# How to subtract pre CT images from post images

**URL:** <https://discourse.slicer.org/t/how-to-subtract-pre-ct-images-from-post-images/22206>\
**Category:** Support\
**Created:** [February 28, 2022, 8:13am UTC](https://discourse.slicer.org/t/how-to-subtract-pre-ct-images-from-post-images/22206 "2022-02-28T08:13:17Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Kawtar\_Lakrad](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/kawtar_lakrad/32/13528_2.png) [@Kawtar\_Lakrad](https://discourse.slicer.org/u/Kawtar_Lakrad)\
**Post date:** [February 28, 2022, 8:13am UTC](https://discourse.slicer.org/t/how-to-subtract-pre-ct-images-from-post-images/22206/1 "2022-02-28T08:13:17Z")

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Operating system: windows 11  
Slicer version: 4.13  
Expected behavior: no idea  
Actual behavior: no idea

I am trying to subtract pre CTs from post CTs, and I was wondering if it is feasible using 3D slicer?  
Thank you so much.

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**Author:** ![pieper](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/pieper/32/8_2.png) [@pieper](https://discourse.slicer.org/u/pieper)\
**Post date:** [February 28, 2022, 9:04pm UTC](https://discourse.slicer.org/t/how-to-subtract-pre-ct-images-from-post-images/22206/2 "2022-02-28T21:04:18Z")

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It depends on what the scans are pre- and post- of. Contrast? Operations?

If it’s same subject in a part of the body that doesn’t deform (e.g. the head or a bone) you can typically register and just subtract (there’s a module in slicer to subtract volumes). Soft body parts are very hard to co-register well enough for subtraction to work.

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**Author:** ![Kawtar\_Lakrad](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/kawtar_lakrad/32/13528_2.png) [@Kawtar\_Lakrad](https://discourse.slicer.org/u/Kawtar_Lakrad)\
**Post date:** [March 3, 2022, 8:05pm UTC](https://discourse.slicer.org/t/how-to-subtract-pre-ct-images-from-post-images/22206/3 "2022-03-03T20:05:26Z")

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Okey, I am going to try this.  
Thank you so much