# How to create new segmentation from growing margins

**URL:** <https://discourse.slicer.org/t/how-to-create-new-segmentation-from-growing-margins/25503>\
**Category:** Support\
**Created:** [September 30, 2022, 2:34pm UTC](https://discourse.slicer.org/t/how-to-create-new-segmentation-from-growing-margins/25503 "2022-09-30T14:34:44Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Ana\_Neves](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/ana_neves/32/16808_2.png) [@Ana\_Neves](https://discourse.slicer.org/u/Ana_Neves)\
**Post date:** [September 30, 2022, 2:34pm UTC](https://discourse.slicer.org/t/how-to-create-new-segmentation-from-growing-margins/25503/1 "2022-09-30T14:34:44Z")

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Operating system: MacOS Monterey 12.2.1  
Sliver version: 5.0.2

Hello. I am currently using 3D slicer for cardiac CT analysis. I’ve been currently struggling with trying to create a new segmentation for periatrial fat.  
At this moment I have segmented the left atrium with the paint tool and filled between slices to gain a model of the left atrium, then I applied the margin tool to grow the segment with the editable intensity range with the hounsfield units for fat to select the periatrial fat.  
However, I would like to create a separate segmentation for the periatrial fat (as in the image below), without including the left atrial segment. How can I “exclude” the left atrium from the periatrial fat segmentation?

 ![segmentation example](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/0/2/022788f91116a9ee7d5c9db7ddacbfdd96aaa5a7.jpeg)

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**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:** [September 30, 2022, 7:34pm UTC](https://discourse.slicer.org/t/how-to-create-new-segmentation-from-growing-margins/25503/2 "2022-09-30T19:34:48Z")

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You can either prevent going inside left atrium during segmentation by masking (Editable area → Outside al segments), or subtract one segment from the other using “Logical operators” effect.

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**Author:** ![Ana\_Neves](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/ana_neves/32/16808_2.png) [@Ana\_Neves](https://discourse.slicer.org/u/Ana_Neves)\
**Post date:** [September 30, 2022, 8:35pm UTC](https://discourse.slicer.org/t/how-to-create-new-segmentation-from-growing-margins/25503/3 "2022-09-30T20:35:29Z")

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Thank you! That is a much better (and simpler) solution than what I was trying to do 😄