows 5 parameters associated with every pixel: X,Y,Index,R,G,B. Since R=G=B for every pixel I assume it ...

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Thread: Index parameter in CT data

  1. Index parameter in CT data

    Hi all,

    I am trying to extract the pixel data from a DICOM CT image slice in
    MATLAB. The Data Cursor option in MATLAB shows 5 parameters associated
    with every pixel: X,Y,Index,R,G,B. Since R=G=B for every pixel I assume
    it is a grayscale image (though "dicominfo" command in MATLAB doesnt
    explicitly mention that). Several pixels in the image have the same
    Index value.I am wondering what Index Value stands for ?

    Any pointers ?

    Thanks, Vidya


  2. Re: Index parameter in CT data

    Another addition, the Index value ranges from 0 to 2718.

    Thanks, Vidya.


  3. Re: Index parameter in CT data

    The terminology you're describing isn't standard for DICOM and no,
    normally CT data would not have RGB values associated except possibly
    by software which has already converted the greyscale data to RGB
    values for rendering on a color display.

    It is possible (but not certain because of the non-standard
    terminology) that the "Index" value relates to a greyscale transform
    normally applied to raw CT image data to convert it to Hounsfield
    units. This is a scale where standard, calibrate-able points for air
    and water in the raw CT pixel data are arbitrarily assigned values of

    -1000 and 0, respectively. The pixel values for various tissue types
    will then fall into some standard ranges on that scale. The index value
    you're seeing could be the CT pixel data value before it was scaled
    into Hounsfield units represented by the RGB pixel value.

    When CT image data are displayed, there is also a contrast and
    brightness transform normally applied to the data. This is called
    "window and level" and may have been included in the RGB display
    values, thus the index value could be unscaled hounsfield value before
    application of the window/level scaling

    The matlab module may also be converting the data into palette color -
    such as is typically used in a GIF file. -ie. rather than supporting
    true 24 bit RGB color, the data pixel data may have been assigned to
    the "closest" 24-bit greyscale value in an 256 entry table and the
    index value is simply the entry number (index) into that table for the
    selected 24-bit RGB value.

    If the image is not a true CT slice but is instead a multiplanar
    reconstruction image (MPR) derived from projecting data from multiple
    CT slices at a oblique angle, the index value could conceivably be a
    reference to the orginal slice number from which a particular pixel in
    the MPR image was taken.

    The discussion above illustrates why we have the dicom standard and
    describe data using well defined attributes. When DICOM standard
    attributes are used its very clear what characteristics of the data are
    being shown. Shame on Matlab for describing that data as "dicom info"
    when it apparently is not using standard attribute identifiers.

    The bottom line is, without further/better identification of the data
    attributes (or experience with the matlab module you're working with),
    no one can really tell you what that value is. Perhaps you want to post
    the exact text the matlab "dicominfo" display is providing you.


  4. Re: Index parameter in CT data

    Your second posting didn't appear in my usenet browser until after I
    wrote the above. With the value range you report, most likely the
    "Index" value is the raw CT pixel data before it is scaled to Housfield
    units and window/level


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