# Z spacing in Image Series Reader
**URL:** https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167
**Category:** Beginner Questions
**Created:** [August 19, 2019, 9:06am UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167 "2019-08-19T09:06:25Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@alkamid](https://discourse.itk.org/u/alkamid)
#### Post date: [August 19, 2019, 9:06am UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/1 "2019-08-19T09:06:25Z")
How is Z spacing determined from a DICOM series, when using SimpleITK’s `ImageSeriesReader`? It looks like X and Y spacing is taken from `Pixel Spacing` (`0028|0030`) field, but I have not been able to find the code for Z spacing. Is it inherited from GDCM, or does ITK have its own algorithm to calculate it?
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### Author:  [@zivy](https://discourse.itk.org/u/zivy)
#### Post date: [August 19, 2019, 1:10pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/2 "2019-08-19T13:10:31Z")
Hello @alkamid
The Z spacing is determined by GDCM. This is done by reading the series and looking at the Image Position Patient tag ([0020|0032](https://dicom.innolitics.com/ciods/ct-image/image-plane/00200032)) per image and then calculating the z spacing.
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### Author:  [@alkamid](https://discourse.itk.org/u/alkamid)
#### Post date: [August 19, 2019, 2:24pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/3 "2019-08-19T14:24:02Z")
Thank you @zivy for the explanation. Do you happen to know where in GDCM’s source is this calculated? The simplest method I can think of would be taking the mean of Z differences between all sorted slices — is this what’s happening? Given that this is ITK forum, treat it as a bonus question! (:
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### Author:  [@zivy](https://discourse.itk.org/u/zivy)
#### Post date: [August 19, 2019, 2:37pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/4 "2019-08-19T14:37:20Z")
Hello @alkamid,
Possibly [here](https://github.com/malaterre/GDCM/blob/b91700a8084a24359a1c0e0259ebdb35894e28c7/Source/MediaStorageAndFileFormat/gdcmImageHelper.cxx#L167), but as you noted this isn’t the GDCM forum, so you should probably ask there to get the authoritative answer.
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### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [August 19, 2019, 3:00pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/5 "2019-08-19T15:00:40Z")
Z spacing is calculated in [ImageSeriesReader](https://github.com/InsightSoftwareConsortium/ITK/blob/45c1147a497585dd33493407e540afa9eb0591fe/Modules/IO/ImageBase/include/itkImageSeriesReader.hxx#L159-L224). Take a look at this [commit](https://github.com/InsightSoftwareConsortium/ITK/commit/23436a04c978d176b32775e97d2701d96d6d2dd6) which updated that calculation for ITK 5.0.
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### Author:  [@alkamid](https://discourse.itk.org/u/alkamid)
#### Post date: [August 19, 2019, 3:06pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/6 "2019-08-19T15:06:16Z")
Thanks a lot @dzenanz, this is exactly what I was looking for!
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### Author:  [@alkamid](https://discourse.itk.org/u/alkamid)
#### Post date: [August 20, 2019, 7:26am UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/7 "2019-08-20T07:26:19Z")
@dzenanz, do I read the code correctly, and there is a default value of Z spacing = 1 if it’s not specified in DICOM metadata? Why is this the case?
```auto
if ( Math::AlmostEquals(dirNnorm, 0.0) )
{
spacing[this->m_NumberOfDimensionsInImage] = 1.0;
}
```
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### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [August 20, 2019, 1:50pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/8 "2019-08-20T13:50:05Z")
If just one image is read, having its thickness be 0.0 is more wrong than having it be 1.0. A choice which was probably made long ago, and now that is backwards compatible behavior.
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### Author:  [@alkamid](https://discourse.itk.org/u/alkamid)
#### Post date: [August 21, 2019, 12:11pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/9 "2019-08-21T12:11:55Z")
Is this fallback only activated when one image is read? Could we have a series with wrong Z spacing which would result in a 3D volume with 1.0 spacing?
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### Author:  [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [August 21, 2019, 12:59pm UTC](https://discourse.itk.org/t/z-spacing-in-image-series-reader/2167/10 "2019-08-21T12:59:35Z")
That will be zero only if the last slice has the same position as the first slice:
>
Which means there is just one slice, or the positions (image position patient tag) are junk.