[Dcmlib] Pixel Representation =1

[Jean-Pierre Roux]

Mathieu Malaterre wrote:

> JP,
>
>     Do you want me to answer that ? Or are you preparing an answer ?


Mathieu.
I remember a discussion we had (with gdcm team), a long time ago.
I suggested to transform the Signed Pixels into Unsigned Pixels, but we 
decided it was up to the user to do that, after reading a post from 
David Clunie, about troubles with 'Pixel Padding value' nad Hounsfield 
density.

Moreover, Nuclear medical images, or PET images are sometimes Signed, 
without any correlation with Hounsfield density.
We *cannot* know automatically what we (gdcm) have to do with Signed values.
Only aware user can know that.

Any comment?

Jean-Pierre Roux
gdcm team

>
> -M
>
> zhou13 at netscape.net wrote:
>
>>  Dear Jean:
>>
>> I find this in gdcmFile.cxx.
>>
>> /**
>>  * \brief   Check whether the pixels are signed (1) or UNsigned (0) 
>> data.
>>  * \warning The method defaults to false (UNsigned) when tag 0028|0103
>>  *          is missing.
>>  *          The responsability of checking this value is left to the 
>> caller
>>  *          (NO transformation is performed on the pixels to make 
>> then >0)
>>  * @return  True when signed, false when UNsigned
>>  */
>> bool File::IsSignedPixelData()
>> {
>>    std::string strSign = GetEntryValue( 0x0028, 0x0103 );
>>    if ( strSign == GDCM_UNFOUND )
>>    {
>>       gdcmWarningMacro( "Pixel Representation (0028,0103) supposed to 
>> be "
>>                       << "mandatory");
>>       return false;
>>    }
>>    int sign = atoi( strSign.c_str() );
>>    if ( sign == 0 )
>>    {
>>       return false;
>>    }
>>    return true;
>> }
>>  
>> So, using GDCM doesn't guarantee that the signess of pixel will be 
>> handled correctly.   People must actively  call this method and  
>> custom  transform  the  value.    A better way would be hiding these 
>> details in side the reader and people would always get the correct 
>> value regardless of the Pixel Representation settings.
>>
>> Regards,
>> Li Zhou
>>
>>  
>> -----Original Message-----
>> From: jpr at creatis.insa-lyon.fr
>> To: zhou13 at netscape.net
>> Cc: dcmlib at creatis.insa-lyon.fr
>> Sent: Mon, 13 Nov 2006 9:21 PM
>> Subject: Re: [Dcmlib] Pixel Representation =1
>>
>> zhou13 at netscape.net <mailto:zhou13%40netscape.net> wrote:  
>>  > Hello list,  >  > Does GDCM correctly handle the Pixel 
>> Representation tag? When the > Pixel Representation value is 1, the 
>> stored value in DICOM is two's > complement ( 16 bit signed short ). 
>> I have been using VTK's DICOM > reader but it can't handle this 
>> situation.  
>> I didn't know VTK's DICOM reader had this kind of trouble. GDCM deals 
>> correctly with pixel representation. GDCM comes with a 
>> vtkGdcmReader() , with the same API than vtk's DICOM Reader.  
>>  > So i am investigating ITK's DICOM reader which is based on GDCM. I 
>> > have some studies from a GE ProSpeed CT which actually set the 
>> Pixel > Representation tag to 1.  
>> If you download gdcm, just use vtkgdcmViewer2 yourDicomFileName to be 
>> sure it works. You can have a look at the dicom header using : 
>> PrintFile filein=yourDicomFileName  
>> HTH  
>> Jean-Pierre Roux from gdcm team.  
>>  >  > Thanks.  > Li Zhou  >  >  > > 
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