[Insight-users] Re: More Qs about DICOM images

[Luis Ibanez]

Hi Soumen,


1) Is likely that your CT image is using a Look Up Table (LUT).

    In that circumstance, the pixels values correspond to
    indexes to entries in the LUT and therefore can be unsigned
    ints.  The real hounsfield units of the image are the values
    translated by the LUT and are only recovered when you read
    the image.

    Please look at the DICOM standard at

         http://medical.nema.org/dicom/2003.html

    in particular to

         http://medical.nema.org/dicom/2003/03_03PU.PDF

    Section  A.1.2.9 MODALITY LUT IE, pdf-page 62.

    In your DICOM header you can check the tag entries

        RESCALE_SLOPE       (0028,1052)
        RESCALE_INTERCEPT   (0028,1053)



2)  About the PET image,
     It not clear what you mean by the image being

              "not quite prominent"

     Note that "image quality" is a very ambiguous
     and therefore dangerous term. An image is an
     information carrier, the fact that the image
     may look nice to the human eye is just a
     secondary factor.

     The question you should probably ask is:

       How to extract the appropriate information
       from the PET image ?

    You are probably trying to locate a tumor or
    track its evolution over time. In this context
    you need to keep in mind the physics behind the
    image acquisition process.



3) In order to register images two with different
    image spacing you must do two things:


    A) Check seven times that the values of origin
       and pixels spacing are correctly specified
       in millimeters in both images. Once you have
       check that seven times, you go for a walk and
       when you come back, check the values for an
       eighth time.


    B) Read the chapter on image resampling from
       the SoftwareGuide.

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

       Section 6.7.1, pdf-page 199.

       Make sure that you run by yourself all the
       examples in

       Insight/Examples/Filtering
                 ResampleImageFilter.cxx
                 ResampleImageFilter2.cxx
                 ResampleImageFilter3.cxx

       Make sure that you understand and can
       interpret correctly the output of these
       examples. The main factor to understand
       is that image registration is performed
       in physical coordinates and not in pixels.
       You have to think in millimeters and
       degrees.





Regards,



    Luis


------------------------------------
soumend at magnum.barc.ernet.in wrote:

> HI!!!!
> Thnx for the reply.
> 
> I have used the minimummaximumimagecalculator to find the pixel value range in 
> the images. And i found that for CT image it is about -1000 to +1200 and for 
> PET images it is very large 0 to 22000(or even 32767 for one set). But muy qs 
> is though the pixel representation field in CT image header shows the value 0 
> i.e unsigned value, still it has signed pixel value ...why it is so?
> 
>  I also want to ask few more things.....
> 1. PET iamges are of very low resolution but has very high intensity 
> range.....is that the reason that PET images not quite prominent. If yes then 
> how to improve the image quality?
> 
> 2. To register two images having diff slice thickness,diff spatial resolution  
> what are the initial things I have to do to make the images ready to register?
> 
> Thnx for the reply....soumen         
> 
> -------------------------------------------------
> 
>