# Change the CT numbers inside contour

**URL:** <https://discourse.slicer.org/t/change-the-ct-numbers-inside-contour/13832>\
**Category:** Support\
**Created:** [October 2, 2020, 9:39pm UTC](https://discourse.slicer.org/t/change-the-ct-numbers-inside-contour/13832 "2020-10-02T21:39:50Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ToufikDZ](https://avatars.discourse-cdn.com/v4/letter/t/7ea924/32.png) [@ToufikDZ](https://discourse.slicer.org/u/ToufikDZ)\
**Post date:** [October 2, 2020, 9:39pm UTC](https://discourse.slicer.org/t/change-the-ct-numbers-inside-contour/13832/1 "2020-10-02T21:39:50Z")

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Operating system: Windows 7  
Slicer version: 4.8.0

Hello,  
I’m using slicer to change the ct numbers inside a contour. Please, I need your help to set the ct numbers (HU) inside a contour to 0 and outside it to -1000. How can I do that with 3d slicer please?

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<div class="post-metadata">

**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [October 3, 2020, 2:46am UTC](https://discourse.slicer.org/t/change-the-ct-numbers-inside-contour/13832/2 "2020-10-03T02:46:49Z")

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You can use Mask volume effect for this (available in Segment Editor module after installing SegmentEditorExtraEffects extension).

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<div class="post-metadata">

**Author:** ![ToufikDZ](https://avatars.discourse-cdn.com/v4/letter/t/7ea924/32.png) [@ToufikDZ](https://discourse.slicer.org/u/ToufikDZ)\
**Post date:** [October 3, 2020, 4:01am UTC](https://discourse.slicer.org/t/change-the-ct-numbers-inside-contour/13832/3 "2020-10-03T04:01:47Z")

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Thank you very much, it worked!