# Why are empty spaces observed after performing Deformable registration ?

**URL:** <https://discourse.itk.org/t/why-are-empty-spaces-observed-after-performing-deformable-registration/1810>\
**Category:** Beginner Questions\
**Tags:** simpleitk, registration, python\
**Created:** [April 24, 2019, 9:49am UTC](https://discourse.itk.org/t/why-are-empty-spaces-observed-after-performing-deformable-registration/1810 "2019-04-24T09:49:02Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![dzenanz](https://discourse.itk.org/user_avatar/discourse.itk.org/dzenanz/32/1093_2.png) [@dzenanz](https://discourse.itk.org/u/dzenanz)\
**Post date:** [April 29, 2019, 4:23pm UTC](https://discourse.itk.org/t/why-are-empty-spaces-observed-after-performing-deformable-registration/1810/2 "2019-04-29T16:23:26Z")

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The “spaces” appear because the images you are registering do not cover the same spatial extent (at least after registration). The missing space has to filled by some value, see [SetDefaultPixelValue](https://itk.org/Doxygen/html/classitk_1_1ResampleImageFilter.html#a227397f3786a0ec4d3107cb73033b4be). The default value is 0 by default, but for CT images more appropriate default is -1000 or -1024.

BSpline transform is not necessarily invertible. See this [discussion](https://discourse.itk.org/t/inverse-of-bspline-transform/496) for possible alternatives.

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