[Insight-users] Wrong reading of spacing in "z" axis by GE low-dose CT?

[Bill Lorensen]

The z spacing is computed from the image position and orientation between
two consecutive  slices.

On Wed, Apr 13, 2011 at 10:06 AM, Bc. Michal Srna <michal at srna.info> wrote:

> Thanks for clarification,
>
> But there are only two values x/y  in header file of those low-dose CT data
> on tag Pixel Spacing (0028,0030).
>
> In link, you sent me:
>
> "Indeed the value for the spacing of the image could be coming from:
>
>    1. Imager Pixel Spacing (0018,1164)
>    2. Nominal Scanned Pixel Spacing (0018,2010)
>    3. Pixel Spacing (0028,0030)
>    4. Pixel Aspect Ratio (0028,0034)
>
> 1. and 2. and 4. are not present in those low-dose CT, so where does it
> take spacing for "z", when in Pixel Spacing tag, there are only two values,
> which ITK read as spacing in x,y?
>
> Does it mean, that spacing in "z" for those low-dose CT data readed by ITK
> is correct?
>
> I am just asking, because I don't know, where did it take that value
> spacing[2] = 4.41962, when there is nothing like that in DICOM header of
> those data.
>
> Firstly I assumed that pixel spacing in "z" is the same as Slice Thickness,
> because in other data set from other imaging system is Slice Thickness equal
> to spacing in "z" when readed by ITK.
>
> Maybe it's just coincidence, when you say, that those two are completely
> different aspects.
>
>
>
> 2011/4/13 Mathieu Malaterre <mathieu.malaterre at gmail.com>
>
>> FYI have a look at:
>>
>>
>> https://sourceforge.net/apps/mediawiki/gdcm/index.php?title=Using_GDCM_API#Where_is_the_value_of_gdcm.Image.Spacing_coming_from_.3F
>>
>> Slice Thickness does not means physical spacing, where did you read
>> that ? It means the tickness of slice during acquisition time. I know
>> some broken DICOM implementation uses it but this is is incorrect.
>>
>> On Wed, Apr 13, 2011 at 2:50 PM, Bc. Michal Srna <michal at srna.info>
>> wrote:
>> > Hello GDCM/ITK developers,
>> >
>> > I just discovered, that ITKv4 (also ITK 3.20) reads probably wrong value
>> of
>> > DICOM tag "0018,050 Slice Thickness" from low-dose CT data provided by
>> > SPECT/low-doseCT multimodal imaging system (manufacturer GE, model
>> INFINIA)
>> > .
>> >
>> > If I am not wrong - when reading CT data ITK takes as value of spacing
>> in
>> > "z" axis value from tag "0018,050 Slice Thickness". In my original
>> low-dose
>> > CT data it is set to 6.1, but ITK reads spacing in "z" as 4.41962 and
>> when I
>> > save those data via ITK, it writes that incorrect value into that tag
>> > "0018,050 Slice Thickness" and also into tag "0018,088 Spacing between
>> > Slices".
>> >
>> > So I am asking, if it's not bug, where does ITK take that value 4.41962,
>> > when "0018,050 Slice Thickness" in original data is 6.1?
>> >
>> > If it could help developers, I can provide example of those data, which
>> are
>> > affected by this problem.
>> >
>> > Thanks for help!
>> >
>> > --
>> > S pozdravem Bc. Michal Srna
>> >
>> > Fotografické portfolio:
>> > http://michalsrna.cz
>> >
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>>
>>
>>
>> --
>> Mathieu
>>
>
>
>
> --
> S pozdravem Bc. Michal Srna
>
> Fotografické portfolio:
> http://michalsrna.cz
>
> _____________________________________
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>
> Visit other Kitware open-source projects at
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>
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> http://www.kitware.com/products/protraining.html
>
> Please keep messages on-topic and check the ITK FAQ at:
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>
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