# Subtracting volumes from two different scans

**URL:** <https://discourse.slicer.org/t/subtracting-volumes-from-two-different-scans/17580>\
**Category:** Support\
**Created:** [May 12, 2021, 1:16am UTC](https://discourse.slicer.org/t/subtracting-volumes-from-two-different-scans/17580 "2021-05-12T01:16:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Shrb\_Show](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/shrb_show/32/10007_2.png) [@Shrb\_Show](https://discourse.slicer.org/u/Shrb_Show)\
**Post date:** [May 12, 2021, 1:16am UTC](https://discourse.slicer.org/t/subtracting-volumes-from-two-different-scans/17580/1 "2021-05-12T01:16:25Z")

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Hi,  
I am a beginner at 3D Slicers and would like some suggestion on how can I subtract volumes from two different CT scans. My goal is obtained the amount (Volume) of bone removed from the first image by comparing to it the second volume.

Thanks in advance

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**Author:** ![JanWitowski](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/janwitowski/32/4512_2.png) [@JanWitowski](https://discourse.slicer.org/u/JanWitowski)\
**Post date:** [May 12, 2021, 1:36am UTC](https://discourse.slicer.org/t/subtracting-volumes-from-two-different-scans/17580/2 "2021-05-12T01:36:29Z")

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To create subtraction images you can use Subtract Scalar Volume module that is pre-built inside 3D Slicer. Alternatively, you can use Python and perform simple subtraction between two volumes. Relevant thread: [Scalar subtraction](https://discourse.slicer.org/t/scalar-subtraction/4401)

However, if you are calculating difference between two segmentations (bone segmented on image 1 - bone segmented on image 2), you don’t need to subtract them, just segment both and calculate their respective volumes with Segment Statistics module.

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**Author:** ![Shrb\_Show](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/shrb_show/32/10007_2.png) [@Shrb\_Show](https://discourse.slicer.org/u/Shrb_Show)\
**Post date:** [May 12, 2021, 1:43am UTC](https://discourse.slicer.org/t/subtracting-volumes-from-two-different-scans/17580/3 "2021-05-12T01:43:04Z")

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Thanks for the reply.  
But I am looking to calculate the amount of bone removed during reaming by comparing the two images

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<div class="post-metadata">

**Author:** ![JanWitowski](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/janwitowski/32/4512_2.png) [@JanWitowski](https://discourse.slicer.org/u/JanWitowski)\
**Post date:** [May 12, 2021, 1:52am UTC](https://discourse.slicer.org/t/subtracting-volumes-from-two-different-scans/17580/4 "2021-05-12T01:52:11Z")

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So to answer your original question – how to subtract volumes from two different CT scans – look at my post above: you can use either Subtract Scalar Volume or python.