[Dcmlib] CT-MONO2-16-chest.dcm / Pixel Padding Value

[Jean-Pierre ROUX]

At 15:55 -0400 17/10/05, Mathieu Malaterre wrote:
>Hello,
>
>	Could someone comment on the use of Pixel Padding Value ? I 
>don't think that gdcm is using it.

Pixel Padding Value : Example.
For some CT images, the 'usefull part' is a circle, not a square.
Supose the ilage is BitsAllocated = 16, BitsStored=12, Pixels are unsigned
(0 <=   <=4095 )
When you process the image, you don't want to loose time making 
computations on 'outside image' pixels.
If the pixels 'out of the image' have a value, say =0, you cannot 
know whether you are on a 'Black' pixel, or on an 'outside image' 
pixel.

Just say that Pixel Padding Value = 4096 (impossible value if you use 
only 12 bits), and user will know whether a Pixel is inside or 
outside image.

BUT :
The 'recomandations' are to use zero for 'unsigned pixels', and 'a 
value equivalent to air' (that's -1000, in Hounsfield density), when 
for signed pixels.
--> You cannot save computation time checking the Pixel Padding Value ...


Anyway, this occurs at processing time; gdcm is not concerned...




> But if one open CT-MONO2-16-chest.dcm there does not seems to be 
>anything wrong...

You could experiment troubles at display time, if 'Pixel Padding 
Value' had a stupid value, say 65000.
Your image would look black, the only part 'non black' would be 
outside the image.

>
>	What it is supposed to do ?

The only place to deal with it would be in 'vtkgdcmViewer' ...

JP

>
>Thanks,
>Mathieu
>
>C.7.5.1.1.2 Pixel Padding Value
>Pixel Padding Value (0028,0120) is used to pad non-rectangular 
>images to rectangular format.
>The native format of some images is not rectangular. It is common 
>for devices with this format to
>pad the images to the rectangular format required by the DICOM 
>Standard with a specific pixel
>value that is not contained in the native image. This attribute 
>specifies the value of this padding value.
>Notes: 1. When the relationship between pixel value and X-Ray 
>Intensity is unknown, it is recommended that the following values be 
>used to pad with black:
>0 if Photometric Interpretation (0028,0004) is MONOCHROME2.
>2BitsStored
>- 1 if Photometric Interpretation (0028,0004) is MONOCHROME1.
>2. When the relationship between pixel value and X-Ray Intensity is 
>known (for example as
>defined by Pixel Intensity Relationship (0028,1040) and Pixel 
>Intensity relationship Sign
>(0028,1041)), it is recommended that a value equivalent to air be used.
>When modifying equipment changes the pixel padding value in the 
>image, it shall change the
>value of Pixel Padding Value (0028,0120).
>
>
>and according to:
>
>http://www.cms-stl.com/products/rtp/dicom/dicom_conform_ver_3_0.htm
>If the DICOM header for a 16 bit CT image file defines a pixel 
>padding value, XiO will convert all pixels equal to that padding 
>value to the minimum pixel value representable by the data type of 
>that image. It is assumed that the CT manufacturer has selected a 
>padding pixel value that is outside the range of the pixel values 
>that make up the image.
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   Jean-Pierre ROUX
   CREATIS - CNRS UMR 5515, INSERM U 630
   Laboratoire de Radiologie Experimentale
   Hopital Cardiologique
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