# How does the Margin (grow/shrink) effect handle anisotropic voxel spacing?

**URL:** <https://discourse.slicer.org/t/how-does-the-margin-grow-shrink-effect-handle-anisotropic-voxel-spacing/48113>\
**Category:** Support\
**Tags:** segmentation, dicom\
**Created:** [September 10, 2026, 8:48pm UTC](https://discourse.slicer.org/t/how-does-the-margin-grow-shrink-effect-handle-anisotropic-voxel-spacing/48113 "2026-09-10T20:48:21Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Farah\_BH](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/farah_bh/32/78099_2.png) [@Farah\_BH](https://discourse.slicer.org/u/Farah_BH)\
**Post date:** [September 10, 2026, 8:48pm UTC](https://discourse.slicer.org/t/how-does-the-margin-grow-shrink-effect-handle-anisotropic-voxel-spacing/48113/1 "2026-09-10T20:48:21Z")

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Hello,

I’m using the Margin effect in Segment Editor to isotropically expand a segment on CT data with anisotropic voxel spacing. When I request a 3.00 mm margin, the UI reports an actual expansion of 2.9 × 2.9 × 2.5 mm, corresponding to a 3×3×1 pixel kernel.

Is this because the Margin effect uses a whole-voxel kernel, rounded per axis, rather than a continuous distance transform?

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**Author:** ![mikebind](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@mikebind](https://discourse.slicer.org/u/mikebind)\
**Post date:** [September 14, 2026, 10:10pm UTC](https://discourse.slicer.org/t/how-does-the-margin-grow-shrink-effect-handle-anisotropic-voxel-spacing/48113/2 "2026-09-14T22:10:31Z")

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Yes, you are exactly correct. I believe this is done because then the filtering can be applied entirely in voxel space. In any case, isotropic expansion will not generally be possible on an anisotropic grid: you can only add or remove whole voxels from the segment, so if the voxels are anisotropic the minimal amount you can add or remove is different in different directions.

If you want to apply an isotropic expansion, you could first resample your image or segmentation to be isotropic. You can resample an existing segmentation using the Segmentation Geometry button:

 ![image](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/e/8/e8fcfd50d11727c56402e9de433958c1d9cf4000.png)

[Segment editor — 3D Slicer documentation](https://slicer.readthedocs.io/en/latest/user_guide/modules/segmenteditor.html#main-options)